An Experimental & Computational Evaluation of the MiniPix(TPX3) Detector for Compton Imaging in Nuclear Medicine

This thesis investigates the feasibility of using the MiniPix(TPX3) (CdTe, 1 mm) detector as a Compton camera for nuclear medicine imaging using experimental and computational methods. Conventional gamma cameras rely on collimators to determine photon positions, attenuating over 99% of incident photons and necessitating higher administered activities. A Compton camera design, eliminating the need for collimators, could significantly reduce patient dose and improve imaging efficiency. The Timepix3 chip, central to this study, is a high-resolution pixel read-out device that can pair with semiconductor materials such as CdTe. This research focused on characterising the MiniPix(TPX3) detector. This was achieved by assessing the energy resolution, detection efficiency, and spatial resolution through the modulation transfer function (MTF). A novel method in the literature was used for determining the vertical position of Compton-scattered electrons and absorbed photons within the detector, an essential step toward realising a functional Compton camera. Monte Carlo simulations using the EGSnrc software were employed to validate the experimental results and simulate scenarios involving point sources, extended sources, and phantoms of various sizes and configurations. Initial characterisation revealed excellent spectral performance for the MiniPix(TPX3), achieving an energy resolution and detection efficiency for 99mTc (140 keV) of 9.5% and 15.8%, respectively, with simulation results matching experimental findings within a 2% error margin. X-ray imaging showed that the MiniPix(TPX3)’s MTF performance approached 30% of a state-of-the-art mammography system, with simulations indicating possible improvements through subpixel event localisation. Tests with single and multiple radionuclide point sources (99mTc and 57Co) demonstrated the detector’s capability as a Compton camera and its wide field of view. Reconstruction using algorithms such as list-mode maximum likelihood expectation maximisation (LM-MLEM) improved spatial resolution to approximately 8 mm for a 99mTc point source, with experimental data aligning with computational reconstructions within a 6% margin of error. A study comparing nuclear medicine imaging of thyroid phantoms using the MiniPix (TPX3) with current gamma cameras highlighted both the detector’s strengths and its limitations. Although the spatial resolution of planar Compton imaging was limited even with advanced algorithms, sensitivity gains of 10-25 times compared to clinical gamma cameras were observed, demonstrating its potential applicability in nuclear medicine imaging. This research addresses a significant gap in the literature regarding experimental Compton camera imaging with a single-layer MiniPix(TPX3) detector (CdTe, 1 mm) for clinically relevant radionuclides. While silicon-based TPX3 detectors dominate the field due to their superior energy resolution, CdTe offers a crucial advantage in detection efficiency, vital for nuclear medicine. However, challenges remain, particularly in imaging radionuclides below 200 keV with a single-layer detector due to Doppler broadening and z-axis localisation constraints. Although the MiniPix(TPX3) detector shows promise, its 2-D design limits depth determination, and its small area restricts imaging of larger objects. Future developments, such as incorporating 3-D depth determination and advanced reconstruction algorithms, are necessary to enhance its clinical utility.