A high resolution prototype small-animal PET scanner dedicated to mouse brain imaging

Abstract

A prototype small-animal PET scanner was developed based on depth-encoding detectors using dual-ended readout of very small scintillator elements to produce high and uniform spatial resolution suitable for imaging the mouse brain. The scanner consists of 16 tapered dual-ended readout detectors arranged in a ring of diameter 61 mm. The axial field of view is 7 mm and the transaxial field of view is 40 mm. The scintillator arrays consist of 14×14 LSO elements, with a crystal size of 0.43×0.43 mm ² at the front end and 0.80×0.43 mm ² at the back end, and the crystal elements are 13 mm long. The arrays are read out by 8×8 mm ² and a 13×8 mm ² position-sensitive avalanche photodiodes (PSAPDs) placed at opposite ends of the array. Standard NIM electronics and a custom designed multiplexer are used for signal processing. The detector performance was measured and the results showed that all except the very edge crystals could be resolved. The average detector intrinsic spatial resolution in the axial direction was measured to be 0.61 mm. A depth of interaction resolution of 1.7 mm was achieved. The sensitivity of the scanner at center of the field of view was 1.02% for a lower energy threshold of 150 keV and 0.68% for a lower energy threshold of 250 keV. The spatial resolution within a field of view that can accommodate the entire mouse brain was ~ 0.6 mm using a 3D ML-EM reconstruction algorithm. Images of a micro hot-rod phantom showed that rods with diameter down to 0.5 mm can be resolved. First in vivo studies were obtained using ¹⁸F-fluoride and confirmed that 0.6 mm resolution can be achieved in the mouse head in vivo. Brain imaging studies with ¹⁸F-fluorodeoxyglucose were also acquired. Future plans are to add more detector rings to extend the axial field of view of the scanner and increase sensitivity.