UBC Theses and Dissertations
Development of a small animal MR compatible PET insert Stortz, Gregory
Positron Emission Tomography (PET) provides in vivo functional information about a living subject by imaging the distribution of biologically meaningful radiotracers such as 18F-fluoro-deoxy-glucose. PET data is complemented by anatomical information from an imaging modality that produces high tissue contrast such as MRI. The work presented in this thesis is a contribution to a collaboration aimed at creating an MRI compatible high-resolution small animal PET insert for simultaneous PET/MR imaging. The PET system was designed with an outer diameter of 114 mm in order to fit inside of a pre-existing 7T small animal MRI. During the design of the PET system, Monte-Carlo simulations were created to estimate the resolution and count rate performance of various iterations of the design. These simulations showed that the proposed dual-layer detector design would be effective in mitigating off-centre spatial resolution degradation, and produced resolution of ~ 1 mm full-width at half-maximum in the centre of the field of view. The effective count rate of our system was estimated to be low in comparison to other small animal PET systems due to the small solid angle subtended by the detectors. A prototype detector block was built incorporating an array of digital photon counters (DPCs) to test the suitability of DPCs as an alternative to silicon photomultipliers for our small animal PET application. Based on the characterization of energy resolution, timing resolution, and rates of count loss as a function of device settings, the most appropriate combination of device settings for our small animal PET application was identified. Once constructed, the prototype PET system was characterized in terms of spatial resolution and count rate performance. Phantom and rodent images were reconstructed using filtered back projection – 3D reprojection (FBP-3DRP) and a novel point-spread-function modelling maximum likelihood expectation maximization (PSF-MLEM) algorithm. The PSF-MLEM reconstruction algorithm was updated to remove non-uniformity artefacts caused by lack of normalization and systematic inaccuracies present in its original implementation. PSF-MLEM resulted in higher quality PET images than FBP-3DRP, resolving feature sizes of 0.7 mm in a resolution phantom and showing contrast between the cortex and ganglia in rodent brains.
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