Image reconstruction for AX-PET. Different approaches to histograming for semi-continuous data

AX-PET is a unique PET device based on axially oriented crystals and orthogonally placed Wavelength shifter strips, both individually readout by Geiger-mode Avalanche Photo Diodes (G-APD, also known as SiPM). The AX-PET design was conceived in order to simultaneously improve spatial resolution and sensitivity by reducing the parallax error. The peculiar design of AX-PET yields discrete transaxial and depth (x and y) coordinates, while the axial coordinate is continuous. In order not to lose resolution, various approaches to treat the axial coordinate were explored to deal with the semi-continuous coordinates. A data set suitable for this task was processed and reconstructed by using different techniques, and the results are presented. This new approach better model the z pdf but, depending on the used technique to estimate the weight, might introduce some blurring in the final image. (orig.)