Physics and technology of time-of-flight PET detectors

Abstract

The imaging performance of clinical positron emission tomography (PET) systems has evolved impressively during the last ∼15 years. A main driver of these improvements has been the introduction of time-of-flight (TOF) detectors with high spatial resolution and detection efficiency, initially based on photomultiplier tubes, later silicon photomultipliers. This review aims to offer insight into the challenges encountered, solutions developed, and lessons learned during this period. Detectors based on fast, bright, inorganic scintillators form the scope of this work, as these are used in essentially all clinical TOF-PET systems today. The improvement of the coincidence resolving time (CRT) requires the optimization of the entire detection chain and a sound understanding of the physics involved facilitates this effort greatly. Therefore, the theory of scintillation detector timing is reviewed first. Once the fundamentals have been set forth, the principal detector components are discussed: the scintillator and the photosensor. The parameters that influence the CRT are examined and the history, state-of-the-art, and ongoing developments are reviewed. Finally, the interplay between these components and the optimization of the overall detector design are considered. Based on the knowledge gained to date, it appears feasible to improve the CRT from the values of 200-400 ps achieved by current state-of-the-art TOF-PET systems to about 100 ps or less, even though this may require the implementation of advanced methods such as time resolution recovery. At the same time, it appears unlikely that a system-level CRT in the order of ∼10 ps can be reached with conventional scintillation detectors. Such a CRT could eliminate the need for conventional tomographic image reconstruction and a search for new approaches to timestamp annihilation photons with ultra-high precision is therefore warranted. While the focus of this review is on timing performance, it attempts to approach the topic from a clinically driven perspective, i.e. bearing in mind that the ultimate goal is to optimize the value of PET in research and (personalized) medicine. Selected abbreviations and symbols BSR Backside readout C d Diode capacitance CFD Constant-fraction discriminator CRLB Cramér-Rao lower bound CRT Coincidence resolving time C q Parallel capacitance of quench resistor DCR Dark count rate DOI Depth of interaction dSiPM Digital silicon photomultiplier DSR Dual-sided readout

Time of Flight in Perspective: Instrumental and Computational Aspects of Time Resolution in Positron Emission Tomography

“…In particular, the application of CRLB theory to weak sources of prompt photon emission, such as done in e.g. [40][41][42][43], tends to yield overly optimistic results, as explained in more detail in [28].…”

“…Another factor not to be overlooked is the reduction in optical transfer time spread that has become possible due to the introduction of SiPMs. Section II.A discussed the three causes of time resolution loss that occur in non-infinitesimal scintillation crystals [28,36]. Each of these effects are minimized when as many of the scintillation photons as possible are transferred to the photosensor as quickly as possible.…”

“…The three-dimensional position-of-interaction in the crystal is decoded from the measured light intensity map(s), while multiple timestamps (typically one per SiPM pixel) are available to estimate the time-of-interaction. The large amount of spatiotemporal information obtained per event makes it possible to reduce all three causes of OTTS-related time resolution loss discussed in section II.A, as well as the influence of the photosensor SPTR [28]. The maximum-likelihood interaction-time estimation (MLITE) algorithm by Van Dam et al, for example, can thus be seen as an advanced form of time resolution recovery [214].…”

“…The TOF resolution of modern PET scanners is primarily determined by the timing performance of the scintillation detectors. An elaborate review of TOF-PET detector technology and the factors that affect time resolution has recently been published [ 28 ]. Here, we briefly review the theory of time resolution, relevant innovations in scintillation materials and photosensor technology, and the way in which time resolution is influenced by the design of the detector.…”

Cross-Talk Issues in Time Measurements

“…Time-of-Flight Positron Emission Tomography (TOF-PET) [18], distances and volumes are measured as the duration of the round trip of particles. To increase the performance in terms of resolution and acquisition time, more and more timestamps are digitized in parallel, in increasingly shorter conversion times, through even larger number of channels and higher speed TDCs [19], [20].…”