CONSIDERATIONS FOR MULTIPLE HEAD SYSTEMS
5. CONSIDERATIONS FOR MULTIPLE HEAD SYSTEMS
5.1. INTRODUCTION Scintillation camera systems are available in different multiple head (also
termed multihead or multidetector) configurations, for example: dual head fixed 180°, dual head 90°, dual head variable angle, triple head fixed 120° and triple head variable angle.
This section addresses the special quality control aspects of these multiple head systems.
A number of the tests that are easily performed on single head cameras are difficult to perform on multiple head cameras because of the configuration of the heads or the difficult physical access to the heads. For example, intrinsic uniformity and spatial resolution measurements for some fixed dual and triple systems are difficult or impossible since the required source–detector distance A number of the tests that are easily performed on single head cameras are difficult to perform on multiple head cameras because of the configuration of the heads or the difficult physical access to the heads. For example, intrinsic uniformity and spatial resolution measurements for some fixed dual and triple systems are difficult or impossible since the required source–detector distance
5.1.1. Multiple head camera planar tests
Section 2 describes the tests necessary for planar camera systems. When a planar system has a second (or third) detector head, essentially all the tests of Section 2 must be repeated for each head. In Section 4.1.3, the following parameters are listed as being applicable to the general scintillation camera (whether or not it is used for SPECT):
(1) Energy resolution (Section 2.1.6.2); (2) Flood field uniformity (Section 2.1.6.3); (3) Spatial distortion (Section 2.1.6.4); (4) Differential ADC linearity (Section 6.1.2.3); (5) Integral ADC linearity (Section 6.1.2.3); (6) Spatial resolution (Section 2.1.6.1); (7) Count rate response (Section 2.1.6.6).
In order to fully characterize a multiple head system, each of these parameters should be determined for each of the detectors. This will increase the amount of work and camera time correspondingly. All the tests described in Section 2.3 should be repeated for each head except for:
(1) Section 2.3.12: Test of basic computer timing: This test is a system test performed with all detectors. Each head should
be exposed equally to the source, with no intervening material between the source and detectors. Each head should collect about the same number of counts during this test. If the disparity is more than 10–15%, the discrepancy should be investigated by repeating the test with each head separately.
(2) Section 2.3.13: Test of computer timing in dynamic acquisition: Extension to multiple heads is straightforward. (3) Section 2.3.14: Test of ECG gated acquisition: Extension to multiple heads is straightforward.
5.1.2. Multiple head tomographic cameras —
non-tomographic parameter tests
5.1.2.1. System planar sensitivity Tomographic systems can place special demands on the matching of
detectors in a multiple head system. In particular, pixel size, detector and collimator alignment (centring) and system sensitivity for each detector are especially important. The test described in Section 2.3.9 (test of system planar sensitivity) must be repeated for each detector. The detectors should match within 3%. If they are not closely matched, artefacts can be seen in SPECT studies and these are associated with having detectors of significantly different sensitivities. The artefact usually manifests itself in the sinogram; one section of which will have reduced intensity, which will result in a corresponding area of reduced counts in the reconstructed slice.
5.1.2.2. System count rate performance When testing the intrinsic count rate performance (Section 2.3.11),
consideration should be given as to whether high count rate studies, for example, first pass cardiac studies, will be performed with the system. If such studies are contemplated and more than one detector is going to be used, then all the heads that are to be used in the high count rate acquisition should be activated and tested together. In some multiple head systems, the count rate performance of a single head is degraded when a second or third detector is used simultaneously. This is caused by delays in transferring data from the detectors to a common data bus in the computer. See Section 2.3.18 for further information.
5.1.3. Multiple head tomographic cameras — tomographic parameter tests
In Section 4.1.4 on performance characteristics, the following performance parameters are listed for tomographic SPECT systems:
(1) Slice thickness; (2) Tomographic signal-to-noise ratio; (3) Tomographic contrast; (4) Tomographic uniformity; (5) Tomographic (in-slice) resolution; (6) Linearity of tomographic response; (7) Quantitative accuracy in tomography;
(8) Precision of estimation of the centre of rotation; (9) Tomographic sensitivity — slice and volume.
When determining these parameters of a tomographic scintillation camera system, separate tests involving each detector would not be required. Only when investigating abnormal performance would a decision be made to test these parameters for individual heads. It is usually possible to acquire the data from all detectors in a single acquisition. Then, at the time of reconstruction, any one, two or three detectors can be included in the reconstructed file. This is sometimes useful when troubleshooting difficult problems.
In Section 4.3 on acceptance and reference tests, there are nine tests used to check calibrations and to assess SPECT performance. The tests that would have to be repeated for each detector are those in Sections 4.3.1 (physical and mechanical inspection of the SPECT system) and 4.3.2 (test to determine the absolute size of a pixel). Some manufacturers may provide special fixtures, source holders and protocols so that pixel size may be conveniently measured and calculated for the multiple detectors.
5.2. TEST SCHEDULE The test schedule for multiple head camera systems should follow the
schedules established in Sections 2.2 for planar cameras and 4.2 for tomographic camera systems.
5.3. ACCEPTANCE AND REFERENCE TESTS In the sections below, each of the tests of Section 4.3 will be discussed in
relation to the testing of individual heads of a multiple head tomographic system. Planar parameters will not be discussed further. For all the tomographic acquisitions, either a 360º/n (where n is the number of heads) or 360º orbit may be used. A 360º orbit is generally preferred, as it allows each detector to acquire a full set of data that can be individually reconstructed. The images then become available for interpretation.
5.3.1. Physical and mechanical tests of the multiple head system
There are 15 steps for the procedure described in Section 4.3.1: Physical and mechanical inspection of the SPECT system, of which the following steps should be performed for each detector:
(2) Rotate all detectors as described. (5) Check mechanical centring for all heads. (6) Check that the Y axis of each head is parallel to the axis of rotation for all
heads. (8) Check all indicators for all heads. (12) Check each collimator of each head for visible damage.
5.3.2. Absolute pixel size
The procedures given in Section 4.3.2: Test to determine the absolute size of a pixel, would have to be repeated for each detector in a system. The pixel sizes for the detectors should be within 5% of each other. It is extremely important that the pixel sizes in the X and Y directions for all detectors are matched. Manufacturers should supply software that will assist the user in maintaining confidence in the matching of the detectors with regard to this parameter. Independent checks of this software, especially on acceptance testing, are necessary.
5.3.3. Tomographic uniformity of the system
In Section 4.3.3: Test of tomographic uniformity of the system, a test for tomographic system uniformity is described that is usually performed with all detectors simultaneously. The only change to the procedure described in Section 4.3.3 is that the orbit may be 360º/n. With single head systems, circular acquisition orbits will produce circular (ring) image artefacts, so-called ‘bullseye’ artefacts, when there are uniformity problems. With multiple heads, these circles will take on the appearance of arcs (see Fig. 48). On occasion, for troubleshooting purposes, 360º orbits should be acquired to help isolate a problem in a single detector. Data from individual detectors may then be reconstructed and images of the tomographic uniformity phantom can be examined.
FIG. 48. Partial ring artefact from a dual head SPECT system. Dual detector SPECT system with detector heads at 180° from each other, 360° total angle of rotation (each detector rotates through 180°), circular orbit. SPECT acquisition of a uniform cylinder, no attenuation correction and no uniformity correction applied. Two images from the set of transverse slices are shown. Different non-uniform artefacts are seen in the transverse slices. In the left image, the centre of the innermost ring corresponds to the centre of rotation. Note that the phantom was positioned off-centre so that this ring artefact does not correspond exactly to the centre of the phantom. Other artefacts are semicircles (arrowed) centred around the axis of rotation and not full rings as would be expected from a 360° rotation of a single detector head SPECT system. In multiple head SPECT systems, each head will contribute to the non-uniformity of the resultant SPECT uniformity. Since the non-uniformity in each head will be different, the resultant pattern observed in the reconstructed images will not be full rings but partial rings (see Ref. [3]).
5.3.4. Tomographic resolution in air
In Section 4.3.4, a test is described that is usually performed with all detectors simultaneously. Either a 360º/n or a 360º rotation may be used. On occasion, for troubleshooting purposes to help isolate a problem in a single detector, individual detectors may be used to acquire and/or to reconstruct data from the point source. When performing this test, it should be borne in mind that the system resolution is dependent upon the distance from source to collimator. It is important to ensure that all heads are set at the same distance from the axis of rotation.
5.3.5. Tomographic resolution with scatter
In Section 4.3.5, a test that is usually performed with all detectors simultaneously is described. On occasion, for troubleshooting purposes to help isolate a problem in a single detector, individual detectors may be used to acquire and/or to reconstruct data from the tomographic phantom.
5.3.6. Centre of rotation and alignment of axes
This test takes on different names depending on the vendor of the multiple head camera system (examples of names include multiple head registration and Rotax). It is a very important test that is always part of the installation procedure and is performed periodically by the user at intervals recommended by the manufacturer (usually weekly or monthly on new systems). The manufacturer provides detailed instructions. Special fixtures to hold multiple sources are usually involved. Pixel size, head registration, as well as centring are determined and corrections calculated and applied using the software supplied by the manufacturer. All detectors are used in this test.
Section 4.3.6 describes a test that can be used if the test provided by the manufacturer is suspect or if the test described in Section 4.3.4 has failed. It is important to ensure that all heads are set at the same distance from the axis of rotation when performing this test on a multiple head system. On occasion, for troubleshooting purposes to help isolate a problem in a single detector, individual detectors may be used to acquire and/or to reconstruct data from the point source.
5.3.7. Slice thickness
The test described in Section 4.3.7 is performed only with all detectors activated. Make sure that all heads are set at the same distance from the axis of rotation when performing this test on a multiple head system.
5.3.8. Variations of uniformity of sensitivity with angle
The test described in Section 4.3.8 may have to be performed for each detector in a multiple head system. This test was found to be very important for detectors built before about 1990. Since then, greater attention has been paid to magnetic shielding for the PMTs. Some instances, such as a scintillation camera installation located near a magnetic resonance imaging scanner, may require that this test be performed. In such a case, the test should be repeated for all detectors.
5.3.9. Total performance test
Typically, the test in Section 4.3.9 would be performed with all detectors. On occasion, it may be informative to perform this test for individual detectors when troubleshooting system uniformity, resolution or contrast problems.
5.4. OPERATIONAL CHECKS Routine operational tests are described in Section 2. These apply to each
head of a multiple head camera. These operational checks comprise: (1) Check of collimator and detector head mountings;
(2) Check of energy calibration of PHA; (3) Check of flood field uniformity and sensitivity; (4) Check of background count rate.
As indicated previously, these tests must be performed daily. With instruments as complex as multiple head tomographic scintillation
cameras, it is certain that failures will occur from time to time. The worst type of failure is one that manifests itself gradually and insidiously in the system. By carrying out routine checks of the SPECT calibrations (centre of rotation and uniformity), it is possible to verify quickly, proper SPECT system performance.
Section 4.4.1 should be performed on multiple heads as well as on single head systems at regular intervals, such as on a weekly basis. Section 4.3.9 is another routine performance test that is used to verify that the system is free from uniformity artefacts and yields reproducible images. The total performance check should be performed at quarterly or half-yearly intervals. If the system is used with different radionuclides, the total performance check should be performed with different radionuclides, perhaps by employing two phantoms, one for short lived radionuclides and one for long lived radionuclides. The phantom need not be sophisticated or complicated. Any tightly sealed jar of water greater than 15 cm in diameter will suffice to prove that reconstructed system uniformity is adequate.
All routine tests should have the instrument configured as it would be used for patients. This would normally require that all detectors be used for these operational tests.
These tests and additional examples are discussed further in Section 3 of Ref. [3].