Design of a High Performances Small Animal PET System With Axial Oriented Crystals and DOI Capability

In most Positron Emission Tomography (PET) systems dedicated to small animal imaging, the geometry of the detector module is based on a block structure where the crystal elements are coupled to a reduced number of photomultiplier tubes (PMT). In this configuration, the spatial resolution and the detection efficiency depend on the crystal dimensions and thus there is a correlation between these two figures of merit. In this paper, we present a method already used by Ter-Pogossian in the 1970s allowing the spatial resolution and the detection efficiency to be independent of each other. The crystals are oriented in the axial direction readout on both sides by individual photodetector channels. The spatial resolution in the transverse plane is driven by the crystal section and the spatial resolution in the axial direction is proportional to the contrast of the light collected on both sides of the crystal. The detection efficiency depends on the number of radial crystal layers and the geometry of the system. With the perfect knowledge of the interaction depth, the inner diameter of the PET system can be reduced to a minimum value leading to an increase in detection efficiency. We investigate two particular geometries dedicated to mouse and whole purpose studies and based on the same detection module. Each module consists on a matrix of 32 $,times,$24 LYSO:Ce crystals of 1.5 mm$,times,$1.5 mm$,times,$25 mm each read at both ends by a Photonis Corp multi-channel plate photodetector. The surface treatment is optimized to reach a volumetric spatial resolution of 1 mm$^{3}$. The detection efficiency of each system is evaluated using Monte Carlo simulation. The mouse and whole purpose systems a- - re based on 4 and 6 modules with an inner diameter of 61.2 mm and 103.2 mm where the axial extent is 25 mm leading to a detection efficiency of 18% and 13%, respectively. This geometrical configuration leads to a detection efficiency close to the system solid angle with a volumetric spatial resolution of 1 mm$^{3}$