Reconstruction of UCL Germanium Compton Camera Data Using ITEM

Various Cs-137 source distributions were imaged with the UCL (University College London) Germanium Compton camera. A reduced camera setup was used consisting of 16 pixels of 4 times 4 times 4 mm³ size each in the front detector and 4 pixels of 4 times 4 times 10 mm³ size each in the back detector. The preamplifier signal was digitised by GRT4 VME readout cards. The deposited energy was extracted online using the moving window deconvolution technique implemented in the FPGAs of the GRT4 cards. A pulse height determination algorithm leading to worse energy resolution was applied offline for comparative studies. For the image reconstruction ITEM (Imaginary Time Expectation Maximization) was used, an iterative algorithm based on quantum mechanics energy minimization. ITEM is very flexible and can easily be applied to every possible Compton camera geometry. An improved backprojection taking into account the changing camera sensitivity in the image space has been used and its limitations investigated. ITEM is shown to work very fast. An image with 10000 pixels can be reconstructed from 40000 events in about 20 s using one 3 GHz Pentium CPU. Data was taken with point, line and circular sources in a plane 2 cm from the front detector. Through analysis of point and line spread functions the angular resolution of the camera was estimated to be around 10 deg.