Detector characteristics of a pixellated germanium Compton camera for nuclear medicine

A pixellated germanium Compton camera is currently being developed for imaging 511 keV sources in nuclear medicine. It was built by ORTEC and consists of two planar Ge detectors (the scatter and the absorption detector) housed within the same cryostat. The scatter and absorption detectors have 152 and 25 4×4 mm² pixels respectively. The readout electronics was developed at Daresbury Laboratory UK and consists of 15 GRT4 VME cards and a PowerPC. The system is controlled by a PC running MIDAS software. This paper reports the current status of camera development. The pixel energy resolution has been measured to give an average of ∼0.5% at 356 keV using a NIM module and ∼2% using a simple digital algorithm. In addition, a centroiding algorithm that takes into consideration the induced charge from the surrounding pixels, is being implemented to improve the intrinsic spatial resolution of the camera, which is restricted by the size of the pixel (4 mm), to a target value of ∼1mm; so far a X-Y position sensitivity of 2 mm has been achieved. The depth of interaction can be provided by analysis of the pulse rise time, theoretical predictions anticipate a value of the order of 0.5 mm. Currently these parameters are determined offline using simple algorithms, but it is the intention to develop the algorithms and then implement them on the FPGAs available on the GRT4 cards for online analysis.