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.
Figure 11. Solid 68Ge Phantom
Another option to do this test is using the intrinsic activity of LSO crystals in digital PET detector systems. Here, the very sensitive detector system counts the events by that very low intrinsic activity of the fraction of 176Lu in the LSO material. Currently, not all manufacturers of digital PET-Systems do offer that option.
PET/CT alignment (1/2 year)
Another option to do this test is using the intrinsic activity of LSO crystals in digital PET detector systems. Here, the very sensitive detector system counts the events by that very low intrinsic activity of the fraction of 176Lu in the LSO material. Currently, not all manufacturers of digital PET-Systems do offer that option.. Figure 12 illustrates an example of this phantom, and Figure 13 illustrates the scan result, ready for visual assessment.
Figure 12. PET/CT alignment Phantom
Figure 13. PET/CT alignment - visual assessment
The EANM encourages the implementation of a program so that centres employ the required quality assurance and QC to gain EARL accreditation (EANM Research Limited) earl.eanm.org. It aims at harmonizing QC performed in different centres that are willing to be EARL accredited, thereby standardizing the QC performed and achieving results that are able to be quantified. Centres that are EARL accredited may be able to exchange findings and data, including patient preparation.
Computed Tomography (CT)
Computed Tomography used for diagnostic purposes needs to undergo QC tests in order to be working within acceptable limits.
D – Daily W/M - Weekly/monthly M - Monthly
Daily x-ray CT QC test of the CT system should be performed according to recommended manufacturer’s procedures and the medical physicists’ expert advice. It is implicit that the following tests apply to standard diagnostic CT applications, QC demand can relax in the case of attenuation correction (AC)-only CT or extend for example in radiotherapy applications.
For instance, it may be recommended to perform daily CT quality procedures which automatically execute a set of CT tube warm up acquisitions, automatic function checks, and different air and water calibration steps for all available voltage settings, in order to guarantee optimum image quality. Laser alignment should be checked at the beginning of the day. Laser alignment should coincide with the grooves on the phantom before performing QC.
According to the manufacturer’s instructions, tube warm up should be performed after a short time when the CT tube has been idle, and a CT tube calibration should also be performed every day, either at the beginning or end of the day, and documented. An example of a homogenous CT image of a water phantom is shown in Figure 14 (A). In Figure 14 (B), a ring artefact is demonstrated, which is unsightly when performing examinations.
Figure 14. Uniform response (A) and ring artefact (B)
Figure 15. Uniformity – section through the water phantom
As shown in Figure 15, axial sections through the water phantom are acquired and CT numbers are calculated in five regions of interest in all slices. Differences between the regions of interest is calculated in one axial section and can be analysed. The standard deviation is produced from the global mean value, so that uniformity can be evaluated. The difference between these values should not exceed more than 2 standard deviations (SD). The water phantom should also not contain any air bubbles, which would result in a difference between the HU when testing for uniformity.
The Radiation Protection in Medicine regulation requires the implementation of regular quality assurance procedures also for dose calibrators. The regular verification of the accuracy, reliability, and constancy of activity measurements in nuclear medicine facilities is essential for patient safety, since incorrect activity readings can directly result in over- or under-dosage and thereby compromise the clinical outcome of diagnostic or therapeutic procedures. As part of the tests, activity measurements are performed using a suitable reference source, and the measured values are compared with nuclide-specific reference values, which are usually established by the manufacturer or supplier of the dose calibrator during the acceptance test or a subsequent safety inspection.
Additionally, verification of molybdenum-99 breakthrough must be observed for activity metes used for the measurement of technetium-99m, which requires the presence of an additional, specific shielding.
Daily constancy testing of activity meters must include verification of both the background and the response in a particular nuclide/energy setting.
The background check serves to exclude excessive background count rates, contamination of the ionization chamber or sample holder, and to verify the proper electronic function of the ionization chamber. For this purpose, an activity measurement must be performed in the most frequently used nuclide setting without any radioactive source in the chamber.
Verification of the constancy of response—and thus the calibration of the signal to the absolute activity value—must be performed daily using the same nuclide setting as that used during the acceptance test. The activity of a long-lived reference source is determined and compared with the corresponding reference value established during the acceptance testing of the dose calibrator. The relative deviation of the measured value must not exceed the action level (usually 3%) or the tolerance limit (usually 5%).
The reference source used must be sealed, have a known activity, a half-life greater than five years, and a high radionuclidic purity. In practice, a cesium-137 reference source (half-life: 30.2 years) is commonly used.
Figure 16. Cs-137 reference source that can be used for most gas filled counter QC. On the right a typical stability check report is displayed
The molybdenum breakthrough test itself is not a constancy test of the dose calibrator, but rather a verification of the radionuclidic purity of eluates of technetium generators. Activity meters used for the measurement of technetium-99m activity must be equipped with a suitable shielding device specifically designed for this purpose. The molybdenum breakthrough test should be performed using the first eluate after delivery of a new technetium-99m generator.
If a generator is used more than about 14 days after the initial elution, a repeat test of the molybdenum breakthrough is recommended.
The eluate is measured once using the technetium-99m setting, and once using the special shielding device supplied for the breakthrough test.
The molybdenum fraction in the eluate should not exceed 0.1 %.
Most manufacturers of activity meters also offer well-type counters (or well counters). These are, in essence, specialized dose calibrators designed for very low activities. They are used to measure the activity concentration in samples such as blood, urine, or similar patient specimens with very low activity levels, typically ranging from a few kBq down to some 10 and 100 Bq.
The daily quality control procedures for well counters largely correspond to those for standard activity meters.
Semi-annual or less frequent tests are the check of the system linearity and must be verified at least every six months using the most frequently used nuclide setting on a particular activity meter.
This test checks the linearity of the activity measurement over a wide activity range by repeatedly measuring a short-lived radioactive source (e.g. Technetium-99m or Fluorine-18) at defined time intervals and comparing the measured values with the expected decay-corrected activities at each point in time.
The relative deviation between the measured and expected activity must be within tolerance (usually ± 5%) with usually 3% being the action level.
Linearity testing must cover an activity range from 1 MBq up to the maximum patient activity measured with the device under test, but at least up to 1 GBq. The measurement intervals should be chosen such that several readings are obtained per decade of activity. Modern activity meters are equipped with measurement protocols that automatically perform data acquisition, analysis, and documentation.
For well-type counters, the calibration factor must additionally be determined every six months. This is defined as the ratio of the activity measured in the activity meter to the count rate measured in the well counter.
Its determination is required as part of the semiannual quality checks, although no specific measurement procedure is prescribed. Various methods may be applied for its determination.
Probes for Radioactivity Guided Surgery (RGS) and dose rate surveillance
For many years, probe type devices have been used for the detection/localizing of areas and structures with radioactivity uptake, for example in the context of sentinel lymph node biopsy (SLNB).
These instruments must undergo daily measurements of background and response constancy.
For this purpose, usually cobalt-57 reference sources with a nominal activity of approximately 200 kBq are used. Commercially available probe systems are supplied with dedicated fixtures and test protocols that allow for reproducible performance of these checks.
At longer intervals—typically every six years—a safety inspection must be carried out by the manufacturer, during which new reference values for the daily constancy checks are usually established.
If fixed (stationary) gamma probes are used—for example, for measuring biokinetics and radionuclide accumulation in the context of therapeutic procedures—these devices must be checked primarily in accordance with manufacturer specifications and inspected by the manufacturer at least once per year.
When such probes are used for monitoring of quantitative parameters, for example:
to verify that ambient dose rates in the vicinity of patients (e.g. at a distance of 2 m) fall below a defined release criterion from radionuclide therapy wards, or to quantify activity accumulations as part of the physical-technical verification of radionuclide therapy, they must be calibrated annually using the radionuclide most commonly employed in therapy.
Radiation protection instruments include Geiger-Müller monitor, contaminator monitors, dose rate meter, electronic personal dosimeter, and personal dosimeter. Dose rate meters and electronic personal dosimeters are normally calibrated with a primary calibration standard by the national metrology institute. Semi-annual tests with a suitable standard radioactive source should suffice to ensure an accurate instrument performance. The sensitivity and accuracy of the Geiger-Müller detector also need to be tested and documented as part of a yearly QC procedure.
An intraoperative probe is usually used to detect gamma radiation during non-imaging procedures. Physical inspection of cables connecting to the actual probe and internal circuit voltage assessment should be performed before each use. The sensitivity and constancy of the probe should be tested at fixed intervals with a reference radioactive source. The manufacturer’s recommendations should be kept at hand, and every result from the QC documented.
The importance of QC in nuclear medicine procedures is essential for good image quality as well as the radiation safety of the patient. By performing routine QC, high quality performance of the equipment used is ensured. Documentation of the tests performed should be kept and are especially important in the recognition of trends. Results of tests that are in the ‘acceptance level section’ but are moving towards the ‘action level section’ could be dealt with as soon as possible and before problems arise.
