A multilayer detector for PET based on APD arrays and continuous crystal elements

Scintillation detectors developed for PET traditionally uses relatively thick crystals (to ensure good efficiency), which are coupled to photomultiplier tubes. The crystals typically measure between 10 and 30mm thick Detectors also require good spatial resolution so the scintillator is normally made up of a densely packed array of long thin crystals. In our design the detection crystal is divided into a number of layers along its length with an avalanche photo diode (APD) inserted between each layer. With thin layers of crystal it is possible to use a continuous rather than a pixellated crystal. The advantages of this design over a conventional PMT based detector are improved light collection efficiency, not dependent on hard crystal to achieve good stopping power, case of crystal manufacture, Reduced detector dead-time and increased count rate, Inherent depth of interaction. We’ve built a 4-layer detector to test this design concept using the Hamamatsu S8550 APD arrays and LYSO crystals. We used the centre 16 pixels of each of the arrays to give an active area of 9.5mm x 9.5mm. The thickness of the 4 crystals increases from 2mm at the front to, to 2.5mm, to 3.1mm and to 4.3mm at the back, to ensure the same count rate in each layer. The energy resolution for each of the layers at 511keV was around 15%, coincidence timing resolution 2.1ns and special resolution less than 2mm using a statistical based positioning algorithm.