Optimizing Rotating Gamma Camera PET for Brain Imaging’

Timothy G. Turkington, Member, ZEEE Box 3949, Duke University Medical Center, Durham, NC 27710

Abstract Several acquisition geometries were investigated to

optimize coincidence count rates and coincidncelsingles rate ratios for brain imaging on a rotating gamma camera PET system. Detection efficiency and singles count rate limits on these systems dictate that keeping unused singles from hitting the cameras should improve coincidence rates if radiotracer doses can be adjusted to match the singles detection efficiency of the system. Radius of rotation and camera field of view were adjusted within the constraints of patient positioning. Acquisitions were done with transaxial septa and with graded absorber alone. Phantoms included a Ge-68 pin source and an extended head-thorax phantom with F-18 solution. Coincidence detection efficiency was best for the absorber only, with a small radius of rotation. Coincidence/singles ratio was best for graded absorber with small radius of rotation and axial camera field limited to 27 cm. The septa yielded a slightly low coincidence/singles ratio, but their reduction of scatter and restriction of incidence angle of detected photons (leading to higher spatial resolution) makes their use favorable.

I. INTRODUCTION Positron Emission Tomography (PET) imaging of the

brain is being used clinically to diagnose and evaluate a variety of diseases, including cancer, memory disorders, and seizures[ 1,2]. The increasing availability of the tracer F-18 2- fluoro-2-deoxy-D-glucose (FDG) makes it feasible for many centers to do PET imaging, in many cases with rotating gamma camera systems modified to do coincidence acquisitions.

The resulting image quality of these hybrid systems is far below that obtainable with dedicated multi-crystal PET systems primarily because of the limited number of coincidence counts that can be obtained. This limit stems from the combination of low detection efficiency of thin NaI detectors and the singles count rate limits imposed by having only two independent detectors, as opposed to hundreds of block detectors in a dedicated system.

Some improvements have been made to hybrid systems. To address the low detection efficiency, manufacturers have replaced the customary 318” thick Nal crystal with 5/8” or 314” crystals. To address the singles rate limits, special pulse processing techniques have been implemented, as well as schemes that allow simultaneous events in different camera regions to be processed, or at least detected and discarded. Such improvements are limited by the constraints that these systems must perform acceptably for low-energy imaging, and costs must not be prohibitive.

‘This work was supported by the Whitaker Foundation.

As in dedicated PET, the sensitivity for detecting radiation is increased on these systems by having open (3D) acquisition geometries. This entails removing any restrictive collimating devices and including all coincidences, regardless of the separation of the photons in the axial direction. Since camera counting rates are limited, however, and since patient doses can be administered which are well above the point of saturating the camera, several manufacturers have opted to use septa on the cameras to limit the axial acceptance of the cameras.

If radiotracer use is to be minimized, then absolute sensitivity of the camera for coincidence events is a good parameter to optimize. However, if minimizing the dose is not the primary goal, then maximizing the ratio of true coincidence counts to single counts is a more direct measure of camera performance, since the singles rate is the best indicator of dead-time and breakdown in spatial resolution due to pile-up, etc. In this scenario, a dose is chosen which puts the singles rates at an acceptable level for the camera, and the design which produces the most coincidences at the established singles rate is optimal.

Hybrid systems have large cameras, since they are designed for multiple purposes, For brain imaging, this extra axial field of view is not necessarily a benefit, and may actually be a detriment. In Figure 1, a configuration is shown that uses a small radius of rotation, which maximizes the detector sensitivity, and which requires that the shoulders be outside the field of view. It can be seen that for activity emanating from the head, photons detected in the region of the camera near the head are much more likely to have their partner photons detected than photons detected in further regions of the camera. If the singles count rate is limited, i t may be beneficial to turn off the camera in that region, thereby reducing single events more than coincidence events.

Figure 1. Some photon pair trajectories for events with one photon hitting a location in the near part of the camera and for events with one photon hitting the far part of the camera.

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FDG imaging of the brain with gamma cameras is currently good enough to identify some tumors, but image quality is far below dedicated PET capability [3], and improvements in image quality are likely to lead to improved diagnosis.

We have investigated a variety of acquisition configurations to determine optimal sensitivity and coincidencekingles ratios. First, we compared a system with no collimation (a graded absorber only) with a system with substantial transaxial septa, which restrict the axial acceptance angle. Secondly, we acquired data at two difference radii of rotation: with the cameras all the way out, which works for all imaging situations, and with the cameras very close to the head. Thirdly, data were acquired with several fields of view, achieved by turning off signals from rows of PMT’s.

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METHODS Data were acquired on a Varicam [4] with 5/8” NaI

detectors. Two levels of collimation were used. The first was transaxial septa (slats), 3 mm thick, extending 6.5 cm from the camera face, and spaced 1.3 cm center-to-center. Between the septa and the detector is a graded absorber to reduce the flux of low-energy photons on the detector [ 5 ] . The other level was a graded absorber alone. This allows the maximum sensitivity to radiation, but allows radiation from outside the field of view to hit the cameras. The absorbers have 1.5 cm thick lead bars extending 4 cm from absorber along the front and back edges to reduce the amount of activity entering from outside the FOV.

Two radii of rotation were used, one with the absorber faces 16 cm from the axis of rotation, and one with the cameras all the way out (31 cm). The outer radius is the one conventionally used on this system for all coincidence imaging. The smaller radius is the smallest feasible radius for head imaging. The different acquisition geometries are shown in Figure 2.

Five different maps for PMT amplifier gains were prepared. All were modifications of the standard high energy gain map, but with some amplifier gains for sets of PMT’s set to zero, to effectively reduce the field of view of the camera. The five patterns used are shown in Figure 3.

PO P1

P2 P3

Figure 2. Camera acquisition geometries used in this work.

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:igure 3. Patterns of PMTs used.

The approximate axial fields of view for PO, P1, P2, and P3 were 40 cm, 33 cm, 27, and 20 cm.

Two source configurations were used. The first was a 10 pCi pin source with activity distributed along a 10 cm line. This source was positioned parallel to the axis of rotation, but 5 cm away. Axially, it was positioned from cm from the edge of the field of view, so it was centered on the front half of the active camera area.

The second source was an extended phantom which included a 20 cm sphere to mimic the head, and an oval torso section, to provide realistic levels of radiation from outside the field of view. The torso is 37 cm wide, 22 cm anterior- posterior, and 40 cm long. The head was filled with 100 pCi of F-18 solution, and the torso with 340 pCi, adjusted to the time of the first acquisition. The phantom and its positioning in the scanner are shown in Figure 4. This phantom was based on body dimensions measured from PET scans from multiple patients [6] . Properly mimicking the attenuating and radioactive distribution properties of the body leads to more appropriate spatial and energy distribution of emitted radiation.

r Figure 4. Head and torso phantom, at top. Phantom positioned for scan, at bottom. The heads are out slightly from the 16 cm radius position.

For each configuration, a 3 min single-rotation coincidence acquisition was done. During this scan, events were acquired in list mode. In addition, detected singles rates were recorded for each camera at 45 degree intervals. These rates represent all events processed by the front end, at all energies. For coincidences, only photopeak events (51 1 keV +/- 10%) were used. (The additional scatter introduced with a lower Compton window is not desirable for brain imaging.) For each configuration, an additional singles acquisition was done at one camera position, to see the spatial distribution of counts in the cameras. These acquisitions were done with two energy windows: a 511 keV +I- 10% photopeak window, as

is used to form coincidences, and a 80 keV + window, to see the distribution of all energies relevant to singles rates. All acquisitions were performed at low enough count rates that random events are dead-time losses were negligible. In the case of the F-18 phantom, a two-hour delay was made between measurements with septa and measurements with graded absorber only, to keep rates comparably low.

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Figure 5. Rates measured with Ge-68 pin source. "GA" refers to graded absorber only, at the various PMT configurations.

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Detected Singles Rates - F-18 phantom RESULTS The results of the rate measurements are shown in Figure 5

for the Ge-68 source and Figure 6 for the F-18 phantom. For the Ge-68 source, the highest singles and coincidence sensitivity comes with the graded absorber, the small ROR, and PO (all PMT’s on). The coincidencelsingles ratio is approximately twice as high for graded absorber as for septa, and there is not much difference in PO, P1, and P2.

For the F-18 phantom, singles and coincidence sensitivity is also maximized for graded absorber, small ROR, and PO. However, for the coincidencelsingles ratio, the septa and absorber values are quite similar. The value is maximized for absorber with the P2 PMT configuration for both RORs, with the small ROR yielding substantially higher ratio.

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Figure 6. Rates measured with F-18 phantom. Values are decay- corrected relative to the setpa measurements so sensivities could be compared; actual recorded rates with graded absorber were much lower.

The 16 singles rate measurements for each coincidence scan (8 angles X 2 heads) were averaged into a single rate. Coincidence list-mode data were binned, and the total number of in-time coincidences recorded. For F-18 data, all rates were decay-corrected.

DISCUSSION The pin source and the extended brain-torso phantom

yielded considerably different results. With the pin source, both absolute sensitivity and coincidencelsingles ratios were better for the graded absorber alone than for the septa plus absorber. Sensitivity improved with the smaller ROR, although the coincidencelsingles ratio did not, consistent with a small source near the radius of rotation and no scattering medium.

With the extended phantom, however, the coincidence sensitivity improved more than the singles sensitivity with small ROR, yielding an improved coincidencelsingles ratio for small ROR. In addition, while the absolute sensitivities for were better for both singles and coincidences at small ROR, the coincidencelsingles ratio was also better. There was some improvement in the coincidencelsingles ratio for the reduced FOV, with the P2 configuration (two rows of PMT’s turned off) yielding the best ratio. The septa produced comparable coincidencelsingles ratios to the absorber, indicating that rejection of the radiation from outside the FOV is important.

These results would indicate the for brain imaging, if the dose is limited, the best option is to use a graded absorber only, with small ROR, and with the axial field of view restricted to approximately 30 cm, since the coincidence sensitivity is approximately 10 times higher than with the septa used in this work. If the dose can be chosen to optimize the imaging, though, comparable coincidence count rates can be obtained with septa, still with some benefit to a small ROR. Work done with patients indicates that the 10 mCi dose typically administered to patients on dedicated multicrystal PET systems is higher than can be withstood even with septa on some gamma camera systems, so use of septa would be appropriate.

Although maximizing coincidence rates or coincidencelsingles ratio was the purpose of this paper, there are other issues to consider in optimizing the performance of gamma camera-based PET systems. For one, the spatial resolution of detected events is worse for photons entering the camera at high incidence angle. A geometry that restricts entrance angle therefore has spatial resolution benefits. In addition, 3D acceptance geometries have varying sensitivity throughout the field of view, leading the varying image

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quality depending on location. Finally, some scattered events are rejected with septa, yielding higher contrast brain images, or less noisy images if a scatter correction is performed. Further work will evaluate noise equivalent count rates [7] and resulting image quality for the configurations investigated in this work.

ACKNOWLEDGMENTS We thank Mr. Edward Wheeler for technical support of the

scanner used in this work. We thank Dr. Charles Laymon for help in configuring the system and discussion. We thank Mr. Michael Daley for help in phantom construction.

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Sostman, S.C. Schold, “Clinical application of PET for the evaluation of brain tumors,” J Nucl Med vol. 32, pp.

[2] R.A. Radtke, M.W. Hanson, J.M. Hoffman, et al., “Temporal lobe hypometabolism on PET: predictor of seizure control after temporal lobectomy,” Neurology

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vol. 43, pp. 1088-1092, 1993.

[3] R.E. Coleman, T.C. Hawk, S.M. Hamblen, C.M. Laymon, T.G. Turkington, “Detection of Recurrent Brain Tumor: Comparison of MR registered Camera-Based and Dedicated PET Images,” Clin. Pos Imag vol. 2, pp. 57- 61, 1999.

[4] The Varicam was sold by Elscint, Ltd., and manufactured by Elgems, Ltd., both of Haifa, Israel.

[5] G. Muehllehner, R.J. Jaszczak, R.N. Beck, “The reduction of coincidence loss in radionuclide imaging cameras through the use of composite filters,” Phys Med Biol vol. 19, pp. 504-510, 1974.

[6] T.G. Turkington, N.E. Williams, S.M. Hamblen, R.E. Coleman, “Regional FDG uptake, attenuation, and geometry measurements for whole body phantom design,” J Nucl Med vol. 40, p. 281P, 1999.

[7] S.C. Strother, M.E. Casey, E.J. Hoffman, “Measuring PET scanner sensitivity: Relating countrates to image signal-to-noise ratios using noise equivalent counts,” IEEE Trans Nucl Sci vol. 3 , pp. 783-788, 1990.

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