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Ultrasound of the Urinary Tract

Author: Matthew Benn Author: Samuel J. Southgate Editor: Stephen W. Leslie Updated: 7/4/2025 8:37:50 PM

Introduction

Ultrasound is a cornerstone imaging modality for evaluating the urinary tract due to its non-invasive nature, absence of ionizing radiation, broad availability, no contrast agents required, point-of-care usage, and real-time diagnostic capabilities.[1][2] It enables effective visualization of the kidneys, ureters (when dilated), urinary bladder, and scrotum, playing a pivotal role in the initial and follow-up assessment of a wide range of urological conditions.[3] Common clinical indications include hematuria, suspected obstruction, urinary tract infections, nephrolithiasis, renal and testicular neoplasms, testicular pain (torsion), suspected prostatic malignancies, and lower urinary tract dysfunction such as urinary retention.[4][5][6]

Ultrasonographic imaging is based on the principle that low-density (hypoechoic) structures will reflect less high-frequency sound waves than surrounding tissue, while high-density (hyperechoic) elements and organs will be highly reflective.[7] Hollow structures like simple cysts are considered anechoic and will not reflect sound waves.[7] 

The amount and magnitude of the returning sound waves are directly related to the density of the scanned tissue.[7] The elapsed time since the sound was generated will be related to the depth or distance from the transducer of the structure causing the reflection.[7] The ultrasound machine then assembles an image based on these reflections, with highly dense entities like bones or stones appearing white and anechoic structures like simple cysts appearing black.[7]

  • Color Doppler presents vascular flow velocity and directional data. A computer uses red to indicate flow towards the transducer, while blue indicates flow in the opposite direction.[7][8][9]
  • Brighter colors indicate a higher flow rate and velocity.[7][8][9] 
  • A third color, usually green or yellow, can be used to indicate turbulence.
  • Pulsed wave Doppler is a way of presenting flow velocity data on a timeline.[7][10]
  • Duplex ultrasound is the continuous presentation of pulsed wave Doppler and standard ultrasound imaging.[7][11]

Transducer shape and frequency are selected based on the depth of the structure being studied, the desired field width, and the resolution required. Higher frequencies will not penetrate as deeply but provide better resolution.[7]

As a first-line imaging tool, ultrasound is especially valuable in populations where minimizing radiation exposure is critical, including children, pregnant individuals, and patients with recurrent renal stone disease.[2][4][5][12][13] Advances in sonographic technology, such as high-resolution probes, color Doppler imaging, contrast-enhanced ultrasound (CEUS), and elastography, have improved the modality’s sensitivity and specificity for various pathologies.[14] Although computed tomography (CT) remains the gold standard for many diagnostic scenarios, ultrasound is often the initial imaging study performed, particularly in emergency, primary care, and outpatient settings. [2][4][5][6]

Point-of-care ultrasound (PoCUS) further enhances accessibility and diagnostic speed, especially in acute presentations such as flank pain, urinary retention, suprapubic discomfort, bladder fullness, hematuria, and in those patients with acute kidney injury (AKI).[2][4][5][15] When performed by trained providers, urinary tract ultrasound facilitates timely diagnosis, reduces unnecessary imaging, and supports safer, more efficient clinical decision-making.[16] This review describes the sonographic assessment of the urinary tract, including indications, techniques, findings, and applications in general, emergency, and point-of-care contexts.

Anatomy and Physiology

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Anatomy and Physiology

The urinary tract consists of the kidneys, ureters, bladder, and urethra, which function collectively to filter blood, regulate fluid and electrolyte balance, and excrete metabolic waste as urine. These structures, particularly the kidneys and bladder, are well-suited for ultrasound evaluation and are routinely visualized with high diagnostic confidence.

The adrenal glands are small, paired, triangular organs located on the superior surface of each kidney.[17] They are approximately 2 cm by 5 cm in size and lie within the superior renal fascia and inside the perirenal space. The right adrenal is more pyramidal in shape, while the left adrenal is more crescentic.[17]

They are roughly located between the 11th and 12th ribs.[17] The right adrenal gland is below the liver and posterior to the inferior vena cava. The left adrenal is superior to the splenic vessels, medial to the spleen, and lateral to the abdominal aorta. Both adrenal glands are below and anterior to the diaphragm.[17]

The kidneys are retroperitoneal organs located between the T12 and L4 vertebrae. Due to the overlying liver, the right kidney is typically positioned slightly lower than the left.[18]

Each kidney is composed of a cortex, medulla, and central collecting system (see Image "Kidney Anatomy").[18]

The renal hilum is composed of (from anterior to posterior) the renal artery, the renal vein, and the renal pelvis.[18][19] It is rotated approximately 30° posteriorly from the coronal plane.[18][19] Sonographically, the renal cortex appears homogeneous and is usually isoechoic or mildly hypoechoic relative to the adjacent liver or spleen.[20][21] 

The medullary pyramids are hypoechoic, triangular structures arranged around the central sinus. The renal sinus, which houses the calyces, renal pelvis, major vessels, lymphatics, and fat, appears hyperechoic on ultrasound owing to the echogenicity of fat.[14] Without obstruction, reflux, or a congenital defect, the collecting system will not be dilated and may be partially obscured within the echogenic sinus fat.[22] The kidney is surrounded by Gerota's fascia and echogenic retroperitoneal fat.[18]

When viewed in the coronal plane, the upper pole of each kidney is angled 15° medial to the lower pole. The mass of the psoas muscle pushes the kidneys' lower poles anteriorly compared to the upper poles.[18] 

The ultrasound probe should be aligned with the long axis of the kidney for optimal longitudinal views and to best identify the midsagittal plane.

The penis is the male sexual organ used for reproduction and urination. It comprises two paired dorsal cylindrical corpora (the erection bodies) and the ventral corpus spongiosum (urethra and glans).[23] The body of the penis has no muscles is mostly comprised of the erection bodies, urethra, glans, supporting skin, connective tissue, nerves, lymphatics, and blood vessels.[23] 

The penile tunica albuginea covers each corpora cavernosa separately and the spongiosum.[23] Buck's fascia (deep fascia of the penis) surrounds both erectile bodies as well as the urethra as a single structure and is immediately superficial to the tunica albuginea.[23]  

The arterial blood supply to the penis derives from the internal pudendal arteries. These include the paired dorsal arteries of the penis located on either side of the deep dorsal vein.[24] A dorsal penile nerve is just lateral to each of the dorsal arteries. All of these structures are beneath Buck's fascia.[24] Another large, midline superficial dorsal vein is found superficial to Buck's fascia but below Dartos fascia.[24] Each corpora cavernosa has a central cavernosal artery running inside at its center. 

The deep dorsal vein of the penis drains the venous blood from the cavernosal bodies, while the superficial dorsal vein receives venous drainage from the skin and penile subcutaneous tissue.[23] 

The prostate gland may be partially visualized transabdominally in male patients, particularly when the bladder is full. The prostate is located just inferior to the bladder and posterior to the pubic symphysis.[25] Sonographic evaluation can provide a general estimate of prostate size and contour, aiding in the assessment of prostatomegaly or bladder outlet obstruction. While not as detailed as transrectal ultrasound, the transabdominal approach offers a non-invasive and accessible method to screen for prostate enlargement in symptomatic patients.[15][25]

The prostate base is located at the bladder neck proximally and the membranous urethra inferiorly. An enlarged median lobe may extend into the bladder, causing an intravesical intrusion visible on the ultrasound.[26] The pubic symphysis and dorsal vein complex are anterior, and the rectum is posterior. The pelvic side walls and levator ani are lateral, while the neurovascular bundles and primary arterial supply are posterolateral.[26] The seminal vesicles are paired hypoechoic sacs adjacent to the posterior prostatic base.[27] 

In male patients, the prostate gland may be partially visualized transabdominally, particularly when the bladder is full. The prostate is located just inferior to the bladder and posterior to the pubic symphysis.[25] Sonographic evaluation can provide a general estimate of prostate size and contour, aiding in the assessment of prostatomegaly or bladder outlet obstruction. While not as detailed as transrectal ultrasound, the transabdominal approach offers a non-invasive and accessible method to screen for prostate enlargement in symptomatic patients and estimate the prostate's volume.[15][25]

The three zones of the prostate include the peripheral zone, the central zone, and the transitional zone. Benign prostatic hyperplasia (BPH) typically occurs in the transition zone.[26]

The scrotum and testicles constitute an external part of the male reproductive system located below the penis and anterior to the rectum and perineum. It is a thick (0.8 mm) sac or pouch composed of skin and muscle that surrounds the testicles in two compartments, separated by a septum.[28] Internally, the tunica vaginalis has two layers. The parietal layer covers the interior surface of the scrotal wall, and the visceral layer covers the testicle.[28][29] Structures inside the scrotum include the external spermatic fascia, testes, epididymides, spermatic cord, and vas deferens.[28] 

The testicles are normally surrounded by a small amount of fluid trapped between the parietal and visceral layers of the tunica vaginalis.[29] Excessive fluid in this area constitutes a hydrocele.[29] 

The ureters arise from the renal pelvis and descend medially and inferiorly through the retroperitoneum. They are normally not visible on ultrasound unless dilated. The distal ureters enter the bladder at the ureterovesical junctions (UVJs), located at the posterior-inferior aspect of the bladder. When imaged in the transverse plane, the UVJs may appear as symmetric protrusions extending into the bladder lumen. Intermittent ureteral jets can be visualized at these sites using color Doppler, reflecting normal urine flow.[30]

The urinary bladder is a muscular, extraperitoneal organ located just posterior to the pubic symphysis. When adequately distended, it appears as a well-defined, thin-walled, anechoic structure on ultrasound.[25] In the transverse view, the bladder is typically rectangular, while in the sagittal view, it takes on a more triangular or dome-shaped appearance (see Image "Ultrasound of bladder in transverse view"). The wall should measure less than 5 mm when fully distended.[31][32] The bladder serves as a key landmark for evaluating lower urinary tract anatomy and assessing urinary retention.

A thorough understanding of normal urinary tract anatomy and its sonographic appearance is essential for accurately identifying pathology and avoiding false-positive or false-negative findings during ultrasound evaluation.

Indications

Ultrasound is frequently used as a first-line imaging modality to guide the clinical management of various urinary tract conditions in emergency and outpatient settings.

Adrenal glands

Adrenal ultrasonography is indicated when possible adrenal neoplasms, incidental nodules, or tumors are seen on other imaging studies, and for suspected adrenal endocrine abnormalities.[33] Ultrasound is convenient, available, avoids ionizing radiation, and can be done at point-of-care, but CT scans and MRI are more definitive.

Indications for adrenal ultrasonography include the following:

  • Abdominal masses in children
  • Adrenal hemorrhage
  • Adrenal insufficiency
  • Adrenal nodules identified during other imaging (incidentalomas)
  • Congenital adrenal hyperplasia
  • Cushing's disease or syndrome
  • Functional adrenal disorders
  • Hyperaldosteronism
  • Hypertension due to endocrinopathies
  • Laparoscopic adrenal ultrasonography for adrenalectomy surgeries
  • Metastatic disease in patients with malignancies
  • Pheochromocytoma
  • Suspected adrenal nodules, masses, tumors, or neoplasms

Foley Catheter Placement and Verification

Ultrasound can be used to confirm the correct placement of a Foley catheter by visualizing the hyperechoic surface of the catheter balloon within the bladder lumen.[21] Fluid can be added through the catheter to distend the bladder and help with visualization.  a Foley catheter by visualizing the hyperechoic surface of the catheter balloon within the bladder lumen.[21] It may also be used to guide Foley placement in cases of challenging transurethral catheterization, especially in men.[34][35] 

Hematuria 

Hematuria, whether microscopic or gross, is a common indication for ultrasound, particularly in the outpatient or initial triage setting. Ultrasound is widely used as the initial imaging investigation of patients with microscopic hematuria (3 or more RBCs/HPF on microscopic urinalysis) due to its safety, convenience, cost-effectiveness, lack of ionizing radiation, no need for potentially nephrotoxic intravenous contrast, wide availability, and ability to detect significant renal and urinary pathology.[36][37][38] 

In the setting of gross hematuria with normal renal imaging, bladder ultrasound may detect intraluminal masses, clots, stones, or bladder wall irregularities, particularly in patients at risk for urothelial carcinoma. However, the resolution of ultrasound is insufficient to identify smaller lesions (<5 mm), and it does not visualize anterior bladder wall lesions well, so a cystoscopy will still be required.[37][39][40]

High-risk individuals (patients aged 60 or over, with a history of >30 pack years smoking, >25 RBCs.HPF, or a history of gross hematuria) will need a CT urogram (CT scan of the abdomen and pelvis without and with IV contrast) for a more comprehensive imaging investigation as well as a cystoscopy.[36][37][39]

Prostate

The base of the prostate is located at the bladder neck proximally and the membranous urethra inferiorly. An enlarged median lobe may extend into the bladder, causing an intravesical intrusion visible on the ultrasound, which is frequently misread as a possible mass.[26] The pubic symphysis and dorsal vein complex are anterior, and the rectum is posterior. The pelvic side walls and levator ani are lateral, while the neurovascular bundles and primary arterial supply are posterolateral.[26] The seminal vesicles are paired hypoechoic sacs adjacent to the posterior prostatic base.[27] 

The three zones of the prostate include the peripheral zone, the central zone, and the transitional zone. Benign prostatic hyperplasia (BPH) typically occurs in the transition zone.[26]

In male patients, the prostate gland may be partially visualized transabdominally, particularly when the bladder is full. The prostate is located just inferior to the bladder and posterior to the pubic symphysis.[25] Sonographic evaluation can provide a general estimate of prostate size and contour, aiding in the assessment of prostatomegaly or bladder outlet obstruction. While not as detailed as transrectal ultrasound, the transabdominal approach offers a non-invasive and accessible method to screen for prostatic enlargement in symptomatic patients and estimate the prostate's volume.[15][25]

The prostate is most easily seen on transrectal ultrasonography, but this is an invasive and uncomfortable evaluation for the patient. Therefore, it is not frequently used for diagnosis.

Although there are a number of indications, it is most often used for prostate biopsies, Aquablation therapy for BPH, and placement of fiduciary markers before radiation therapy for prostate cancer. 

Indications for transrectal ultrasound of the prostate include the following:

  • Aquabation therapy for BPH
  • Estimation of prostate size when treatment is needed for BPH
  • Evaluation and aspiration of prostatic abscesses
  • Evaluation for elevated PSA or a hard nodule suspicious for prostatic cancer
  • Evaluation of selected cases of hematospermia
  • High-intensity focused ultrasound (HIFU) treatment of prostate cancer
  • Male infertility
  • Placement of fiduciary markers before external beam radiation therapy for prostate cancer
  • Prostatic brachytherapy for cancer
  • Prostatic cryotherapy
  • Transrectal or transperineal prostate biopsy

Penis

Ultrasound can be used in some penile emergencies, such as possible penile fractures, traumatic injuries, and in the evaluation of Peyronie's disease.[41][42] Color Doppler ultrasound is sometimes done for the evaluation of penile vascular flow in patients with erectile dysfunction (ED), especially in those who have failed oral and topical therapy.[43][44][45]

It may also be useful in dorsal penile vein thrombosis, foreign bodies in the penis or urethra, unexplained penile masses, priapism, and urethral abnormalities such as diverticula, cysts, or abscesses.

Kidneys

Ultrasound is very good at identifying hydronephrosis, simple or complex cysts, calculi >4 mm in size, and solid renal masses, as well as tracking their progress over time.[46][47] It is often the first imaging modality used on the kidneys due to its convenience, availability, low cost, lack of ionizing radiation, painless usage, and immediate results. 

  • Simple renal cysts appear as anechoic, well-circumscribed lesions with posterior acoustic enhancement.[4][46][47] 
  • Solid or complex cystic masses may exhibit internal echoes, septations, calcifications, or vascular flow on Doppler, warranting further imaging with contrast-enhanced ultrasonography, CT, or MRI.[38][48]
  • The vascularity of renal masses can be determined using Doppler technology.

Contrast-enhanced ultrasonography can help characterize renal neoplasms, especially complex cystic lesions, but is not as useful for solid lesions due to overlapping diagnostic characteristics.[49] Ultrasound intravenous contrast uses microbubbles to enhance the visualization of blood flow with ultrasound.[50] The microbubbles are composed of a shell of albumin, galactose, lipid, or suitable polymer filled with a dense, hydrophobic gas (such as perfluorocarbons and sulfur hexafluoride).[51] The microbubbles visualize readily on ultrasound and are designed so that the gas diffuses slowly from their capsule but remains long enough for clinical use.[51]

Characteristics such as the degree and type of enhancement, degree of heterogeneity, washout rate, and the presence of ring or annular enhancement can help characterize solid renal lesions.[52][53]

Contrast-enhanced ultrasonography has been shown to be superior to standard ultrasonography and CT scanning in evaluating complex cystic renal lesions.[54]  It is better at identifying wall thickness, visualizing internal septa, and detecting solid components of these complex cystic lesions.[54] It is particularly well suited for patients with significant renal failure, as the contrast agents used for ultrasonography are usually not nephrotoxic.[54] 

Superb microvascular imaging (SMI) is a newer Doppler ultrasound technique with enhanced sensitivity for detecting slow blood flow in renal masses.[55][56][57][58] This includes detecting blood flow characteristics inside microvessels within the neoplasm.[56][57][58][57] SMI has the potential to differentiate between benign and malignant renal neoplasms just based on the vascular patterns.[56][57][58] It may also have a role in evaluating the vascularity of transplanted kidneys.[59]

Ultrasound is very operator-dependent, and visualization of the retroperitoneum, renal vasculature, and hilum is limited.[50] Visualization of renal lesions can also be challenging in patients with a high body mass index (BMI), bowel gas interposition, isoechoic neoplasms, or smaller masses (<3.5 cm).[50] The detection rate and visualization of renal lesions are far superior with CT scanning compared to ultrasound until the lesions reach at least 3.5 cm in size, at which point the two modalities are roughly equivalent.[50] 

The renal resistive index is calculated from arterial vascular flow readings in the kidney using spectral Doppler measurements at the corticomedullary junction (arcuate arteries) or next to the medullary pyramids (interlobar arteries).[60] The formula is: (peak systolic velocity - end-diastolic velocity) / peak systolic velocity, with a normal range of 0.50-0.70.[61] Increased renal resistive indices are most closely associated with ureteral obstruction from calculi but may also be elevated in various other kidney disorders, such as renal vascular disease and kidney failure.[60][62][63]

A difference of more than 0.1 in the resistive index between the two kidneys indicates obstructive uropathy, such as from an obstructing ureteral calculus.[5][60][61][64][65][66][67][68] Immediately after an acute ureteral obstruction from a stone, the immediate renal reaction is prostaglandin-mediated vasodilation. This typically lasts less than two hours, followed by a decrease in renal arterial blood flow and increased vascular resistance in the kidney due to mechanical factors and the release of regulatory pathways (kallikrein-kinin, prostaglandin–thromboxane, and renin-angiotensin) vasoconstrictors.[60][69][70] A higher unilateral renal resistive index reflects this difference.[60]

The renal resistive index, especially the difference between the resistive indices of the two kidneys, is a particularly sensitive and specific test in diagnosing obstruction from acute renal colic from a ureteral calculus in pregnant women, where intravenous urography and ionizing radiation are discouraged.[61][71][72][73] A renal resistive difference of 0.04 or more is considered significant in this population.[71]

Ultrasonography and renal resistive index can be used to verify the patency of double J ureteral stents, to differentiate obstructive from non-obstructive hydronephrosis, and to detect signs of early kidney transplant rejection or acute kidney injury.[60][62][63][74][75][76][77][78][79] An increased renal resistive index indicates decreased renal vascular perfusion and may also suggest changes in systemic hemodynamics and possible subclinical atherosclerosis, which makes renal ultrasonography valuable in the workup of primary hypertension.[63][80]

The pulsatility index is similar to the resistive index. It is calculated using the following formula: (peak systolic velocity-minimum diastolic velocity) / mean velocity and has a normal range of 0.50 to 0.70.[81][82] 

Indications for renal ultrasonography include the following:

  • Abdominal trauma
  • Active surveillance for renal lesions, such as an angiomyolipoma
  • Acute kidney injury
  • Back or flank pain that is unexplained or acute (possible abscess, pyelonephritis, or renal colic from ureterolithiasis)
  • Confirmation of double J stent placement
  • Febrile UTI (to rule out hydronephrosis and possible obstructive pyelonephritis)
  • Follow-up evaluations after reimplantation surgery
  • Further evaluation of a renal lesion seen on other imaging
  • Hematuria (gross or microscopic)
  • History of hydronephrosis or ureteral reimplantation surgery
  • Possible abscess, pyelonephritis, or renal colic
  • Preoperative evaluation of kidneys before renal procedures (percutaneous nephrostomy, PCNL), renal tumor ablation, cyst aspiration/sclerotherapy, and biopsies.
  • Renal transplant evaluation and monitoring
  • Right sided varicocele
  • Tracking of known urinary and renal calculi 

Testicular Pain and Scrotal Masses

Ultrasonography is the imaging modality of choice to visualize and diagnose intrascrotal lesions and masses, as well as investigate and evaluate testicular pain, such as a possible testicular torsion.[83][84] Color Doppler ultrasonography has proven to be very accurate for diagnosing testicular torsion.[85][86][87] 

Ultrasonography can also differentiate epididymal cysts (spermatoceles) from solid masses and tumors. It can measure the diameter of dilated scrotal varices in varicoceles, examine impalpable testicles in patients with hydroceles, determine if a traumatized testis is ruptured, diagnose testicular torsion, identify testicular malignancies, indicate testicular infections (orchitis, epididymitis), visualize the testicle after trauma, and verify vascular flow to the testicle.[88][89][90][91] 

Scrotal (testicular) ultrasonography is the diagnostic procedure of choice for testicular pain or scrotal masses, and can also be useful in the evaluation of male infertility.[83][84]

Indications for scrotal ultrasonography include the following:

  • Abnormal scrotal sac (Hydrocele or spermatocele)
  • Abnormal scrotal wall lesion 
  • Unexplained retroperitoneal lymphadenopathy
  • History of testicular cancer, leukemia, or lymphoma
  • Infertility
  • Possible testicular torsion
  • Scrotal abscess
  • Scrotal mass identified by other imaging
  • Scrotal swelling
  • Scrotal or testicular mass on examination 
  • Scrotal surgery follow-up
  • Testicular nodule or lump
  • Testicular pain (chronic and acute)
  • Testicular trauma
  • Varicocele

Ureteral Obstruction, Hydronephrosis, and Vesicoureteral Reflux

One of the most common indications for urinary tract ultrasound is the evaluation of suspected ureteral obstruction, particularly in the setting of flank pain due to nephrolithiasis.[14][92][93] Patients often present with acute, colicky pain radiating to the groin, which may be accompanied by hematuria, nausea, or vomiting.[94] The hallmark sonographic finding of obstruction is hydronephrosis, which appears as an anechoic dilatation of the renal collecting system within the echogenic renal sinus and possibly the associated ureter (see Video "Nephrolithiasis, Ultrasound").[14][93]

Hydronephrosis is typically graded as:

  • Mild: Preservation of renal papillae with early calyceal dilatation.[4][15][20]
  • Moderate: Blunting of calyces and obliteration of papillae, often producing a "bear paw" appearance.[4][15][20]
  • Severe: Coalescence of dilated calyces into a single fluid-filled space, with compression of a thinned renal cortex (<1 cm).[4][15][20]

Hydronephrosis is not specific to ureterolithiasis and may also occur due to extrinsic compression of the ureter (such as from pregnancy, a retroperitoneal tumor, or a pelvic mass), an anatomical defect such as a ureteral stricture, or in the context of lower urinary tract dysfunction (such as urinary retention, benign prostatic hyperplasia, or neurogenic bladder).[95][96] 

In children, contrast-enhanced voiding urosonography is a safe, sensitive, well-established method of diagnosing and following vesicoureteral reflux without patient radiation exposure.[97][98][99][100][101][102][103][104][105] There is evidence that contrast-enhanced voiding urosonography may be superior to conventional voiding cystourethrography in diagnosing and grading pediatric vesicoureteral reflux, although the reported false negative rate of 3% is a concern.[101][104][106][104]

Contrast-enhanced ultrasonography can also be used to evaluate the urethra and lower urinary tract, as well as a possible alternative to fluoroscopy in video urodynamics.[107][108][109][110][111]

Differentiating Hydronephrosis from Similar Appearing Mimics

Clinicians must distinguish hydronephrosis from normal anatomic variants or other anechoic structures to avoid misinterpretation, misdiagnosis, and inappropriate treatment. 

  • An extrarenal pelvis, found in about 10% of the population, may appear as an isolated anechoic structure medial to the hilum and should not be mistaken for collecting system dilatation (see Image "Extrarenal pelvis on ultrasound").[112] 
  • Color Doppler can help identify hilar vessels and rule out other structures that can simulate hydronephrosis.[113]
    • It does this by identifying blood flow in major arteries and veins, distinguishing stagnant dilated calyces from blood vessels with blood flow, and diagnosing pseudohydronephrosis that suggests dilation but without obstruction.[113]
  • Hydronephrosis will demonstrate an elevation in the renal resistive index not found in mimicking structures like an extrarenal pelvis or a parapelvic cyst.[114][115]
  • Hydroureteronephrosis technically refers to the dilation of the renal pelvis and its associated ureter, although the term hydronephrosis is commonly used for both conditions.
  • Parapelvic cysts, found in up to 1.5% of individuals, are well-defined, non-communicating anechoic cystic structures in the renal sinus.[4][15] 
  • Renal pyramids are triangular, hypoechoic areas found in the medulla, bounded by the cortex, which do not distort renal architecture.[20][116]

Urinary Bladder and Retention of Urine

The urinary bladder is frequently scanned with ultrasound for a volume determination (post-void residual) and to diagnose urinary retention in patients with suprapubic pain and/or urinary difficulties.[117][118][119] Ultrasound is a rapid and reliable method for evaluating bladder volume in such situations. In the transverse plane, the width (W) and depth (D) of the bladder are measured; in the sagittal plane, the height (H) is recorded.[120] Bladder volume is estimated by multiplying these dimensions together with a correction coefficient for the bladder shape (discussed later in the Technique or Treatment section).[121]

This information aids in determining the need for catheterization and monitoring post-void residual volumes. Generally, a post-void residual volume of 150 mL or less is acceptable, while 400 mL or more is exceptional and may represent urinary retention.[117] A significantly distended bladder in the presence of hydronephrosis may indicate a lower urinary tract obstruction.[122]

A ureterocele is a congenital birth defect where the distal ureter balloons out, extending into the bladder lumen, forming a sac-like structure.[123] This can cause ureteral obstruction, which can progress to kidney damage, urinary tract infections, and other problems.[123] It is often diagnosed during prenatal ultrasonography or other imaging during a UTI workup.[123]

Indications for bladder ultrasonography include the following:

  • Assessment of posterior urethral valves using contrast-enhanced voiding urosonogram
  • Bladder neoplasm
  • Bladder volume measurement
  • Benign prostatic hyperplasia (to identify changes associated with outlet obstruction)
  • Confirmation of Foley catheter position
  • Evaluation of intravesical clot burden (hematoma)
  • Foreign bodies in the bladder
  • Hematuria evaluation
  • Investigation for distal ureteral dilation and/or distal ureteral calculi
  • Measurement of bladder volume in urinary retention
  • Measurement of post-void residual urine volume
  • Pelvic fluid collections
  • Prostate imaging when the transrectal approach is not available (such as after rectal cancer(
  • Suprapubic tube placement assistance
  • Ureterocele evaluation and tracking
  • Urinary retention
  • Vesicoureteral reflux assessment with contrast-enhanced voiding video urosonogram

Urinary Calculi

Although most of the ureter is not reliably visualized due to overlying bowel gas and stool, stones can sometimes be detected at the ureteropelvic junction (UPJ), within the renal pelvis, or at the ureterovesical junction (UVJ).[124][125] Stones appear as hyperechoic foci with posterior acoustic shadowing.[4][126] Color Doppler may reveal a “twinkle artifact,” a rapidly alternating signal that enhances stone detection, which is highly predictive of urolithiasis, especially when combined with a finding of acoustic shadowing in patients with urinary stones sized 5 mm or more. (see Video "Nephrolithiasis, Ultrasound").[127][128][129][130][131][132][133][134]

Ultrasound can reliably identify larger renal calculi (>5 mm), but its sensitivity is significantly reduced for stones <4 mm, and it tends to overestimate the size of smaller stones it detects.[127][135][136][137][138] Adding a flat plate abdominal X-ray (KUB) to ultrasonography can substantially enhance the usefulness of the ultrasound, as the KUB is more likely to visualize smaller calcific stones, while ultrasound can detect even radiolucent calculi, such as those composed of uric acid.[127][137][139][140] Sizeable calculi easily seen on ultrasound that are not visible on the KUB would strongly suggest a radiolucent stone.

Stones at the ureterovesicle junction (UVJ),  the most common location for impaction of ureteral calculi due to their narrow ureteral caliber, may cause an identifiable focal protrusion into the bladder wall and can occasionally be directly visualized on a transverse bladder scan.[14] 

The absence of a ureteral jet on Doppler may suggest complete obstruction, although variability in jet timing limits its reliability as a standalone finding.[30][141][142][143]

Ultrasound has significant limitations in the assessment of urinary tract calculi, particularly in directly visualizing small stones (<4 mm) as well as overestimation of calculus size, so a non-contrast CT (the gold standard) should be pursued if clinical suspicion is high or when ultrasound findings are inconclusive.[126][135]

Hydronephrosis from a ureteral calculus can be seen on ultrasonography, even if the stone itself may not be visualized with this modality. A high renal resistive index would suggest an obstruction in a patient with apparent renal colic with or without hydronephrosis.[5][60][64][65][66] Ureteral dilation from hydronephrosis may also be seen. The lack of hydronephrosis may suggest that there is no acute obstruction, but this is not definitive.

If the clinical symptoms and/or the resistive index suggest a stone and the ultrasound is negative, consider a CT scan for confirmation.

7. Ultrasonography of the Adrenal Gland

Due to the small size of the adrenal glands, extracorporal sonography of the normal adrenal gland is challenging. In contrast, laparoscopic sonography facilitates excellent visualization of the glands, especially to identify surgical planes.44

Adrenal glands in adults are not normally visualized on transabdominal ultrasound, especially in obese patients. Any prominent adrenal gland seen on ultrasound should be considered abnormal and further imaging with CT or MRI should be considered. Intra-operative sonography of the adrenal gland is useful during adrenalectomies, particularly in the following cases: small nodules, right-sided adrenal tumors, obese patients, or during partial adrenalectomy.

During transabdominal sonography, a curved-array 2.5-6 MHz transducer offers a wide, deep field of the view that is necessary to image the adrenal glands deep in the abdomen. For laparoscopic intra-operative ultrasonography, a linear side-viewing or a flexible side-viewing (preferred) transducer 7.5-10 MHz probe provides excellent ultrasonographic visualization.

Transabdominal subcostal or intercostal views both provide adequate visualization of the adrenal glands. The lateral decubitus position may improve visualization when overlying bowel gas is present. After visualizing the upper pole of the kidney, the probe is directed medially to identify the adrenal gland.

During laparoscopic sonography, the transducer is placed through a standard 10mm port. Typically, the most medial or lateral port is used for the transducer. Rarely, an additional trocar is needed to evaluate the surgical margins of a benign adrenal mass that may be considered for laparoscopic partial adrenalectomy. The upper pole of the kidney should be scanned from medial to lateral in the longitudinal plane until the adrenal gland is identified. If surgical margins are not grossly evident, sonography can be used to determine relationships with other adjacent structures. Doppler ultrasound can also be used to identify vascular structures.

Adrenal masses or hyperplasia may be found incidentally during ultrasound for other indications, but transabdominal ultrasound to specifically assess the adrenal glands may be useful in following patients with an adrenal mass that has not been treated surgically. Additionally, laparoscopic ultrasound is a valuable adjunct to laparoscopic adrenalectomy, facilitating tumor localization and identification of adjacent structures. This technique is particularly helpful for partial adrenalectomies.45,46,47,48,49,50

7.2 AdenomasAdrenal adenomas are often found incidentally and in most cases are non-functioning. Sonographically, adrenal adenomas appear as well-demarcated, solid nodules, attached to the adrenal limb. Adrenal adenomas are usually isoechoic to the adrenal gland and are hypoechoic relative to the perinephric fat. A left adrenal adenoma can be misdiagnosed as an accessory spleen if its relationship with the adrenal limb is indeterminate.51

7.3 MyelolipomasAdrenal myelolipomas are rare, nonfunctioning tumors composed of varying proportions of bone marrow elements and fat. They arise from the zona fasciculata of the cortex and appear sonographically as echogenic masses. Lesions can be heterogeneous as a result of internal hemorrhage, and if they are predominantly composed of myeloid (as opposed to fat), they can appear iso- or hypo-echoic.

7.4 PheochromocytomaPheochromocytomas are typically large and well-defined on ultrasound, but can appear as homogenous solid structures or can be more heterogenous (Figure 34). Bleeding may occur in the medulla, and typically appears as a round to oval echogenic mass in the central part of the gland.52

Figure 34. Pheochromocytoma7.5 Primary Adrenal CarcinomasAdrenal cortical carcinomas are rare, and the sonographic appearance is variable, depending in part on the size of the lesion. Smaller lesions are homogeneous and appear sonographically similar to adenomas. The mass may have a thin, capsule-like echogenic rim and some calcifications. Larger lesions may contain necrotic material and hemorrhage and can mimic a pheochromocytoma. In some cases, ultrasound can be more informative than CT or MRI. For example, when a large adrenal mass abuts the liver and the origin of the mass is uncertain, anterior displacement of the retroperitoneal fat reflection is strongly suggestive of adrenal origin.51

7.6 LymphomaPrimary adrenal lymphoma is rare, but secondary involvement of the adrenal glands is present in as many as 25% of lymphoma patients. Nearly half of all adrenal lymphomas are bilateral, and the most common type of lymphoma to involve the adrenal gland is non-Hodgkin’s lymphoma. Sonographically, these tumors are echo-poor. However, echogenic areas of necrosis or hemorrhage may be present.

7.7 MetastasisThe adrenal glands are a common site of metastatic disease. Metastatic lesions < 3 cm in size may appear as a solid homogenous mass, indistinguishable from an adenoma. Larger lesions may demonstrate heterogeneity due to central necrosis or hemorrhage.

Contraindications

There are no absolute contraindications to performing abdominal or pelvic ultrasound, including urinary tract ultrasound. The procedure is non-invasive, does not involve ionizing radiation, and is generally safe for all patient populations, including pregnant individuals and children.[12][13]

Relative limitations may arise if inadequate acoustic windows are encountered due to obesity, excessive bowel gas, or poor patient cooperation.[144] Ultrasonography is very operator-dependent, which may lead to suboptimal or variable imaging quality at times. These factors can all hinder diagnostic accuracy by reducing image quality.

In such situations, alternative imaging modalities, such as CT or MRI, should be considered to complete the diagnostic evaluation.

The only relative contraindication to ultrasound evaluation would be the need for urgent surgical intervention, where even a brief delay for an ultrasonic assessment is inadvisable.

Equipment

Ultrasound of the urinary tract is typically performed using a low-frequency (2–5 MHz) curvilinear transducer, which provides adequate depth penetration for evaluating the kidneys, bladder, and surrounding structures in most adult patients.[25][144] This transducer is optimal for imaging retroperitoneal organs due to its wider footprint and deeper tissue penetration.[15][22]

In pediatric patients or in the assessment of superficial structures—such as the bladder in thin individuals or transplant kidneys—a high-frequency linear transducer (5–12 MHz) may be used to achieve improved spatial resolution.[25]

Modern ultrasound systems often incorporate color and spectral Doppler capabilities, which are essential for assessing renal perfusion, evaluating suspected vascular abnormalities, and visualizing ureteral jets. Some systems also offer contrast-enhanced ultrasound (CEUS) functionality for characterizing renal masses.[145] Additionally, while still emerging in routine practice, elastography may be available for research or advanced diagnostic applications, particularly in patients with chronic kidney disease or renal transplant follow-up.[146]

For point-of-care applications, portable ultrasound machines with curvilinear probes and Doppler functionality are commonly used and offer sufficient image quality for most diagnostic needs when performed by trained providers.

Personnel

Ultrasound of the urinary tract may be performed by a range of trained healthcare professionals, including radiologists, sonographers, urologists, nephrologists, emergency physicians, intensivists, and other clinicians credentialed in PoCUS. Competency in performing and interpreting renal and bladder ultrasound requires appropriate training and supervised experience to ensure diagnostic accuracy and patient safety.

According to the American College of Emergency Physicians, clinicians should complete a minimum of 25-50 supervised renal and bladder ultrasound examinations to achieve basic competency in these studies.[147] Additional practice and correlation with radiologic interpretations are recommended to refine skills, particularly in recognizing less common or subtle findings.

In institutions with formal ultrasound credentialing programs, ultrasound studies may be performed by certified sonographers and interpreted by board-certified radiologists. In point-of-care settings, clinicians performing bedside urinary tract ultrasound must maintain proficiency through continued education, quality assurance programs, and adherence to institutional guidelines for documentation and image storage.

Preparation

Ultrasound of the urinary tract should be performed with the patient in a supine position, with the abdomen and pelvis fully exposed to allow unobstructed access for transducer placement.[4] A towel or drape may be used to protect the patient’s clothing and maintain modesty. Generous application of ultrasound gel helps to eliminate air between the transducer and skin surface, improving acoustic coupling and image quality.

Use warmed ultrasound gel whenever possible for patient comfort.

Applying the gel only to the location being scanned is usually preferable. This helps minimize air bubbles, which can interfere with ultrasound transmission and detection.

Right-handed operators should position the ultrasound machine on the patient's right side for optimal ergonomics. This allows for easy visualization of the screen while scanning with the right hand and adjusting machine controls with the left.

The room should be dimly lit to enhance screen visibility and reduce glare. The patient should be instructed to remain still and breathe normally during the scan.

When assessing the urinary bladder, the patient should ideally have it full to improve visualization of the bladder contour, wall thickness, interior lining, neoplasms, foreign bodies, and the ureterovesical junctions. Ultrasound scanning can be performed in urgent or emergent scenarios regardless of bladder volume, but the sonographer should note suboptimal conditions.

Cooperation may be improved in pediatric patients by involving parents, providing clear instructions, and using distraction techniques when appropriate.

Technique or Treatment

Ultrasound of the urinary tract is typically performed using a low-frequency curvilinear transducer (2–5 MHz) to achieve adequate depth penetration and resolution for abdominal and pelvic structures.[15] A phased-array transducer may be used if a curvilinear probe is unavailable, particularly in limited acoustic windows. The ultrasound system should be set to the “Abdominal” preset, which optimizes image parameters for structures such as the kidneys and bladder.

BLADDER

Bladder ultrasonography typically uses a low-frequency curvilinear transducer (3.5-5 MHz). This provides a wider field of view with sufficient penetration to visualize the entire bladder. A phased array transducer may also be used.

Bladder imaging begins with the transducer placed just superior to the pubic symphysis, with the indicator directed superiorly to obtain a sagittal view.[22] The probe is then rotated 90 degrees counterclockwise, with the indicator pointing to the patient’s right, to acquire a transverse view. Fanning through the bladder in this plane allows assessment of the full volume and contour.

The ureterovesicle junction, located at the posteroinferior aspect of the bladder, should be examined for distal ureteral stones, which may appear as hyperechoic foci with posterior shadowing. Color Doppler imaging may assist in visualizing ureteral jets, indicating ureteral patency.

In a full bladder, the urine appears anechoic and fills most of the anterior screen. The bladder wall should be evaluated for smoothness, thickness, and the presence of any mural or intraluminal abnormalities such as clots, diverticula, or masses.[4][25] 

Bladder stones are readily visualized on bladder ultrasound examinations.[148] They will appear as well-demarcated, highly echogenic structures with intense posterior acoustic shadowing, that are mobile and move as the patient changes position.[148]

Bladder tumors are generally too small to be seen on ultrasound unless they are sizable. This is why cystoscopy is required to complete a urological workup for hematuria.

The bladder wall should be <5 mm thick.[149] A uniformly thickened bladder suggests bladder overactivity, cystitis, or outlet obstruction, such as from BPH or a urethral stricture.[150] The normal maximum bladder capacity is about 500 mL, and the post-void residual urine volume should be <150 mL.[117]

An enlarged median lobe of the prostate can extend into the bladder lumen. If large enough, this can cause obstructive uropathy or create a "ball-valve" effect, making urination difficult. A mass at the bladder neck or base, on imaging of the bladder, is often an extension of the prostatic median lobe, which is frequently overread by radiology.

Trabeculations of the urinary bladder wall typically occur with bladder outlet obstruction but may also be seen with prolonged overactivity. They are caused by hypertrophied individual muscle bundles in the bladder wall that have hypertrophied at different rates, creating irregularities. These can progress to deep cells and eventually diverticula, outpouchings of the bladder mucosa and submucosa extending outside the external contour of the bladder, and are devoid of muscle. Ultrasound can detect trabeculations and diverticulae, as the neck of the diverticulum is easily seen, and color Doppler imaging can determine if there is internal stagnation.

Ureteroceles are congenital birth defects where the distal ureter balloons out, extending into the bladder lumen, forming a sac-like structure.[123] This can cause ureteral obstruction, which can progress to kidney damage, urinary tract infections, and other problems.[123] It is often diagnosed during prenatal ultrasonography or other imaging during a UTI workup.[123]

A ureterocele will appear as a thin-walled, fluid-filled cystic mass of variable size located inside the urinary bladder near the ureterovesical junction (UVJ).[123] They are often associated with duplicated kidneys.[123] Ureteroceles may be obstructive and cause hydroureteronephrosis with ureteral dilation.[123] On imaging, ureteroceles can give a "bladder-in-bladder" appearance or demonstrate a "Cobra Head Sign."[123] If the ureterocele is causing symptoms or renal deterioration, treatment is generally surgical.[123]

For bladder volume estimation, especially in cases of urinary retention, height (superior-inferior), width (lateral), and depth (anterior-posterior) are measured in cm. (see Image "Bladder Dimensions Visualized on Ultrasound") A volume formula is applied by multiplying these values with a correction coefficient, as determined by the overall bladder shape.[120][121] A universal or standard correction coefficient of 0.7 can be used. However, more accurate measurements are obtained using a customized coefficient value based on each patient's bladder shape, ranging from 0.56 (spherical bladder) to 0.92 (cuboid).[121] 

The standard formula for calculating the bladder volume is: [Bladder volume (mL) = Height (cm) x Width (cm) x Depth (cm) x Correction coefficient (standard is 0.7)].[121]

KIDNEYS

For renal ultrasonography, a low-frequency (2-5 MHz) curvilinear transducer is preferred for most adults. A higher frequency (5-12 MHz) may be used for children, superficial transplanted kidneys, pelvic kidneys, or very thin patients.

If applicable, a renal evaluation should begin with the asymptomatic side to establish a normal baseline and identify patient-specific anatomical features.

  • For the right kidney, the patient is supine. The transducer is placed in the mid-axillary line, just below the costal margin, with the probe indicator directed superiorly. The right kidney is best visualized in a coronal oblique plane using the liver as an acoustic window, appearing medial and inferior to the liver on the screen.[4][15]
  • For the left kidney, the transducer is positioned in the posterior axillary line, typically higher than the right side due to the superior position of the left kidney. The patient is typically in the right lateral decubitus position to minimize bowel interference.  Visualization is often more challenging due to overlying bowel gas and the absence of a large acoustic window, like the liver. The spleen may assist by providing a partial acoustic window.[15][22]

To improve image acquisition and reduce rib shadowing, the transducer can be rotated obliquely to align with intercostal spaces, rotating counterclockwise on the right and clockwise on the left. Having the patient take and hold a deep breath may widen the intercostal spaces and push the kidneys into a more favorable position. Placing the patient in the right lateral decubitus position in complex left kidney visualization cases can improve access.

Each kidney should be scanned in longitudinal and transverse planes, using a slow fanning motion to evaluate the entire organ from anterior to posterior. Key assessment features include renal size (dimensions), cortical thickness, corticomedullary differentiation, echogenicity, presence of hydronephrosis, cysts, stones, or masses.

  • Both kidneys should be roughly the same size. A difference in length of more than 1.5 cm is considered abnormal.
  • The renal cortical thickness should be measured. 1.5 cm is considered normal, while a cortical thickness of less than 1 cm is worrisome for cortical thinning.
  • The parenchyma of the renal cortex should be homogeneous and hypoechoic or isoechoic compared to that of the spleen or liver. The renal hilum is more echogenic than the renal cortex due to its large vessels, sinus fat, and collecting system. The kidney's medullary pyramids are typically hypoechoic compared to the renal cortex.
  • The upper poles of the kidneys are usually closer together than the lower poles, as the upper pole of each kidney is tilted or angled towards the midline. If this is reversed, with the lower poles closer together, it suggests a horseshoe kidney.[151]
  • Renal ultrasound is not very sensitive for visualizing renal calculi smaller than 3 mm. However, it is the preferred imaging study for pregnant and pediatric patients who present with possible renal colic.

Parenchymal renal scarring is almost always seen over the renal pyramids, frequently affecting compound calyces, and often appearing at the poles. The renal cortex is thin or absent when there is a parenchymal scar, which is typically visualized as an acute depression of the otherwise smooth renal outline.[152] 

Such scars are almost always related to kidney infections and are often associated with vesicoureteral reflux in childhood.[153] A renal nuclear scan is a much more sensitive imaging modality for imaging parenchymal scarring, although contrast-enhanced ultrasonography may be helpful.[154][155][156] Standard ultrasound alone is considered insufficient to detect renal parenchymal scarring.[157]

Cysts of the kidney are often found incidentally on ultrasound examinations of the kidneys. The following three criteria identify and define a simple cyst: [158]

  1. Anechoic interior
  2. Thin, smooth, and well-demarcated walls
  3. Posterior acoustic enhancement beyond the cyst

Simple renal cysts do not need additional imaging or testing, but solid masses and complex cysts typically require a CT (preferred) or MRI scan without and with IV contrast or contrast-enhanced ultrasonography to determine vascularity and look for tissue vascular enhancement.[159][160][161][162][163][164] Such vascular enhancement suggests a likely malignancy.[163][165]

The Bosniak classification scoring system was developed for characterizing renal cystic lesions on CT but has been expanded and updated to describe cystic neoplasms seen on ultrasound, especially when IV contrast enhancement is used to determine vascularity.[166][167][168][169][170][171][172][173][174] The Bosniak classification system is described as follows: 

  • Class I: A benign simple cyst with a very thin wall without septa, calcifications, or solid components. Density similar to water (≤15 HU) with no enhancement after IV contrast administration. Malignant potential is approximately 0%. No follow-up imaging is required. 
  • Class II: There are two sub-categories:
    • A benign simple cyst with a few thin walls and septa. May have fine calcifications but no vascular enhancement after IV contrast.
    • Homogeneous high-attenuating (>70 HU) masses ≤3 cm in size with sharp margins. No vascular enhancement after IV contrast. Malignant potential is extremely low at <1%. Follow-up imaging is unnecessary but may be suggested 6 or 12 months later in selected patients to confirm the diagnosis and classification.
  • Class IIF: An indeterminate designation between classifications II and III, with two subtypes:
    • Cysts with multiple thin septa with smooth, minimally thickened walls that may have nodular calcifications. There may be an impression of some mild enhancement.
    • High attenuation masses >3 cm in size but with no vascular enhancement after IV contrast administration. The estimated malignant potential is 5%. Follow-up imaging with CT or ultrasound is recommended at 6 months and then annually for 5 years.
  • Class III: The cyst demonstrates a multilocular structure with thickened walls or septa, irregular calcifications, or measurable vascular enhancement >10 Hounsfield units. These cysts have an estimated malignancy rate of 55%. Close follow-up or surgical excision with partial or total nephrectomy is recommended based on lesion size as follows:
    • Active surveillance if the lesion is ≤2 cm
    • Active surveillance, percutaneous ablation, or surgical excision if the lesion is 2–4 cm
    • Surgical excision if the lesion is >4 cm
  • Class IV: These cysts are typically large nodular lesions with enhancing solid or necrotic components. They have an extremely high malignant potential of 91% to 100%. Surgery consisting of a partial or total nephrectomy is recommended. 

Parapelvic cysts can sometimes be mistaken for hydronephrosis. One way to differentiate a parapelvic cyst from hydronephrosis is to look at its orientation. Parapelvic cysts have their long axis pointing between the renal pyramids, while in hydronephrosis, the long axis points to the renal pyramid and papilla. Renal resistive index can also differentiate a parapelvic cyst and an extrarenal pelvis (with normal or low resistive index) from true hydronephrosis with its higher resistive index.

Pyelonephritis typically appears as a unilateral enlargement of the affected kidney with increased (brighter) echogenicity and decreased (darker) corticomedullary differentiation. Doppler ultrasonography may show decreased blood flow to affected areas, and abscesses may also be visible. Hypoechoic areas will look darker on ultrasound, representing edema or inflammatory changes. Hyperechoic areas that appear brighter may indicate hemorrhage.

Ultrasound is highly recommended for patients with acute pyelonephritis, especially when they are unresponsive to appropriate medical therapy, as it can identify hydronephrosis and an increased renal resistive index, suggesting potentially life-threatening obstructive pyelonephritis (pyonephrosis) requiring urgent surgical drainage.[175]

Ultrasound in patients with renal failure typically shows more hyperechoic parenchyma (relative to the liver), reduced cortical thickness, and a smaller kidney overall.[176][177] However, older individuals will naturally have a thinning of the renal cortex and smaller kidneys.[178][179][180] 

Solid renal masses are often initially detected by ultrasonography, which is frequently the first imaging modality used for suspected kidney neoplasms. As ultrasound is a non-invasive, convenient, economical imaging modality that avoids radiation exposure, it is recommended for initial imaging of suspected renal neoplasms before resorting to CT or MRI.

Doppler ultrasonic imaging can assess tumor vascularity, and the diagnosis can be initially made by ultrasonography, such as benign angiomyolipomas, which demonstrate characteristic hyperechoic fatty components that can be confirmed by other imaging.[163][181]

CT and MRI are more sensitive, but ultrasound is more convenient and less costly, avoids unnecessary radiation exposure, and performs extremely well for tracking renal neoplasms monitored closely under an active surveillance protocol. The latest American Urological Association guideline on evaluating microhematuria recommends renal ultrasonography as the suggested imaging modality of the kidneys for low and intermediate-risk patients with microhematuria.[182][183]

Renal ultrasound also has an intraoperative role. It can be used to localize intraparenchymal masses in partial nephrectomy surgery, as contrast-enhanced ultrasonography and Doppler imaging improve tumor visualization.[184][185][186][187][188] Imaging is projected onto console screens for real-time visualization.

Ultrasound guidance is sometimes utilized to establish percutaneous access to the renal collecting system for nephrostomy tube placement or for percutaneous nephrolithotomy. It has the advantage of not using ionizing radiation but requires some degree of hydronephrosis as a target and may not be feasible in very obese patients.[189]

PENILE URETHRA

For the evaluation of the penis and urethra, the preferred probes are linear transducers with a maximum frequency of 12-15 MHz.[24]

To visualize the urethra, sterile ultrasound gel or 2% lidocaine jelly is injected gently through the urethral meatus using a 60 mL catheter-tip syringe. A small urethral catheter may also be used. About 20 mL is needed to fill the urethra. Filling the bladder provides a useful landmark. If any strictures are found, their location, length, diameter, and distance from the external sphincter or other recognizable landmark should be noted.[190]

There is increasing use of contrast-enhanced urethral ultrasonography, particularly in pediatrics. The contrast agent is instilled into the bladder for voiding studies or directly into the urethra for retrograde urethrosonography.

A urethral lesion may be found in up to 15% of all penile traumas.[24][191][192] If blood is observed at the urethral meatus, an imaging evaluation of the urethra is necessary.[193] This is often a retrograde urethrogram, but ultrasound with contrast enhancement can also be used, avoids ionizing radiation, and can be done more quickly, even while waiting for X-ray availability.[192] Magnetic resonance imaging may also be used, but it is time-consuming, costly, and generally restricted to equivocal cases when alternative imaging is inconclusive.[24]

PENIS

As an external, superficial organ, the penis is particularly well suited to ultrasonic examination.[24][194] As such, it is probably underutilized. Linear transducers with a higher frequency range of 12-15 MHz are typically used for penile ultrasonography.[24]

The corpora cavernosa appear as well-delineated cylindrical structures with moderate homogeneous echogenicity.[45] They are less echogenic than the corpus spongiosum.[45] The penile septum is visualized as an area of posterior acoustic shadowing between the two corpora.[45][195][196] Buck's fascia and the tunica albuginea appear as bright (dense) lines, with the tunica measuring about 2 mm thick when the penis is flaccid.[24][45]

The cavernosal arteries can be seen in the center of each corpora as thin tubular structures with echogenic walls.[45] Their normal diameter is 0.3-0.5 mm (flaccid) and >0.7 mm when erect.[45] The dorsal penile veins appear as anechoic compressible tubules and usually have detectable flow on color Doppler US (Fig. 3).[45] 

In the evaluation of erectile dysfunction, particularly before consideration of a penile prosthesis surgical implant, an intracorporal injection of vasoactive drugs (such as alprostadil, papaverine, or a combination such as trimix: alprostadil, papaverine, and phentolamine) is given.[197] Cavernosal arterial diameters are measured before and 5 minutes after the injection, and the clinical response is noted.

Left and right cavernosal peak systolic velocity (PSV) and end-diastolic velocity (EDV) are recorded, and the resistive index (RI) is calculated.[43][44][45]

  • The PSV is the maximum flow rate during systole.[44][45]
  • The EDV is the residual flow at the end of diastole.[44][45]
  • The RI indicates the peripheral resistance to penile blood flow. The formula is: Resistive Index (RI) = (PSV − EDV) / PSV.[44][45]
  • The PSV is the most accurate indicator of penile arterial disease.
  • A PSV value of 35 cm/sec or more is considered normal, while a PSV determination of less than 25 cm/sec is diagnostic of severe arterial insufficiency.[44][45] 
  • PSV values between 25 and 35 cm/sec are considered intermediate or marginal for diagnosing arteriogenic ED.[24][44][45][194][198]
  • A PSV of 30-35 cm/sec is probably acceptable or borderline, while a PSV of 20-25 cm/sec is marginal and may be abnormal.[194]
  • A difference in PSV readings between the two cavernosal arteries of more than 10 cm/sec is suggestive of erectile dysfunction due to arterial insufficiency.[44][45][199]
  • The EDV and RI reflect the activity of the veno-occlusive mechanism.[44][45] Normal cutoff values would be an EDV of less than 5 cm/sec with a resistive index (RI) greater than 0.8.[44][45][200][201]
  • In patients with arterial insufficiency, venous competence cannot be assessed using Doppler US.[24]
  • Faster arterial acceleration time (>110 milliseconds) and/or failure to visualize the helicine arteries with power Doppler and incomplete or inadequate penile rigidity after intracorporeal vasoactive injection indicate arteriogenic ED.[194]
  • Damped waveforms and high-velocity jets on penile Doppler ultrasonography suggest proximal artery stenosis.[199]
  • Arteriogenic erectile dysfunction is also an independent risk factor for cardiovascular disease, especially as ED typically occurs several years before any cardiac events.[45][202][203][204][205][206][207][208][209]

Central cavernosal shear wave elastography has been used to objectively quantify penile rigidity in the evaluation of ED.[210][211][212][213] Patients with vasculogenic ED demonstrated much higher central shear wave elastography values both before (flaccid) and after vasoactive intracorporal injections than those with ED from other causes.[210][211][212][213]

Priapism is the abnormal condition where the penis develops a prolonged involuntary erection in the absence of appropriate stimulation.[214] The immediate diagnostic need is to identify whether the priapism is ischemic (low-flow) or non-ischemic (high-flow).[24][214] Ischemic priapism is a true surgical emergency and requires prompt treatment to prevent permanent damage to the penis, such as cavernosal fibrosis.[214][215] 

Penile ultrasound demonstrates fibrosis of the corpora cavernosa as fine, echogenic strands close to the cavernosal arteries.[45] The fibrosis replaces the normal sinusoids.[45] There are generally minimal changes after the injection of vasoactive drugs, and Doppler ultrasound typically shows venous dysfunction.[45]

Penile ultrasound can be very useful in helping differentiate ischemic from non-ischemic pathologies.[216] It is so effective that it has been suggested that penile ultrasound be utilized instead of cavernosal blood gas determinations in the differentiation of ischemic from non-ischemic priapism.[216] Penile ultrasonography is particularly helpful when used for confirmation of blood flow after a shunting procedure for ischemic priapism, as penile edema and inflammation of the cavernosa can simulate a residual erection.[43][216]

  • In ischemic priapism, the cavernosal arteries will demonstrate minimal EDV due to very high vascular resistance.[217][218]
  • In non-ischemic priapism, the cavernosal artery PSV will be extremely elevated (>50 cm/sec), along with a high EDV.[200][201] Venous drainage is enhanced, and internal fistulas show high-velocity flow with turbulence.
  • These results can quickly differentiate ischemic (low-flow) from nonischemic (high-flow) priapism.[24]

Penile trauma can be usefully evaluated by ultrasound.[24][219] It is safe, available, inexpensive, and can provide information not immediately available otherwise.[24][219] Any interruption of the echogenic line of the tunica albuginea would indicate a penile corporal rupture (penile fracture), signifying a tear in the tunica albuginea.[220] Ultrasound can quickly establish this diagnosis and identify the location and extent of the injury, which can otherwise be difficult to determine due to the swelling and hematoma accompanying this injury.[24][221] Ultrasound will also demonstrate any associated injury to the urethra and/or penile dorsal vein.[24][191][192][222]

Peyronie's disease is a benign, progressive disorder of the penile tunica albuginea that causes localized fibrosis, scarring, plaque formation, and curvature of the penis during erections.[223][224] Penile ultrasonography is the imaging modality of choice in Peyronie's disease.[224][225] The presence, size, and thickness of Peyronie's plaques in the penile tunica albuginea and any calcifications can be detected by penile ultrasound.[224][225][226] Peyronie's plaques appear as hyperechoic localized thickening of the penile tunica albuginea.[24] Calcifications greatly increase the echogenicity of the plaques and will demonstrate acoustic shadowing.[24][224][227]

In addition to identifying, measuring, and tracking Peyronie's plaques over time, ultrasound can also be used to determine the involvement of the neurovascular bundle within the lesion, which is an important consideration for patients considering surgery.[24][224]

Elastography can identify Peyronie's plaques even if they are non-palpable and not visualized on standard ultrasonography.[228]

Using vasoactive intracorporeal injections allows for an assessment of the penile vascular potential.[197] Those patients identified with arterial insufficiency and venous leak disorders are unlikely to have good erectile function from grafting procedures alone and will likely require a penile prosthesis.[225][226] Ultrasonography can also track the condition's progress when using non-surgical therapies.[225][226] 

PROSTATE

Biplanar high-frequency (7.5-10 MHz) endorectal linear or curved array transducers are generally used.[229] Biplanar transducers allow for simultaneous sagittal and transverse views without switching probes.[229] Higher frequency probes are usually preferred.[229]

Benign prostatic hyperplasia (BPH) is a common ultrasound finding and will be identified in most men aged 50 or older. BPH presents as an asymmetrical enlargement or multiple hypoechoic nodular areas of the transition zone and periurethral glands.[150][230][231][232] This may progress to a more homogenous echogenicity.[230] There may be deviation of the urethra and intrusion of the prostate into the bladder.[230] The peripheral zone becomes compressed, as demonstrated by its hyperechoic appearance.[150] Smoothed wall adenomas with decreased internal vascularity are also a common ultrasonic finding in BPH.[150][230] Transrectal ultrasonography is also valuable for estimating prostatic size before surgical procedures for BPH.[233][234][235] 

The prostatic volume is customarily calculated by measuring the length, height, and width and multiplying by the ellipsoidal coefficient of π/6 (or 0.52).[233][234] However, this formula underestimates the prostatic volume by 10% to 18% in most (80%) cases, especially in larger prostates >60 grams.[233][236] A revised formula using a coefficient factor of 0.66 instead of the traditional ellipsoidal coefficient of 0.52 has proven superior in more reliably estimating prostatic volumes but has not yet become the clinical standard.[233] This is very similar to the proposed "bullet" formula, which uses a coefficient factor of 0.65.[237]

Prostatic cysts are often found on prostatic ultrasonography. They become clinically significant if they interfere with male fertility. Congenital cysts may derive from a Müllerian or Wolffian origin. Persistent Mullerian duct cysts appear as midline, anechoic cystic structures with a teardrop shape when viewed in the sagittal plane. These cysts are often associated with hypospadias (11% of hypospadias patients) but can be as high as 40% in patients with posterior hypospadias.[238] The incidence of Mullerian duct cysts rises as the hypospadias grade increases (urethral meatus becomes more proximal or posterior).[238][239][240]

Uncommon hypoechoic prostatic lesions include granulomas, hematomas, infarcts, and lymphomas.[241] Cysts of the vas deferens and seminal vesicles may be associated with renal cystic disease and renal agenesis.[242][243] Prolonged hypogonadism can make the prostate appear hypoplastic.

Prostatitis tends to make the normally ovoid prostate more rounded with reduced echogenicity from increased edema. Small hypoechoic areas may be seen, which may represent tiny early abscesses, and calcifications or fibrosis may be visualized in the apical peripheral zone. Color Doppler will show increased vascularity.[230][244] A periurethral hypoechoic halo is characteristic of prostatitis, but can sometimes be confused with the periprostatic sphincter, which is also hypoechoic.[230][244] 

Transrectal ultrasonography has not been considered a particularly reliable diagnostic imaging modality for prostate cancer, although it is extremely helpful for prostatic biopsies. Hypoechoic lesions have classically been associated with prostate malignancies, but up to 40% of prostate cancers are isoechoic, and large recent series have not indicated a substantially increased detection rate of cancer in hypoechoic nodules.[245][246][247] 

Current Guidelines Recommend That All Hypoechoic Lesions in the Peripheral Zone of the Prostate Should Be Biopsied.[241][248][249]

Artifacts in transrectal prostatic ultrasonography can affect the image quality and analysis.[220] These include the following:[220][250]

  • Pseudomasses from imaging rectal folds or reflection from an intraluminal fluid level give a mirror image.
  • Size artifacts are produced by lesions too small to otherwise characterize.
  • False positive malignancy (due to beam thickness, attenuation, or refraction)
  • Narrow or incomplete field of view (due to reverberation, shadowing, or mirror-imaging)
  • Complex echo patterns (artifacts from rectal balloon reflections, mirror-imaging, or side lobes)

Transrectal Ultrasound-Guided Prostate Biopsies

Transrectal ultrasound-guided prostate biopsies are frequently used for patients suspected of having prostate cancer due to hard nodules on digital rectal examinations or elevated PSA levels.[230] Specific indications for a prostate biopsy are described elsewhere, in the companion StatPearls' reference on "Prostate Cancer Screening" at https://pubmed.ncbi.nlm.nih.gov/32310541.[230]

While prostatic biopsies are usually done using the biopsy channel of the transrectal probe, they can also be performed with a transperineal approach while still using transrectal ultrasonography for guidance.[251] This approach significantly reduces infection risk and can still be performed in a physician's office or clinic. A dorsal lithotomy position is used. This approach is particularly suited for patients at high risk for infection. Antibiotic prophylaxis may not be necessary with a perineal biopsy approach.[252]

The patient should be instructed to stop taking blood thinners before the procedure and to use a Fleet enema two hours before the biopsy.

The following techniques are employed for transrectal ultrasound-guided prostate biopsies:

Prophylactic Antibiotic Management and Selection:

  • The patient should be instructed to take an oral antibiotic before the procedure, possibly starting a full day prior, or he may be given an IM antibiotic injection in the clinic at the time of the biopsy, as these measures significantly reduce the risk of a post-procedure infection and are highly recommended.[253][254][255]
  • Fluoroquinolones are the most commonly used antibiotics for prophylaxis, but growing resistance is a concern.[256][257]
  • As a general guide, if more than 20% of local Escherichia coli strains are resistant to fluoroquinolones, a different class of antibiotics is suggested for prophylaxis.[253]
  • Previous recent use of fluoroquinolones or third-generation cephalosporins increases the risk of resistance to these antibiotics when used for prophylaxis.[258]
  • A full day of preoperative antibiotics and targeted antibiotic therapy is suggested in areas with high fluoroquinolone resistance.[256][259]
  • The preferred antibiotic alternative to fluoroquinolones may well be fosfomycin.[256] 
  • Fosfomycin has many favorable beneficial qualities: [256][257][260][261] 
    • Good prostatic tissue penetration after a single 3-gram oral dose.
    • Bacterial resistance is low.
    • It is effective against many highly drug-resistant organisms.
    • It is readily available.
    • It has an excellent safety profile.
    • Efficacy is improved when fosfomycin is taken together with a fluoroquinolone.[262]
  • Alternative prophylactic antibiotics that can be used alone or in combination include aztreonam, amikacin, ceftriaxone, ceftriaxone + gentamicin, metronidazole, cefuroxime, and cefpodoxime.
  • There is no consensus on the best combination, so antibiotic selection depends on local bacterial resistance patterns and patient clinical characteristics.[263] 
  • Targeted antibiotics (based on rectal swab cultures prior to the biopsy) are suggested when feasible, but especially for high-risk patients.[264][265][266][267][268][269][270][271]
  • Targeted preoperative antibiotics can reduce post-operative infections by about 42% compared to empiric antibiotic use.[272]
  • Rectal preparation with a povidone-iodine antiseptic solution, swabs, or gel has also been recommended immediately before transrectal biopsies and has shown very good efficacy in reducing postoperative infections and sepsis.[273][274][275]
  • A transperineal approach should be considered for higher-risk patients.

Preoperative Patient Preparation, Positioning, and Probe Insertion:

  • Optimal patient positioning is the left lateral decubitus position.
  • The buttocks should be aligned with the side of the table or just beyond, and both legs flexed.
  • The lithotomy position may also be used but has been associated with greater patient discomfort.[276]
  • Rectal anesthetic and/or povidone-iodine may be applied just before probe insertion.
  • Biplanar transrectal probes are recommended.
  • The condom-covered, lubricated ultrasonic probe is slowly and gently inserted into the rectum until the prostate can be seen.
    • Using sterile ultrasound gel is important, as infections have been reported with multi-use gel dispensers that have become contaminated.[277]
  • B (Brightness) mode imaging is used. (B-mode is grayscale imaging that uses each dot's brightness to represent the returning echo's strength. Denser tissues reflect more echoes and give a brighter, more echogenic image.)
  • The prostate is visualized, and its measurements are recorded.
  • The prostatic volume is calculated.

 Periprostatic Block

The procedure requires a periprostatic block to anesthetize the neurovascular bundles containing the nerves supplying sensory innervation to the prostate.[278] The technique is described as follows: 

  • Using transrectal ultrasonography, a triangular hyperechoic fatty area (called the "Mount Everest" sign) can be identified at 5 and 7 o'clock on either side of the prostate at the junction between the seminal vesicle, rectum, and the prostatic base on transrectal longitudinal sagittal imaging.[279][280][281][282] 
  • 5  to 10 mL of 1%-2% lidocaine is injected into each of these triangular areas using a long (22-gauge, 7-inch) spinal needle passed through the biopsy channel of the transrectal ultrasound probe and monitored by ultrasound.[279][280][281][282]
  • The observation of an appropriately sized hypoechoic wheal on ultrasound in the correct location confirms an adequate anesthetic injection.
  • Local anesthetic should also be administered to the prostatic apex, between the prostate and the rectum, as this area contains sensitive somatic nerves that can cause pain despite the earlier block.[279][280][281][282][283][284]
    • The apex of the prostate is composed of peripheral zone tissue and is a common site for positive biopsies.
    • It is also the most painful location to biopsy as it is located below the rectal dentate line and requires a separate local anesthetic injection for pain control.[285] 
    • The apex is the most common site for "missed cancers," as it may be skipped due to the expected intense pain.[286][287] 
    • Basilar prostatic blocks alone do not control sensation at the apex well, so it should be anesthetized separately.
  • There is evidence that a direct injection into the prostate offers some additional benefit, but there is a risk of increased systemic lidocaine absorption or inadvertent intravascular administration.[283][284][283][288][289][290][291][292] 
    • Such injections should, therefore, be done cautiously.
  • A combination of injections at both the prostatic base and apex is recommended for optimal pain control, as demonstrated by several large meta-analyses.[284][293][294][295][296][297]
  • When added to the periprostatic nerve block described above, using intrarectal local analgesia, such as lidocaine-prilocaine cream or lidocaine gel, further improves pain control during transrectal ultrasound-guided biopsies.[292][298][299][300]
  • The anesthetic block takes about 5 to 10 minutes [301]to be fully effective.

Biopsy Technique:

  • Biopsies are typically taken with an 18-gauge needle using a spring-operated biopsy gun. The needle is long enough to pass through the biopsy channel of the transrectal probe and into the prostate. 
  • The biopsy needle shows up easily on ultrasound, and its expected path can be outlined as a dotted line by a setting on the sagittal view of most modern ultrasound machines.
  • The use of simultaneous axial and sagittal viewing makes performing the biopsy technically easier and ensures that both sides are sampled.
  • When activated, the biopsy gun advances the needle 0.5 cm and samples the next 1.5 cm of prostatic tissue immediately proximal to that point.
  • After taking the biopsy, the biopsy gun is removed from the probe and handed to an assistant who removes the sample, labels it, dips the tip in formalin (optional), resets the spring, and returns it carefully to the surgeon.
  • Dipping the end of the biopsy needle in formalin between taking samples disinfects the needle tip, helping reduce the risk of infection.[302] 
  • Standard prostate biopsies typically include about 12 samples.
  • One common protocol is to take two samples each from the prostatic base, mid-gland, and apex on either side, for a total of twelve samples.
  • Biopsy samples should preferentially be taken from the lateral aspects of the prostate with adequate sampling of the apex.[303][304][305][306] (Additional samples from the transition zone do not increase the cancer detection rate.)
  • Any peripheral zone hypoechoic areas should also be biopsied.
  • Saturation biopsies (typically 25 core samples) are recommended when initial standard biopsies have been negative, but there remains significant suspicion of an underlying malignancy.[307][308][309][310]
  • They should be done with MRI fusion technology and usually require regional or general anesthesia.[311] They are not appropriate for initial prostatic biopsies.
  • When the biopsy procedure is finished, the probe can be slowly removed.
  • A small gauze pad, pack, or other dressing is applied to the rectum.
  • The patient is told to expect some bleeding and to notify the physician if the bleeding is excessive, if he develops a fever, or if he has any other problems.
  • If PiRAD 3, 4, or 5 lesions have been detected, the ultrasound biopsy technique can be combined with MRI by digital overlay, allowing visualization for a fusion biopsy. (See below) 
  • The patient is usually given a return appointment to review the biopsy results about one week later.

MRI Fusion Biopsies

  • When MRI imaging shows suspicious lesions, the radiologist can outline and color them digitally.
  • This digital image can be electronically overlaid over the real-time transrectal prostatic ultrasound image, making the suspicious areas visible.
  • With the MRI correlated and superimposed on the ultrasound image, directed biopsies of the suspicious lesions can now be performed using transrectal ultrasonography.
  • At least 2 but usually 3 or 4 samples are taken from every suspicious lesion identified by MRI.
  • Usually, a standard 12-core biopsy sample is performed in addition to the suspicious lesions identified on the MRI.

Shear wave elastography has similar sensitivity and specificity for prostate cancer detection as MRI.[312][313][314] Prostate cancers tend to be harder and denser than normal tissue. Elastography can measure malignant prostatic tissue's increased firmness and stiffness with good diagnostic results, similar to prostatic MRI.[312][313][314] Although multiparametric prostatic MRI has become the standard of care for screening patients with elevated PSA levels and possible prostate cancer, elastography may still play a useful role in equivocal or borderline cases.[315] 

Combining several imaging modalities, such as elastography with contrast-enhanced ultrasonography or MRI, provides superior diagnostic results compared to either method when used alone.[316][317][318][319] The combination of multiparametric prostatic MRI and Shear Wave Elastography resulted in superior cancer detection compared with either modality alone, with an overall sensitivity of almost 95% and a specificity of 96%.[318] 

Micro-ultrasound technology, using an extremely high-frequency transducer (29 MHz), provides exceptional resolution and detail (three times more than standard transrectal ultrasound) as it provides visualization of lesions as small as 70 micrometers.[320] Preliminary results indicate that micro-ultrasound-guided biopsies can identify prostatic malignancies not detected on standard transrectal ultrasound or prostatic MRI and provide results comparable to systemic biopsies.[320][321][322][323][324][325][326] 

Micro-ultrasonography is a promising improvement over conventional transrectal prostatic ultrasound.[320] It could eventually replace systemic biopsies in clinical practice as it reduces the diagnosis of clinically insignificant, indolent cancers while increasing the detection rate of treatable prostatic malignancies.[327]

SCROTUM, TESTICLES, AND EPIDIDYMIS

Ultrasonography is the preferred diagnostic imaging modality for evaluating scrotal masses and testicular pain.[301] A high-frequency (7.5-10 MHz) linear array transducer is usually used as it provides a very detailed image of the scrotal contents, which are relatively superficial.[301] 

Epidermoid cysts are rare, benign testicular neoplasms that are characterized by multiple concentric layers of keratinizing squamous epithelium and keratinous debris, giving them a solid hypoechoic appearance.[328][329] They are typically round with concentric hyperechoic layers internally, giving them an "onion skin" appearance, and have no internal vascular flow that can be detected on color Doppler ultrasonography.[328][329] Although they are the most common benign neoplasm of the testes, they only account for 1%-2% of all testicular masses.[328][330][331] 

While they can mimic testicular cancers on imaging, the lack of tumor markers, characteristic "onion skin" appearance on ultrasound, and lack of internal vascularity on color Doppler imaging is usually sufficient to make the diagnosis.[328]

Epididymal cysts (spermatoceles) will be seen as anechoic structures, usually near the head of the epididymis. The "falling snow" sign may be seen using color Doppler, and internal septations may be present.[332] Hydroceles are also anechoic and will surround the testicle completely, while spermatoceles are usually smaller and will be separate from it.[29][333]

Scrotal ultrasonography has a role in the evaluation of male infertility and varicoceles as well.[334][335][336][337] Generally, a varicose vein diameter of >2.95 mm indicates a clinically significant varicocele.[338][339][340][341]

Testicular microlithiasis was once thought to be associated with testicular cancer but has since been found to be a relatively common incidental finding that has no such malignant association.[342][343][344]

Superb microvascular imaging has been found helpful in evaluating vascularity in undescended testes.[345][346][347]

Epididymitis is visualized on ultrasound as an enlarged, hypervascular epididymis that may be hypoechoic or heterogeneous.[84] Increased blood flow on color Doppler imaging is characteristic.[84] Reactive hydroceles and scrotal wall thickening may also be seen, and suggest the diagnosis when present.[84] Spread of these findings into the testis is termed epididymoorchitis.[84] 

Solid paratesticular tumors of the epididymis include adenomatoid neoplasms (73%), leiomyomas (11%), and cystadenomas (9%).[348][349][350][349] These paratesticular tumors are rare and comprise only about 5% of all intrascrotal neoplasms.[348]

Paratesticular neoplasms generally have well-defined borders, but other characteristics like size, echogenicity, and vascularity are variable, making a definitive diagnosis difficult solely from ultrasonography.

  • Adenomatoid tumors are the most common benign neoplasms of the epididymis. They are usually homogeneous on ultrasound with minimal internal vascularity on color Doppler imaging, although this is variable.[351][352] They are most often found in the tail of the epididymis, are asymptomatic, and typically occur in middle-aged patients.[353]
  • Epididymal papillary cystadenomas are often associated with von Hippel-Lindau disease. It is reported that 67% of patients with cystadenomas will have von Hippel-Lindau disease. Sonographically, it may appear solid (when its cystic components are small) or primarily cystic. These neoplasms tend to be hypervascular.[354][355][356]
  • Leiomyomas are benign epididymal tumors that are often bulky.[348] 

While ultrasound alone cannot reliably distinguish between these different paratesticular neoplasms, sonography can reliably differentiate intratesticular neoplasms (high likelihood of malignancy) from paratesticular lesions (overwhelmingly (95%) benign).[357][358] Tumor size >1.5 cm and identifiable color Doppler vascular flow are generally suggestive of possible malignancy.[359] When a solid mass involves intratesticular and paratesticular structures, the differential diagnosis includes leukemia, lymphoma, metastatic disease, sarcoidosis, and tuberculosis, which can also affect both regions.[360][359]

MRI imaging can be performed if further investigation of the lesion is necessary.[352][361] If the benign nature of these paratesticular lesions cannot be reliably determined otherwise, surgical exploration with an intraoperative frozen section will be necessary.[362]

Testicular Cancer

Testicular neoplasms tend to be heterogeneously hypoechoic solid lesions with lobulations, well-defined borders, and identifiable internal blood flow on Doppler ultrasound imaging.[84] Cystic structures and irregular margins may also be seen. They may contain internal echogenic foci from calcifications, fibrosis, or hemorrhage.[84] These features are not diagnostic for testicular malignancies and may be found in various benign entities.[84]

The sonographic appearance of testicular cancer is highly variable, and it is not currently possible to reliably characterize histological types based on ultrasonographic appearance alone, as this requires radical orchiectomy surgery and careful histological examination.[84] However, as summarized below, several histological types of testicular neoplasms tend to demonstrate specific ultrasonographic characteristics.  

  • Choriocarcinomas are relatively solid, heterogeneous masses with internal calcifications, necrosis, and hemorrhage. 
  • Embryonal cell cancers are typically heterogeneous with poorly defined borders that blend into the adjacent normal testicular tissue. 
  • Leydig cell tumors are generally small and hypoechoic. Cystic changes may be present.
  • Lymphomas of the testicle are visualized as discrete hypoechoic masses with increased blood flow on color Doppler imaging. If the testicle is completely involved with lymphoma, this may only be apparent by comparison with the contralateral testicle. 
  • Seminomas will generally appear hypoechoic and uniform, although bigger neoplasms will show more lobulations and heterogeneity. They are the most common germ-cell testicular cancers.
  • Sertoli cell tumors will tend to be round, lobulated, and well-demarcated.
  • Teratomas appear as well-defined complex masses with cystic changes and may contain calcifications.
  • Yolk sac neoplasms may have calcified regions or cystic areas.

Patients with extensive metastatic disease may infrequently develop areas of calcification or fibrosis in a normal-sized or atrophic testis, which develops from involuted malignancy.[363]

Testicular cysts are well-demarcated, homogeneously hypoechoic lesions with posterior acoustic enhancement on ultrasound imaging.[84] Complex testicular cysts have some degree of internal echogenicity. However, all cysts lack internal vascularity on color Doppler imaging.[84] 

Testicular torsion is most commonly seen in adolescents who develop acute unilateral scrotal pain.[364] It is caused by inadequate fixation of the testicle inside the scrotum with the tunica vaginalis joining the spermatic cord higher (more superiorly) than normal.[364] This allows the superior end of the testicle to drop down into the scrotal sac, causing the "bell clapper deformity," which is usually bilateral.[84][365] 

Intrascrotal rotation of the testicle compromises the blood supply, leading to ischemia, acute pain, and necrosis if not repaired expeditiously (optimally, within six hours of onset).[364] A definitive diagnosis can be made when there is no detectable blood flow on color Doppler imaging of the affected testicle with normal blood flow on the contralateral side.[84][364] Color Doppler imaging of the testicle is far superior to radionuclide imaging, which was used previously. Ultrasound is not only more reliable diagnostically, but it is readily available in emergency departments and can be performed much more quickly.[364][365]

The "whirlpool sign" is a reliable sonographic finding characteristic of testicular torsion.[366][367][368][369][370][371] It is seen as an acute, sudden change in the course of the spermatic cord with a spiral or twisting appearance between the testicle and the external inguinal ring.[366][367][368][369][370][371]

A tortuous and redundant spermatic cord may also be seen.[366] In patients with acute unilateral scrotal pain, the redundant cord is typically found bunched up and lying on top of the testicle or in the scrotal sac.[366] This suggests abnormal attachment of the tunica vaginalis.[366]

Torsion of the appendix testis is usually diagnosed clinically from the history and the "blue dot sign," but ultrasonography can be helpful.[84][372] Normally, the appendix testis will be <5.6 mm in size and demonstrate minimal or no detectable blood flow on color Doppler.[372][373][374] As the appendix testis may not be easily visualized on ultrasound, normal blood flow to the affected testicle can at least effectively rule out testicular torsion. The appendix testis is generally visualized in about 80% of cases.[374]

A torsed appendix testis will appear enlarged (>5.6 mm), avascular, and may be surrounded by a hyperemic epididymis, often with posterior enhancement.[372][375]

Varicoceles are dilated veins of the pampiniform plexus of the spermatic cord caused by dysfunctional valves of the internal spermatic veins, usually on the left side.[376] Normal-sized veins are typically 0.5-1.5 mm in diameter, while veins larger than 3 mm are considered pathological and clinically significant.[376][377][378][379][378] Varicoceles are usually found above and lateral to the testicle. Ultrasonography is usually not required for varicoceles, as the diagnosis is usually made clinically based on clinical examination findings such as a "bag of worms" appearance.[376]

Ultrasonography should be performed with the patient supine.[376] The scrotum should be elevated and supported with a rolled towel placed underneath.[380] Warm ultrasound gel is suggested. Higher frequency transducers (>7.5 MHz) will provide better images and are suggested.[380]

Varicoceles are usually found above and lateral to the testicle. A Valsalva maneuver may show reflux (reverse blood flow) on color Doppler and accentuate the veins, making them easier to identify.[377] On ultrasound, varicoceles appear as multiple hypoechoic (dark) tortuous, tubular structures and may show a complex pattern.[376] They have been described as “multiple, anechoic, serpiginous, tubular structures."[91][380]

The finding of an isolated right-sided varicocele has traditionally been associated with the possibility of a right renal carcinoma with a tumor thrombus extending into the inferior vena cava, resulting in the varicocele.[376] Although this is quite unlikely, performing a quick renal ultrasound examination is recommended either separately or at the time of the varicocele ultrasonography.[340][376][381][382][383][384][385][386][387]

ULTRASOUND ARTIFACTS

  • Anisotropy describes the phenomenon where ultrasonic reflections only occur when the transducer is exactly at a right angle to the studied structure. This happens with particularly smooth objects. They will appear bright when the transducer is at 90 degrees, but dark at other angles.[7][388][389][390][391]
  • Edging artifacts occur when ultrasonic waves hit a curved surface at a critical angle. This unique surface reflection of sound waves along its surface does not return any reflections to the transducer and is seen on the ultrasound image as a dark line.[392] This edging artifact is very helpful in differentiating the upper pole of the testis from the head of the epididymis.[388][392]
  • Enhancement occurs when tissue immediately behind a very hypoechoic structure appears unusually bright due to increased transmission.[388]
  • Resolution artifact leads to a loss of detail and can make two adjacent structures look like one. Most notable in kidney stones.[393]
  • Shadowing (acoustic shadowing) is the opposite of the enhancement artifact. This occurs when a structure absorbs or reflects such a large proportion of the sound waves that all tissues behind this structure do not receive enough ultrasonic waves to be visible and will appear extremely dark or black.[127] Acoustic shadowing is caused by refraction and reflective effects at the margins between a curved object and its surrounding tissues.[388][394] Phase cancellation also contributes to this effect.[394]
  • Speckle artifacts occur close to the transducer due to interference from tissue ultrasound scatterers.[393]
  • When using color Doppler imaging, twinkling artifacts appear as rapidly alternating colors behind an irregular hyperechoic structure, such as a kidney stone.[128][395][396][397][398] It is caused by ultrasonic sound waves interacting with the irregular stone surface, producing temporary microbubbles.[395][396][397] This twinkling artifact is the most sensitive ultrasound modality for identifying small stones.[396]
  • Shadowing and twinkling artifacts are particularly useful in identifying calcifications in urological organs such as kidney stones.[5][398]
  • Other artifacts include comet tail, focal enhancement, grating lobe, multipath, range ambiguity, refraction, ring down, section thickness, side lobe, and speed errors.[393]

7. Ultrasonography of the Adrenal Gland7.1 Anatomical ConsiderationsSee section 3 of "Kidney, Adrenal, Ureter" core curriculum chapter for details on the anatomy of the adrenal glands.

Due to the small size of the adrenal glands, extracorporeal sonography of the normal adrenal gland is challenging. In contrast, laparoscopic sonography facilitates excellent visualization of the glands, especially to identify surgical planes.44

Adrenal glands in adults are not normally visualized on transabdominal ultrasound, especially in obese patients. Any prominent adrenal gland seen on ultrasound should be considered abnormal and further imaging with CT or MRI should be considered. Intra-operative sonography of the adrenal gland is useful during adrenalectomies, particularly in the following cases: small nodules, right-sided adrenal tumors, obese patients, or during partial adrenalectomy.

During transabdominal sonography, a curved-array 2.5-6 MHz transducer offers a wide, deep field of the view that is necessary to image the adrenal glands deep in the abdomen. For laparoscopic intra-operative ultrasonography, a linear side-viewing or a flexible side-viewing (preferred) transducer 7.5-10 MHz probe provides excellent ultrasonographic visualization.

Transabdominal subcostal or intercostal views both provide adequate visualization of the adrenal glands. The lateral decubitus position may improve visualization when overlying bowel gas is present. After visualizing the upper pole of the kidney, the probe is directed medially to identify the adrenal gland.

During laparoscopic sonography, the transducer is placed through a standard 10mm port. Typically, the most medial or lateral port is used for the transducer. Rarely, an additional trocar is needed to evaluate the surgical margins of a benign adrenal mass that may be considered for laparoscopic partial adrenalectomy. The upper pole of the kidney should be scanned from medial to lateral in the longitudinal plane until the adrenal gland is identified. If surgical margins are not grossly evident, sonography can be used to determine relationships with other adjacent structures. Doppler ultrasound can also be used to identify vascular structures.

Adrenal masses or hyperplasia may be found incidentally during ultrasound for other indications, but transabdominal ultrasound to specifically assess the adrenal glands may be useful in following patients with an adrenal mass that has not been treated surgically. Additionally, laparoscopic ultrasound is a valuable adjunct to laparoscopic adrenalectomy, facilitating tumor localization and identification of adjacent structures. This technique is particularly helpful for partial adrenalectomies.45,46,47,48,49,50

7.2 AdenomasAdrenal adenomas are often found incidentally and in most cases are non-functioning. Sonographically, adrenal adenomas appear as well-demarcated, solid nodules, attached to the adrenal limb. Adrenal adenomas are usually isoechoic to the adrenal gland and are hypoechoic relative to the perinephric fat. A left adrenal adenoma can be misdiagnosed as an accessory spleen if its relationship with the adrenal limb is indeterminate.51

7.3 MyelolipomasAdrenal myelolipomas are rare, nonfunctioning tumors composed of varying proportions of bone marrow elements and fat. They arise from the zona fasciculata of the cortex and appear sonographically as echogenic masses. Lesions can be heterogeneous as a result of internal hemorrhage, and if they are predominantly composed of myeloid (as opposed to fat), they can appear iso- or hypo-echoic.

7.4 PheochromocytomaPheochromocytomas are typically large and well-defined on ultrasound, but can appear as homogenous solid structures or can be more heterogenous (Figure 34). Bleeding may occur in the medulla, and typically appears as a round to oval echogenic mass in the central part of the gland.52

Figure 34. Pheochromocytoma7.5 Primary Adrenal CarcinomasAdrenal cortical carcinomas are rare, and the sonographic appearance is variable, depending in part on the size of the lesion. Smaller lesions are homogeneous and appear sonographically similar to adenomas. The mass may have a thin, capsule-like echogenic rim and some calcifications. Larger lesions may contain necrotic material and hemorrhage and can mimic a pheochromocytoma. In some cases, ultrasound can be more informative than CT or MRI. For example, when a large adrenal mass abuts the liver and the origin of the mass is uncertain, anterior displacement of the retroperitoneal fat reflection is strongly suggestive of adrenal origin.51

7.6 LymphomaPrimary adrenal lymphoma is rare, but secondary involvement of the adrenal glands is present in as many as 25% of lymphoma patients. Nearly half of all adrenal lymphomas are bilateral, and the most common type of lymphoma to involve the adrenal gland is non-Hodgkin’s lymphoma. Sonographically, these tumors are echo-poor. However, echogenic areas of necrosis or hemorrhage may be present.

7.7 MetastasisThe adrenal glands are a common site of metastatic disease. Metastatic lesions < 3 cm in size may appear as a solid homogenous mass, indistinguishable from an adenoma. Larger lesions may demonstrate heterogeneity due to central necrosis or hemorrhage.

Complications

Ultrasound of the urinary tract is a safe, non-invasive diagnostic modality with no known biological risks at standard diagnostic energy levels. Unlike imaging modalities that involve ionizing radiation or intravenous contrast, ultrasound carries virtually no systemic complications and is well tolerated across all patient populations, including children, pregnant individuals, and those with renal impairment.

Minor patient discomfort may occur during the examination, particularly when firm transducer pressure is applied over tender areas such as the flanks or suprapubic region. Additionally, the application of gel may feel cold or mildly uncomfortable to some patients, though this is easily mitigated by warming the gel beforehand and removing any residue promptly after the examination.

There are no procedural complications specific to urinary tract ultrasound, and adverse effects are exceedingly rare. Proper patient communication, gentle technique, and attention to comfort help ensure a well-tolerated and effective examination.

Clinical Significance

Ultrasound of the urinary tract is a rapid, cost-effective, and non-invasive imaging modality that plays a critical role in the evaluation of a wide range of urological conditions. It is particularly valuable in the diagnosis of hydronephrosis, where it demonstrates high specificity for moderate to severe obstruction.[5] When performed by experienced providers, its sensitivity in detecting obstructive uropathy and nephrolithiasis is significantly improved, making it a reliable tool in both emergency and outpatient settings.[4][5][6][399][400]

Importantly, ultrasound can expedite clinical decision-making, especially in the assessment of flank pain, hematuria, urinary retention, and suspected renal or bladder masses. It enables prompt identification of pathology without the risks associated with ionizing radiation or intravenous contrast, which is particularly advantageous for pregnant patients, children, and those with impaired renal function.

By serving as a first-line diagnostic tool, urinary tract ultrasound can reduce reliance on CT, minimize radiation exposure, and support more judicious use of advanced imaging. Its real-time nature also allows for immediate bedside application in point-of-care settings, facilitating faster diagnoses and more efficient care delivery.

Enhancing Healthcare Team Outcomes

Ultrasound of the urinary tract is a powerful diagnostic tool that supports timely, patient-centered decision-making across a broad range of clinical settings. Its safety, accessibility, and effectiveness make it an ideal first-line modality for evaluating flank pain, hematuria, urinary retention, and suspected urinary tract obstruction. However, the effective use of ultrasound depends not only on the technical skills of individual clinicians but also on the strength of interprofessional collaboration among the healthcare team.

Competent performance and interpretation of urinary tract ultrasound require the coordinated effort of physicians, advanced practice providers, nurses, sonographers, radiologists, and allied health professionals. Each team member plays a vital role in delivering accurate and efficient care. Physicians and advanced practitioners must develop strong ultrasound acquisition and interpretation skills or collaborate closely with imaging specialists. Nurses contribute to patient preparation, positioning, and comfort, while also ensuring continuity of care before and after imaging. Radiologists and sonographers bring advanced expertise in identifying subtle or incidental findings, helping to distinguish clinically relevant pathology from benign variants such as parapelvic cysts or extrarenal pelvis.

Effective interprofessional communication ensures that ultrasound findings are contextualized appropriately and followed by timely management decisions. For example, incidental findings—such as renal cysts or bladder wall irregularities—often require nuanced judgment and shared decision-making between emergency physicians, radiologists, urologists, and primary care providers to avoid unnecessary testing while ensuring patient safety. 

The adoption of point-of-care ultrasonography (PoCUS) dramatically expands immediate access to diagnostic imaging, especially in time-sensitive situations or underserved environments. When integrated into structured workflows, PoCUS allows for faster diagnoses, reduces length of stay, limits exposure to ionizing radiation, and enhances the overall efficiency of care delivery.[15]

By fostering a team-based culture, encouraging cross-disciplinary training, and prioritizing shared accountability, the healthcare team can ensure that ultrasound of the urinary tract is applied effectively and ethically. This interprofessional approach leads to more accurate diagnoses, safer imaging practices, improved patient satisfaction, and better clinical outcomes across diverse healthcare environments.

Media


(Click Video to Play)

Nephrolithiasis, Ultrasound. The image shows a normal kidney with grades of hydronephrosis, twinkle artifact, and ureterovesical junction stone.

Contributed by MK Herbst, MD

(Click Image to Enlarge)
Extrarenal pelvis on ultrasound
Extrarenal pelvis on ultrasound Contributed by Meghan K. Herbst, MD

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<p>Kidney Anatomy</p>

Kidney Anatomy

Blausen.com staff. Medical Gallery of Blausen Medical 2014. WikiJournal of Medicine. doi: 10.15347/wjm/2014.010.
ISSN 2002-4436. [CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)] via Wikimedia Commons.

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Ultrasound of bladder in transverse view
Ultrasound of bladder in transverse view Contributed by Meghan K. Herbst, MD

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<p>Bladder Dimensions Visualized on Ultrasound

Bladder Dimensions Visualized on Ultrasound. Bladder dimensions in sagittal and transverse views to determine PVR using ultrasound.

Contributed by MK Herbst, MD

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