Nuclear Medicine Quality Assurance

Function

Nuclear medicine equipment, both imaging and nonimaging, requires regular quality control procedures at different intervals, including daily, monthly, semiannually, annually, and biannually. The nuclear medicine technologist (NMT) is primarily responsible for most of these procedures. Service engineers and medical physicists also play a key role, particularly in performing tests on an annual and biannual basis.[4][5] Various tests are conducted at these intervals to ensure equipment reliability and safety. The clinical and radiation safety aspects of these tests are discussed in greater detail in the following sections.

Dose Calibrator

The dose calibrator, a nonimaging device, is essential for measuring the radiation dose injected into patients or used in phantoms prior to imaging procedures. While not an imaging instrument itself, the dose calibrator plays a critical role in ensuring that doses are accurate and reliable. Improper calibration of the dose calibrator would compromise the integrity of calibration studies and impact diagnostic quality.

Most dose calibrators use ionization chambers connected to circuits that convert and display ionization currents from radioactive sources into digital units of activity. Several factors can influence the accuracy and performance of a dose calibrator, including environmental conditions and device-specific variations. To ensure proper functioning, regular checks on parameters such as precision, accuracy, and linearity, as well as operational tests for reproducibility and background, must be performed daily.

Routine checks typically include measuring the background radiation to ensure the device is free from contamination. An increase of more than 10% requires further investigation. Additionally, the consistency of the voltage supply to the ionization chamber should be checked, with any deviation of more than 5% warranting a report.

Precision and accuracy tests are conducted quarterly. The precision test involves measuring 2 different radioisotopes 10 times, calculating the mean and standard deviation to determine precision variation. If the variation exceeds 5%, further investigation is required. Accuracy is assessed by measuring the activity of radioactive sources such as cobalt-57 (Co-57), cesium-137 (Cs-137), germanium-68 (Ge-68), and cobalt-60 (Co-60), with any variation of more than 5% from previous readings needing to be addressed.

The linearity of the dose calibrator is checked quarterly using a technetium-99m (Tc-99m) source, typically with an activity near 100 mCi or the maximum routinely measured activity. The process begins by eluting the molybdenum-99 (Mo-99) generator to obtain at least 100 mCi (3,700 MBq), ensuring the amount equals or exceeds the calibrator's maximum capacity. Radiation is measured and recorded every hour over a 36-hour period, with results documented in an Excel sheet to trace the response curve. A straight line should appear in a linear response system, and any variation exceeding 10% requires investigation. Additionally, the Tc-99m eluate must be checked to ensure it remains uncontaminated by Mo-99.

Geometric evaluations are carried out annually or after maintenance. For this check, Tc-99m activity is measured in syringes of varying volumes—3 ml, 5 ml, and 10 ml—with any variation of more than 5% from expected values requiring attention. Records of all quality control procedures should be maintained, both in hard and soft copies, for future audit purposes.[6]

Radiation Survey Meter

Another nonimaging instrument, the radiation survey meter, plays a crucial role in detecting and measuring radiation contamination during routine surveys. Daily checks include background measurements to ensure the meter is free from contamination, as well as battery checks to confirm proper functionality. Weekly checks focus on constancy and sensitivity, ensuring consistent performance and the ability to detect low levels of radiation accurately. On an annual basis, accuracy tests are performed to confirm that the meter provides precise readings. Regular checks ensure reliable radiation safety monitoring. Radiation safety relies on detecting hazards and maintaining meter functionality for routine surveys.

Gamma Camera 

SPECT-CT uses a gamma camera that acquires both 2D and 3D images. The 3D images are captured as the detector rotates around the patient from 0° to 360°, allowing for better anatomical detail and reducing ambiguities from planar imaging. The addition of computed tomography (CT) improves attenuation correction, image resolution, and anatomical localization.

To ensure consistent image quality, quality control is performed at different intervals. Daily checks include energy peaking, background radiation, extrinsic uniformity (crystal uniformity with a collimator), sensitivity, CT warmup, and hardware features like the touchpad and emergency lock. For daily uniformity checks, a Co-57 flood source is preferred. This device emits gamma rays at 122 keV, close to Tc-99m’s 140 keV, and has a half-life of 272 days, making it suitable for regular use.[7][8][9] Uniformity issues must be corrected promptly to avoid compromising diagnostic accuracy.

Weekly quality control includes assessment of the center of rotation (COR) to ensure accurate alignment between mechanical rotation and software reconstruction. Intrinsic uniformity, measured without a collimator, evaluates the crystal directly and should be assessed weekly. Integral uniformity (%IU) should be calculated for both the central field of view (CFOV) and the useful field of view (UFOV) using the following formula:

%IU = [(Maximum pixel count - Minimum pixel count) / (Maximum pixel count + Minimum pixel count)] × 100 [10]

Annual tests evaluate tomographic uniformity, spatial resolution, and overall system performance. A bar phantom is used for 2D spatial resolution, while a Jaszczak phantom is used for 3D SPECT resolution and uniformity. Additional checks include laser alignment, dosimetry, SPECT-CT image registration, and CT number uniformity. SPECT-CT registration is verified using a gadolinium-153 (Gd-154)source.[11][12][13] Failure in any of these checks can lead to inaccurate image acquisition, requiring repeat scans and increasing patient radiation exposure.

Positron Emission Tomography-Computed Tomography

PET-CT is a hybrid imaging modality that exclusively acquires 3D images and provides higher spatial resolution than a gamma camera. This modality plays a critical role in oncology, particularly in diagnosis, staging, and follow-up. Daily performance checks should include uniformity, coincidence timing, and energy peaking to ensure accurate image acquisition. Most vendors recommend the use of a prefilled Ge-68 phantom or an integrated source for these evaluations, which helps assess detector crystal performance. In modern systems, these tests are often automated, and the results can be logged for audit and longitudinal review.

CT-related assessments, such as x-ray tube warm-up and CT number verification for various materials, should also be performed routinely. Weekly evaluations include checking the consistency of the standardized uptake value (SUV), which quantifies tracer uptake based on the administered dose and the patient's body mass index. Ensuring consistent SUV measurements requires cross-calibration between the dose calibrator and the PET scanner, in accordance with Society of Nuclear Medicine and Molecular Imaging recommendations.[14]

Annual assessments should include normalization procedures, tomographic uniformity, CT number uniformity, and verification of dosimetry to confirm that the radiation dose delivered during scans remains within safe parameters. Calibration of the well counter, which is used for verifying radiopharmaceutical activity, is also included annually. Failures in these evaluations can lead to inaccurate reconstructions, potentially affecting clinical decisions. Additionally, repeat scans due to technical errors can increase cumulative radiation exposure to patients.

Issues of Concern

Routine quality assessments in nuclear medicine departments maintain consistent image quality and minimize radiation exposure for both patients and staff. Environmental factors such as temperature, humidity, equipment mishandling, and radioactive contamination can directly impact the performance of imaging and nonimaging equipment. Degraded images should always prompt an immediate review, as they often signal equipment-related issues or failed quality procedures.[15][16]

Tracking all assessment results over time is necessary to detect significant trends or deviations. Most inconsistencies resolve by repeating the standard procedures, but involving a trained service engineer or medical physicist may improve diagnostic reliability. An annual meeting dedicated to quality oversight provides a structured approach to managing performance issues and auditing the results of all assessments. This meeting should involve radiologists, nuclear medicine technologists, pharmacists, medical physicists, radiology department managers, and nurses, emphasizing that quality assurance remains a shared responsibility across the entire team.

Clinical Significance

Even minor flaws in the performance of imaging instruments can affect image quality and compromise diagnostic decisions. To prevent such issues and ensure consistently high standards in image production, each nuclear medicine department should implement a comprehensive and well-structured quality system. This framework must undergo review and revision every 2 years to remain effective. A biannual assessment of quality-related protocols is strongly recommended, with oversight by local regulatory authorities. All staff members should be encouraged to participate in continuous education and training programs focused on maintaining equipment performance and ensuring accurate imaging outcomes.

Other Issues

The quality of diagnostic imaging goes beyond the instruments and includes several factors that can influence the overall accuracy and clarity of the acquired images. Incorrect radiopharmaceutical administration techniques, issues with labeling, such as failed production, and improper handling of radiopharmaceuticals can all significantly degrade image quality.[17][18][19] Additionally, pre-examination factors like fasting status, blood glucose control, hydration, and uptake times, particularly in fluorine-18 fluorodeoxyglucose (18F-FDG) PET scans, must be optimized to ensure accurate SUV quantification. Patient movement during the imaging process and the injection of radiopharmaceuticals into the wrong patient are also critical factors that can compromise diagnostic quality.

To minimize or eliminate these issues, several rules must be followed. The patient's identity must be confirmed and the requested scan verified before administering a radiopharmaceutical. The radiopharmaceutical should undergo thorough quality checks before injection to ensure it meets necessary standards. Correct injection techniques are crucial to prevent unnecessary radiation exposure to the patient and preserve the quality of the images. Proper patient education about the procedure can also help reduce movement during the scan, further enhancing image accuracy.

Enhancing Healthcare Team Outcomes

Quality control requires a team effort and is not the responsibility of a single individual. In a nuclear medicine department, where radiation exposure is a concern, even minor flaws can significantly affect diagnostic image quality and increase radiation exposure for patients. Every team member, including radiologists, nuclear medicine technologists, radiology managers, nurses, service engineers, medical physicists, and administrative staff, must contribute to maintaining quality standards. Regular meetings, along with an evidence-based approach, help minimize the risk of flaws and ensure consistent service quality.