Nuclear Medicine Quality Assurance

Function

Nuclear Medicine equipment (both imaging and non-imaging) QC must be performed in various frequencies (daily, monthly, semi-annually, annually, and bi-annually). The nuclear medicine technologist (NMT) is in charge of most of the QC procedures. Service engineers and medical physicists also assist in the process of quality control and quality assurance (typically annual and bi-yearly tests).[4][5] Different QC tests are carried out at various time intervals, and a brief explanation of their roles is provided in Table 1. The clinical and radiation safety aspects of these tests are discussed in greater depth in the following sections.Table1

Routine quality control procedures for Nuclear Medicine equipment (imaging and non-imaging equipment)

Dose Calibrator

While a dose calibrator is not an imaging device, every dose injected into a patient or used to fill a phantom is measured with a dose calibrator before it is used in an imaging device. If the dose calibrator is not functioning properly, calibration studies would not be reliable.

Ionization chambers connected to circuits that convert and display the ionization current produced by radioactive sources in digital units of activity account for the vast majority of dose calibrators. Numerous factors influence the accuracy and working performance of a dose calibrator, and regular quality control parameters, such as precision, accuracy, and linearity of its response to activity, as well as operational checks of reproducibility and background, must be performed daily to ensure that the calibrator is operating correctly.

Sealed radioactive sources including cesium-137 ( Cs-137), cobalt-57 ( Co-57) or germanium-68 (Ge-68), cobalt-60 (Co-60), and unsealed radioactive isotopes such as technetium-99m (Tc-99m) are routinely used for checking the reproducibility of dose calibrator. 

The following routine checks need to be performed to check the functionality of the dose calibrator:

1. Background reading to make sure that the dose calibrator is contamination-free. An increase of more than 10% needs to be investigated.
2. Consistency in the voltage supply to the ionization chamber should be evaluated, and any change of more or less than 5% should be reported.

Precision and accuracy tests should be performed quarterly. This test provides information about any loss of precision due to background changes, any sudden change in the pressure of filled gas in an ionization chamber, or fluctuation in power supply to the ionization chamber. The precision test is typically carried out by measuring two different radioisotopes ten times and calculating the mean and standard deviation (SD) of the results to calculate precision variation (or variation in precision) (PV). The ratio of standard deviation to mean is typically calculated for each radioactive source, and any variation of more than or less than 5% must be investigated further.

The accuracy of the dose calibrator is accessed to difference mean measure activity of the source (post decay correction). It is measured with the help of Co-57, Cs-137, Ge-68, or Co-60 radioactive sources. Therefore, it is essential to investigate any variation of more or less than 5% from the previous reading.  

Linearity of dose calibrator is checked with the help of Tc-99m sources. It should be performed quarterly by using the following steps:

1. Elute Mo-99 generator and try to get at least 100 mCi (3700 MBq) or more equal to or higher than the maximum activity capacity of the calibrator.
2. Measure and record the reading for 36 hours with a gap of 1 hour.
3. Record in an excel sheet and trace a curve. A straight line is expected for a linear response system, and any variation of more or less than 10% should be investigated.
4. It is essential to check that Tc-99m elute should not contaminate molybdenum-99 (Mo-99).

Geometric evaluation (volume-based) and accuracy should be performed once a year or after the system has been serviced or post power down. For geometric evaluation, Tc-99m activity should be filled in various syringes (3 ml, 5 ml, and 10 ml) in the same volume and measured. Any variation of more or less than 5% should be reported for further evaluation. It is essential to maintain a record  (hard and soft copies) of each QC procedure required for future audit purposes.[6]

Gamma Camera (SPECT-CT) 

The gamma camera (GC) is imaging equipment that can capture both 2D and 3D images. The GC acquires 3D images by rotating around the patient at various angles, known as projection from zero to 360 degrees. These features help to see more details to overcome any uncertainty presented by 2D images. Furthermore, in SPECT-CT, computed tomography (CT) components improve the attenuation correction to enhance image resolution and anatomical localization. For the GC, it is critical that all aspects of the QC procedure, including 2D and 3D, must be performed frequently on various frequencies to upkeep the consistent reproducibility of acquired images. Daily checks for energy peaking, background radiation, and crystal uniformity with a collimator are recommended (extrinsic uniformity). It is always good to use a flood field source made of Cobalt-57 (Co-57), which has a half-life of 272 days and gamma energy of 122 keV for daily uniformity checks of crystal (extrinsic uniformity).[7][8][9] 

Co-57 based flood field source is recommended as its gamma energy is near Tc-99m (140 keV). Any issue with uniformity should be rectified as it can deteriorate the overall diagnostic quality of images. CT warm-up should be performed daily to ensure that the x-ray tube is working correctly. The SPECT-CT mechanical and software center should be evaluated weekly by checking the Center of Rotation (COR). Intrinsic uniformity (uniformity assessed without a collimator) should be checked once a week to ensure that the crystal performs appropriately. The integral uniformity (IU) at the center field of view (CFOV) and valuable field of view (UFOV) is calculated using the following formula:

%IU=[Maximum counts of pixel- Minimum counts of pixel/ maximum counts of pixel + minimum counts of pixel]X100[10] 

GC  spatial resolution should be checked annually with the help bar phantom (2D spatial resolution) and Jaszczak phantom (3D spatial resolution and SPECT performance). In addition, SPECT-CT registration also needs to be checked with the help of a gadolinium-153 (Gd-153) radioactive source.[11][12]

PET-CT

PET-CT imaging equipment can only acquire 3D images but have a higher spatial resolution than the GC. It is essential to regularly check equipment, including constancy and coincidence, and energy peaking. Most vendors recommend using a Ge-68 pre-filled phantom or in-built source for daily QC testing to ensure optimal crystal performance. The processing is often automated in modern systems, and the results can be recorded for further auditing. PET-CT plays an essential role in oncology regarding diagnosis and follow-up. The advantage of PET-CT-based imaging is that it provides quantification of the acquired images in the form of a special uptake value (SUV), which is calculated based on injected radiation dose and body mass index (BMI). It is mandatory to maintain SUV consistency, and this needs to be evaluated by well counter calibration manually. CT-related QC is also required to be performed regularly, including tube warm-up, CT number normalization, and PET-CT image registration.

Issues of Concern

Any NM department's routine QC procedure is essential as this practice helps maintain high-quality images consistently while also reducing the risk of radiation exposure to patients and staff. Temperature, humidity, equipment mishandling, and radioactive contamination are all factors that can affect the imaging and non-imaging equipment functionality directly. Therefore, it is critical to closely monitor images that appear degraded, as this could indicate faulty QC results.[13][14] 

In addition, it is crucial to keep track of all QC results and look for any significant shifts in trends. Most faulty QC results can be corrected by repeating the QC steps; however, reviewing results with trained service engineers or physicists can be beneficial. Once a year, a QA committee meeting should be held to manage the issue and audit the QC results. Quality is not an individual responsibility and is a collective effort. A QA committee should include radiologists, NMT, pharmacists, medical physicists, radiology managers, and nurses.

Clinical Significance

Any minor QC flaw in imaging instruments can impact the overall outcome of the image and the diagnostic decision-making process. Every NM department must have a robust QA plan to avoid any minor imaging instrument flaws and maintain the highest standards of image production for a more extended period. The NM department QA plan should be reviewed and updated every two years. A bi-annual review of the department's quality control protocols is highly recommended, and local regulatory bodies should evaluate the results. All employees should be encouraged to participate in any ongoing training opportunities to learn more about QA procedures.

Other Issues

NM QA is not limited to instruments and includes other factors that can reduce the overall diagnostic quality of acquired images. These factors include:

1. Incorrect radiopharmaceutical administration techniques 
2. Incorrect radiopharmaceutical administration
3. An issue with the labeling of radiopharmaceuticals such as failed production[15][16]
4. Pre-exam instructions not being followed by patients, especially in the setting of 18-F-FDG PET scans
5. Patient movement during acquisition
6. Injection of radiopharmaceuticals to the incorrect patient

All these factors can be avoided or minimized by following specific rules, which include:

1. Confirming patient identity and the requested scan before injecting a radiopharmaceutical
2. A radiopharmaceutical's QC results should be checked before administering it
3. Ensure injection techniques are followed correctly as any tissue radiopharmaceutical can give an unnecessary radiation burden to the patient and degrade the overall image quality
4. Proper explanation to the patient about the procedure to avoid any movement during the scan

Enhancing Healthcare Team Outcomes

QC is not an individual responsibility but rather a team effort involving everyone in a department. It becomes even more critical in the setting of an NM department, where there is a risk of radiation exposure. Any minor QC flaw can negatively impact the overall diagnostic quality of images, resulting in a higher radiation burden for patients. Therefore, every team member should actively participate in the QA plan, including a radiologist, NMT, radiology manager, nurses, service engineers, medical physicists, and administrative staff. Regular meetings and an evidence-based approach can help reduce the likelihood of QC failure and help to upkeep standard services.