European Nuclear Medicine Guide
Quality Control (QC) tests are an important part of the nuclear medicine routine work. They should be performed at designated time intervals to maintain proper functionality of the gamma camera. QC is a constituent part of the nuclear medicine’s department quality management program. A list of tests involved in QC for gamma camera and CT follows. Guidelines on the procedures of QC should be available to all the technologists who perform these tests.
Constancy tests: those tests performed to determine variations to reference data, which describe the equipment, its individual components, and initial state. These are performed by the equipment’s operator.
Acceptance tests: assert that the equipment’s performance parameters satisfy technical, legal, and/or manufacturer specifications. These are performed by the equipment’s manufacturer.
Action levels: values used in constancy tests. If those values are exceeded, an investigation must be conducted according to the quality management framework.
Tolerance limits: if these are exceeded, the use of the equipment in the clinical routine is limited or possibly not allowed. Violations of tolerance limits, their causes and consequences must be documented and justified by the radiation protection officer.
Record keeping: documentation of QC procedures and results should be recorded, along with the date and the initials of the person performing the test.
Gamma Camera: Single Photon Emission Computed Tomography (SPECT and SPECT/CT)
D - Daily, W/M - Weekly/monthly, M – Monthly, Q - Quarterly
Daily tests
Visual and physical inspection of the SPECT gamma camera should detect external mechanical or electrical defects or damages. A touch pad test should be performed on a daily basis and after each collimator change. An additional operational check should be performed on emergency stop buttons, if available, which should light and shut-down all motor-driven system movements when pressed.
As shown in Figure 1, the window setting for the radionuclide to be used should be performed, for example 99mTc, to ensure that the peak for the selected radiopharmaceutical coincides with the object under the camera. If this is not performed, degradation of the image quality and loss of spatial resolution may result.
Figure 1. Energy window setting for 99mTc
Operational check of the background count rates with or without collimators and within one or more energy windows should be performed daily to detect radiation caused by possible radioactive contamination of the scintillation camera or surroundings, external radiation from a neighbouring unshielded source or an excess of electronic noise.
Extrinsic uniformity of the imaging equipment should be performed daily in order to access the system’s response to spatially uniform flux of 99mTc, 57Co or 153Gd photons. Such a flood field uniformity may be tested qualitatively by visual inspection or quantitatively by calculation of the integral and differential image uniformity within camera’s central field of view (CFOV) and useful field of view (UFOV). Figure 2 demonstrates an example of extrinsic uniformity using 57Co, which is an external flood source.
Figure 2. Extrinsic uniformity using 57Co
Weekly/monthly tests
Intrinsic uniformity involves performing the QC test without any variables such as collimators in order to check the systems’ response to spatially uniform flux of 99mTc. Figure 3 shows an example of the end result achieved after performing an intrinsic uniformity test.
Figure 3. An example of a report of an intrinsic uniformity test
Centre of Rotation (COR) is the point at which the axis of rotation of the gamma camera and the perpendicular centre of the detector plane intercept. The transaxial alignment of the acquired projection images with the system’s mechanical centre of rotation is critical for accurate generation of tomographic images reconstructed from acquired projection images. For the multi-head SPECT system, it is crucial that the electronic centre of each angular projection used in the image reconstruction process is consistently aligned with the centre of mechanical rotation. As shown in Figure 4, point sources are placed in a phantom whereby the gamma camera rotates around this phantom, producing a result as shown in Figure 5 (this example illustrates a specific procedure required by the manufacturer, other configurations may exist).
Figure 4. Phantom for the COR test
Figure 5. Report of the COR test
Spatial resolution is the ability of a scintillation camera to accurately resolve spatially separated radioactive sources. The purpose of checking spatial resolution and linearity is to detect gradual long term deterioration of spatial resolution, and to display image linear objects as exactly linear as possible, as compared to acceptance and reference measurements, shown in Figure 6. Bar phantom image acquisition may or may not be required by imaging equipment manufacturers, but is done at the discretion of the user. An example of the bar phantom used for spatial resolution and linearity is shown in Figure 7.
Figure 6. Spatial resolution and linearity
Figure 7. Bar phantom
SPECT/CT alignment
Performing SPECT/CT examinations presents challenges such as mismatch, which degrade the accuracy with which the SPECT is aligned with the CT. One example of one manufacture will be described here:
QC of SPECT/CT usually involves several sources (i.e. point sources, a phantom with spheres or oblique line sources) that contain the contrast material used for CT and a point source of the radionuclide 99mTc, placed in plastic point sources, as shown in Figure 8. A SPECT/CT acquisition is performed, followed by the usual reconstructions involved, as shown in Figure 9. The permitted deviation between SPECT and CT should not exceed 5mm. Manufacturer specifications are respected.
Figure 8. Phantom for SPECT/CT alignment
Figure 9. Image of SPECT/CT alignment
SPECT Image quality
A SPECT total performance phantom is designed to provide a qualitative evaluation of the tomographic images and a QC procedure to demonstrate the limit of performance of the SPECT system. The most used SPECT phantom, the ‘Jaszczak’ phantom, contains a cylindrical container to simulate the abdomen of the patient. Inside the phantom, there are a number of solid spheres and rods of varying diameters. The phantom´s uniform container is usually used for detecting ring artefacts (which arise from detector non-uniformity), while the spheres and rods are used for assessing contrast and estimating resolution. In a gamma camera setting, the phantom is typically filled with around 300MBq of 99mTc (according to the collimator to be used) to create a background of uniform activity, the phantom is then scanned over approximately 15-30 minutes to obtain a high-count SPECT acquisition. The spheres could also be filled with fixed amounts of activity to perform a ‘hot’ test. By both methods, contrast resolution and spatial resolution of the system can be determined from the results. An example of the ‘Jaszczak’ phantom is shown in Figure 10. Other phantoms such as the Carlson phantom can also be used to achieve the same results as above.
Figure 10. ‘Jaszczak’ SPECT phantom with the rods and spheres visible
Positron Emission Tomography (PET/CT)
D - Daily W/M - Weekly/monthly M - Monthly Q - Quarterly
A uniformly filled 68Ge/68Ga phantom is used to perform daily QC tests for a PET system (Figure 11). The daily QC PET testing consists in the acquisition of a predetermined number of counts, which will be sufficient to evaluate the clinical performance.
