Design considerations for an HPGe-based brain SPECT

Objectives: The complexity of the brain results in many instances in which it is desirable to be able to study more than one property simultaneously, such as measuring radiotracer neuroreceptor binding at the same time as transporter abundance or cerebral perfusion together with receptor occupancy. While SPECT is often touted for its possibility of imaging multiple radiotracers simultaneously, multi-tracer SPECT studies are rarely done in practice due to the limited energy resolution of conventional gamma cameras that necessitates complicated correction schemes to account for crosstalk between the different energy channels. Fully depleted single-crystal planar high-purity germanium (HPGe) detectors have been demonstrated for biomedical imaging of single-photon emitters (1, 2). These mechanically cooled detectors have been shown capable of providing spatial resolution <1.5 mm in all three dimensions and FWHM energy resolution of ~1% at 140 keV. This energy resolution would allow for easy separation of relevant photopeaks and suppression of scatter contamination in dual-tracer brain studies utilizing ^99mTc and ¹²³I. Here we consider possible designs for a brain SPECT system based on modular HPGe detectors employing pinhole collimation.

Methods: In a manner similar to previous theoretical studies of pinhole SPECT designs for brain imaging (3-5), we employed the standard analytical expressions for the spatial resolution and sensitivity of a single pinhole camera, including the intrinsic spatial resolution of the detector, to explore the impact of different design parameters on the spatial resolution and sensitivity on the system level. Fixing the field of view to be 19 cm in diameter and the target resolution at the center of the field of view to be 7.0 mm, we then calculate the point sensitivity at the center of the field of view as a function of pinhole radius (equivalent to radius of rotation for a rotating gantry system) for different combinations of detector size and intrinsic spatial resolution. Results: For a fixed combination of intrinsic spatial resolution and detector size, the sensitivity shows discontinuous increases as the pinhole radius increases, where the jumps in sensitivity correspond to the ability to add another pinhole camera to the ring as it increases in size. The HPGe detectors we have previously demonstrated have active areas 8 cm in diameter and intrinsic resolution of 1.3 mm. Using these values returns a peak sensitivity of 1.49x10^-4 cps/Bq at a pinhole radius of 17.4 cm for a single ring made up of thirty pinhole cameras. This model also shows that increasing the size of each HPGe detector to 14-cm diameter without any change in intrinsic spatial resolution would result in a sensitivity of 3.08x10^-4 cps/Bq, slightly more than doubling the sensitivity while utilizing only twenty cameras due to the ability to increase the pinhole diameter on each camera because of the greater magnification possible with a larger detector. Alternatively, improving the intrinsic spatial resolution to 0.5 mm while retaining the 8-cm detector size would allow for a sensitivity of 1.98x10^-4 cps/Bq employing 25 pinhole cameras, where the improvement in sensitivity comes from the reduction in pinhole radius made possible by the improved intrinsic spatial resolution. Conclusion: This analytic model demonstrates the potential for an HPGe-based brain SPECT system to provide comparable spatial resolution and sensitivity to conventional systems based on Anger cameras, while the outstanding energy resolution would facilitate improved multi-tracer imaging. The model can be extended to include volume sensitivity and multi-ring configurations, but it already provides a convenient means to assess the impact on system performance of key detector parameters under current development.

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