Project Summary
Clinical time-of-flight positron emission tomography (TOF-PET) systems capable of excellent coincidence time
resolution (CTR) promise to drastically enhance effective 511 keV photon sensitivity. The ability to more precisely
localize annihilation origins along system response lines constrains event data, providing improved signal-to-
noise ratio (SNR) and reconstructed image quality by associating 511 keV photons more closely to their true
origin. This SNR enhancement increases as CTR is improved, and a major goal of ongoing PET instrumentation
research and development is to push system CTR ≤100 ps full-width-at-half-maximum (FWHM). At this level of
performance, events are constrained ≤1.5 cm, providing a ≥five-fold increase in SNR relative to a system with
no TOF capability. Advanced systems capable of ≤100 ps FWHM CTR would effectively more than double or
quadruple the effective 511 keV system sensitivity, in comparison to state-of-the-art, clinical TOF-PET systems
(250-400 ps FWHM CTR). Thus, advancing CTR is also a pathway for greatly improved system sensitivity without
increasing detection volume and system material cost. Standard PET detectors comprising segmented arrays of
high-aspect-ratio scintillation crystal elements and aggressive electronic signal multiplexing cannot achieve this
level of performance and are ultimately limited by poor light collection efficiency, depth-dependent scintillation
photon transit time jitter seen by the photodetector, and poor electronic SNR for optimal discriminator time pickoff
and 511 keV photon time of interaction estimation. To address this, we are developing a new detector readout
concept for monolithic scintillation detectors which allows scintillation photons arriving at each photosensor pixel
to be counted and directly digitized. The spatiotemporal arrival time of scintillation photons in monolithic detectors
intrinsically carries all information on 511 keV photon energy, three-dimensional (3D) position and time of
interaction, and 3D position of interaction dependent scintillation photon transit skew. [Thus, this new detector
readout concept’s ability to directly digitize the temporal scintillation light maps on photosensor arrays coupled
to monolithic scintillators offers a unique opportunity for machine learning (ML) techniques to extract 3D
positioning and time of interaction estimators in large area, thick (high 511 keV photon detection efficiency)
detector modules that are at the statistical limit of performance. We will leverage this new advancement to
investigate the performance of ML applied to the digitized photon data streams from a prototype detector module
to demonstrate high resolution, three-dimensional positioning capabilities and CTR in a design that also makes
no sacrifices on detection efficiency. The proposed PET detector technology can have a significant impact on
quantitative PET imaging. The image SNR enabled by the significant boost in effective sensitivity can be
employed to substantially reduce tracer dose and shorten scan time/increase patient throughput, or to better
visualize and quantify smaller lesions/features in the presence of significant background, which are important
features that can make PET more practical and accurate, as well as help to expand its roles in patient
management.]
