Designing a compact high performance brain PET scanner—simulation study

Abstract

The desire to understand normal and disordered human brain function of upright, moving persons in natural environments motivates the development of the ambulatory micro-dose brain PET imager (AMPET). An ideal system would be light weight but with high sensitivity and spatial resolution, although these requirements are often in conflict with each other. One potential approach to meet the design goals is a compact brain-only imaging device with a head-sized aperture. However, a compact geometry increases parallax error in peripheral lines of response, which increases bias and variance in region of interest (ROI) quantification. Therefore, we performed simulation studies to search for the optimal system configuration and to evaluate the potential improvement in quantification performance over existing scanners. We used the Cramér–Rao variance bound to compare the performance for ROI quantification using different scanner geometries. The results show that while a smaller ring diameter can increase photon detection sensitivity and hence reduce the variance at the center of the field of view, it can also result in higher variance in peripheral regions when the length of detector crystal is 15 mm or more. This variance can be substantially reduced by adding depth-of- interaction (DOI) measurement capability to the detector modules. Our simulation study also shows that the relative performance depends on the size of the ROI, and a large ROI favors a compact geometry even without DOI information. Based on these results, we propose a compact ‘helmet’ design using detectors with DOI capability. Monte Carlo simulations show the helmet design can achieve four-fold higher sensitivity and resolve smaller features than existing cylindrical brain PET scanners. The simulations also suggest that improving TOF timing resolution from 400 ps to 200 ps also results in noticeable improvement in image quality, indicating better timing resolution is desirable for brain imaging.

Concept of an upright wearable positron emission tomography imager in humans

“…In the current prototype, the spatial resolution decreased substantially at the periphery versus the center region, and portions of the cortex may have been outside the field of view of the imager, both effects due to very tight geometry. Loss of spatial resolution toward the edge of the imager could be partially mitigated with the introduction of depth of interaction (DOI) correction (Gong et al, 2016). …”

“…As such, these trade‐offs would need to be considered based on research and clinical need and whether the main priority is image quality or participant mobility. Most recently implementation of Time of Flight (TOF) capability in brain imaging is discussed and analyzed in simulations as a method to improve S/N ratio, reduce the background effects, including scatter of emissions from the body without introducing shields, and effectively to increase sensitivity even with less scintillator material and lower total weight of the imager (Gong et al., 2016). …”

“…We expect that the next‐generation device will yield a significant improvement in image quality as well as more functional and versatile mechanical support, eventually including the ability of the participants to stand and walk with the imager attached to the head. We have plans for several iterations of next‐generation devices (Bauer, Brefczynski‐Lewis, Lewis, Mandich, & Majewski, 2014; Brefczynski‐Lewis, Bauer, Lewis, Mandich, & Majewski, 2014; Gong et al., 2016; Harrison et al., 2015; Kinahan et al., 2015) to further explore our theoretical concept. Ultimately, we envision multiple designs that could be focused either on full brain coverage and high efficiency with the drawback of less mobility, or more on enabling free movement at the expense of full coverage or maximum sensitivity.…”

Simulation study of a high‐performance brain PET system with dodecahedral geometry

“…Recently several designs of dedicated brain systems have been presented. Gong et al simulated a helmet structure system that consists of six side rings with different diameters, a top panel, and a bottom panel. A slightly different helmet PET was first presented and then upgraded to the helmet‐chin system .…”

Sinogram Blurring Matrix Estimation From Point Sources Measurements With Rank-One Approximation for Fully 3-D PET

“…The system model can be obtained by different approaches, including analytical calculation, Monte Carlo simulation, and real measurements. Some of the analytical methods are based on a parameterization of the detector response function [5]–[9]; others focus on multi-ray tracing implementations [10]–[12]. Most of the analytical methods only modeled inter-crystal penetration and could not consider inter-crystal scatter.…”