Quality control

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)

Test	D	W/M	M	Q
Physical inspection	x
Collimator touch pad and gantry emergency stop	x
Energy window setting for 99mTc	x
Energy window setting – other radionuclides to be used	x
Background count rate	x
Extrinsic uniformity (99mTc, 153Gd, or 57Co)	x
Intrinsic uniformity		x
Centre of Rotation		x
Spatial resolution and linearity			x
SPECT/CT alignment			x
Jaszczak phantom				x

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

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

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 and linearity

Spatial resolution is the ability of 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 imaged 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. 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)

Test	D	W/M	M	Q
Acquire 68Ge Cylinder scan	x
Generation of Normalization factors	x
Evaluation of Daily QC Components	x
Block noise	x
Block efficiency	x
Measured random counts	x
Scanner efficiency	x
Scatter ratio	x
Calibration factors and image plane efficiency	x
Timing Offset	x
Width and Time Alignment Fit	x
Cross Calibration		x
PET/CT alignment (every half year)

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

PET/CT alignment (1/2 year)

This is usually performed by placing two ⁶⁸Ge/⁶⁸Ga line sources inserted in a suitable phantom and a scan is performed. The results obtained are assessed visually, and in the event of a deviation, the medical physicist is informed, together with the manufacturer. 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.

Quality control	D	W	M
Laser Alignment	x
Water phantom and standard deviation	x
Artefact evaluation	x
Uniformity	x
Slice thickness	x
Noise	x
Modulation Transfer Function			x
Wet laser printer Quality Control		x
Visual Checklist		x
Dry laser printer Quality Control			x
Display monitor Quality Control			x
Constancy			x

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.

Non-imaging QC procedures performed in Nuclear Medicine

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

 

Radionuclide calibrators and well counters are examples of equipment that are used in nuclear medicine, whereby QC also needs to be performed. Dose calibrator tests include clock accuracy, background tests, constancy, stability, accuracy, and linearity to be repeated at reasonable intervals (e.g. background/constancy daily; accuracy whenever a reference source is available; linearity every six months).Except for accuracy and linearity, all tests should use a long-lived source (normally Cs-137, figure16) The Linearity shall be determined with an high enough starting activity, decaying over some days following a manufacturer deigned sequence of automatically repeated measurements. The accuracy shall me measured with a commercially available calibrated source the activity of which is traceable to the national metrology institute.

Radiation protection instruments include Geiger-Müller monitor, contaminameters, 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. Semiannual 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.

Wrap-up recommendations

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.

For further information on QC in Nuclear Medicine the reader is referred to the following publications.

Busemann Sokole E., Plachćinska A., Britten A., Georgesopoulou M. L., Tindale W., Klett R., “Routine quality control recommendations for nuclear medicine instrumentation”, European Journal of Medical Molecular Imaging, 2010;37:662-71, [Available Online]

European Association of Nuclear Medicine (EANM), (2017) “Quality Control of Nuclear Medicine Instrumentation and Protocol Standardisation”, [Available Online]

German Commission on Radiological Protection, (2011-2018), “Quality Control of Nuclear Medicine Equipment – Definition of Action Levels and Tolerance Limits”, [Available Online]