Innovations in Instrumentation for Positron Emission Tomography

Abstract

PET scanners are sophisticated and highly sensitive biomedical imaging devices that can produce highly quantitative images showing the 3-dimensional distribution of radiotracers inside the body. PET scanners are commonly integrated with x-ray CT or MRI scanners in hybrid devices that can provide both molecular imaging (PET) and anatomical imaging (CT or MRI). Despite decades of development, significant opportunities still exist to make major improvements in the performance of PET systems for a variety of clinical and research tasks. These opportunities stem from new ideas and concepts, as well as a range of enabling technologies and methodologies. In this paper, we review current state of the art in PET instrumentation, detectors and systems, describe the major limitations in PET as currently practiced, and offer our own personal insights into some of the recent and emerging technological innovations that we believe will impact the field. Our focus is on the technical aspects of PET imaging, specifically detectors and system design, and the opportunity and necessity to move closer to PET systems for diagnostic patient use and in vivo biomedical research that truly approach the physical performance limits while remaining mindful of imaging time, radiation dose, and cost. However, other key endeavors, which are not covered here, including innovations in reconstruction and modeling methodology, radiotracer development, and expanding the range of clinical and research applications, also will play an equally important, if not more important, role in defining the future of the field.

Figure 1

Illustration of relative geometry of a) standard whole-body clinical PET scanner where the patient is translated through the scanner to create a whole-body image from multiple bed positions; b) dedicated brain PET scanner; c) dedicated breast PET scanner with patient lying in prone position; d) preclinical PET scanner for rodent imaging; and e) extended field of view scanner for total-body PET imaging. The physical extent of the PET scanners are indicated in blue and typically consist of rings of detectors around the subject or organ.

Figure 2

Photograph of the EXPLORER total-body PET scanner which is currently under construction (courtesy United Imaging Healthcare, Shanghai, China).

Figure 3

Illustration of different detector designs used in PET. a) Light sharing clinical block detector, b) high resolution light sharing pre-clinical detector, c) one-to-one coupled detector, and d) monolithic detector. Dark blue represents the scintillator crystals, light blue represents the light guide, brown represents the photodetectors (i.e. SiPMs), and green represents the electronic circuits. Crystal dimensions represent common detector designs.

Figure 4

Overview of common DOI encoding methods for PET detectors. a) Dual-ended readout, where photodetectors (i.e. APDs or SiPMs) are fastened to both ends of a pixelated scintillator array to measure DOI-dependent differences in light collection. b) monolithic detectors, where an array of photodetectors measures DOI-dependent differences in the spread of scintillation light. c) the phoswich detector which uses two or more layers of scintillators that vary in their scintillation emission properties (e.g. decay time). d) a stacked scintillator configuration where the top scintillator array is offset by ½ crystal pitch so that the distribution of light from the top layer is unique from the bottom layer as measured by a set of photodetectors. e) phosphor-coated detector, where a thin layer of phosphor coating applied to the sides of the scintillator crystal induces DOI-dependent signal shape changes. Reproduced with permission from [38].