New Scintillation Light Detection Concepts for PET
This project investigates a new high performance scintillation detector technology for PET. The basic concept is currently being employed to build position sensitive photon detector modules. These basic block detector modules can be used to build high performance clinical or pre-clinical (small animal) PET systems and is the design employed in the breast-dedicated PET system described in a previous section. The module is in the form of many detector layers. Each layer comprises two planar arrays of 1x1x1 mm3 lutetium-ytrium-oxyorthosilicate (LYSO) scintillation crystals coupled to two specially designed, planar and extremely thin (200 micron) position-sensitive avalanche photodiodes (PSAPD), each with a 8x8 mm2 sensitive area. There are alternating layers of planar crystal arrays and flat PSAPD detectors, coupled together and the layers are configured “edge-on” with respect to incoming photons. The PSAPD has both high light sensitivity and an intrinsic resolution that is finer than 1 mm, the selected width of the LYSO crystals. The PSAPD generates large electronic signals and allows the precise positioning of the light flashes resulting from the absorption of a 511 keV in any crystal; the resolution of each LSO-PSAPD compound layer in any direction is determined primarily by the 1-mm individual crystal dimension. The PSAPDs replace the photomultiplier tubes (PMT) used in standard PET designs. The PSAPD reads the crystals from their relatively large side faces, rather than from their tiny end faces for better scintillation light collection efficiency and directly-measured photon interaction depth.
This approach allows the extraction of a factor of 5-10 times greater light signal from the long and thin scintillation crystals than with the standard end-extraction; the crystal scintillation light collection efficiency is now nearly complete (>95% vs. <20% for the current ‘state-of-the-art’ in 1 mm resolution detectors). Light collection per event is now largely independent of crystal length or surface treatment, and of location within a crystal of the point of light creation, so that there is lower noise from light signal intensity variation from event to event. These features also yield a factor of 2-5 better energy and time resolution performance, compared to existing state-of-the-art small animal PET detectors, for better background (“scatter” and “random” coincidence) event rejection. These properties enable better contrast resolution and molecular probe target-to-background ratio visualization in the images.
Coincidence detection efficiency (i.e., fraction of emitted coincident 511 keV photon counts collected from a given probe concentration) is increased by bringing the detectors closer to the subject and by using thicker scintillation crystal material (2 cm effective thickness proposed vs. 1 cm standard for small animal imaging). Spatial resolution is enhanced by using finer scintillation crystals (1 mm proposed vs. 1.5-6.0 mm standard). Uniformity of spatial resolution improves by direct measurement of the photon interaction depth within the crystals using the side-coupled PSAPDs; this depth measurement reduces position-dependent parallax positioning errors (hence loss of resolution) due to photon penetration into crystals. Standard PET system detectors are incapable of this photon interaction depth resolution. These novel features will help to enhance the “molecular sensitivity” of PET.