Banoqitah, EMO; (2014) Enhancing the Performance of the UCL Pixellated HPGe Compton Camera using Split Events for Nuclear Medicine Studies. Doctoral thesis , UCL (University College London).

Abstract

A pixellated High Purity Germanium Compton camera has been developed at the University College London (UCL) for nuclear medicine studies using two planar detectors which both have 4×4mm2 position sensitivity for each pixel. The potential benefits of this imaging system lie with a wider field of view, 3D image reconstruction without tomography, and portability. This thesis presents the use of split (charge sharing) events, which occur at the interpixel area between adjacent pixels, to enhance the spatial resolution from 4mm to 0.5mm for lateral positioning. In addition, this method increases the system efficiency, resulting in more photons in the reconstructed image for a given integration time. The Compton camera has a unique planar orientation of 152 scatter pixels at the front and 25 absorber pixels at the back. The two detectors are separated by 9.6cm apart in the same vacuum housing. The camera was built by ORTEC and (Gamma Ray Tracking 4 channel) GRT4 cards were employed to control the data acquisition. The spatial resolution improvement was evaluated theoretically and experimentally using simulated and experimental reconstructed images. The camera’s overall spatial resolution was estimated theoretically to be 5mm intrinsic and 1.8mm after using split events for a 662keV gamma ray placed at 2.5cm from scatter detector. The equivalent the experimental result showed a FWHM of 2.2mm. The camera was also evaluated using different isotopes and phantoms from points, line and simulated clinical anthropomorphic distributed sources. In conclusion, according to the mathematical calculation, the geometrical spatial resolution profoundly affects the overall spatial resolution. However, utilising split events has enhanced the spatial resolution to submillimetre and increased the efficiency by 30%. Including split events as a high spatial resolution knowledge of source positioning has improved reconstructed images. The current system limitations include offline data processing, slow data transfer rate and limited readout electronics.

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