UCLA

Positron emission tomography (PET) is a noninvasive imaging technique based on the detection of annihilation gammas that provides the spatial and temporal concentration of radiolabeled probes in the scanned subject. Following the interaction of a positron and an electron, a pair of gammas is emitted in opposite directions. By detecting this gamma pair and tracking back its location of origin, the radioisotope distribution can be reconstructed. The main two components of PET detectors are the scintillator crystal and the photosensor. The scintillator is responsible for stopping the annihilation gammas while the photosensor detects visible light photons generated by the scintillator as a product of a gamma interaction. A few of the many challenges on the process of determining the origin of the annihilation gammas will be addressed in this manuscript.

One issue in PET detection comes from lost scintillation light that is unable to exit the scintillator crystals. With this in mind a number of crystal geometries and surface treatment configurations were compared in terms of the fraction of scintillation light that exits the scintillator. Another problem rises from the position assigned to the gamma interaction within the crystal. In traditional detector systems every gamma-crystal interaction is assigned to a specific location in the crystal, usually either the average depth or the front face of the crystal, regardless of how deep in the crystal the interaction occurred. This method can lead to mispositioning of the origin of the annihilation gammas. With the purpose of obtaining more information on the gamma interaction location within the crystal, a slanted crystal geometry was evaluated in terms of depth of interaction resolution. The third issue addressed in this work refers to the identification of crystals at the edge of the crystal/detector array. Light exiting the edge crystals cannot be spread in all directions because there are no detector pixels in all directions at the edge of the array. This condition compresses the light information from the edge crystals making it very difficult to distinguish an edge crystal from its neighbors. With the goal of reducing the edge effect while balancing the cost of our detectors, pixel binning configurations were evaluated in terms of crystal resolvability.

This work explores alternatives to improve the accuracy of the spatial information at the detector level.