Rebinning and reconstruction of point source transmission data for positron emission tomography

Transmission scans using a single-photon point source on a positron emission tomography (PET) scanner are inherently acquired in a cone-beam geometry. Traditionally, these data are binned into radially parallel oblique projections and then rebinned using single-slice rebinning (SSRB) into a set of two-dimensional parallel sinograms. For axially oblique lines of response (LORs), the SSRB approximation results in axial and tangential blurring of the transmission image, especially at large radial distances. For the animal PET scanner recently completed at the University of Pennsylvania, there is interest in placing the transmission source outside the scanner in order to maximize the transverse field of view (FOV). For this extremely asymmetric configuration, however, the LORs are very oblique (axial angle=19-42 deg). We compared three methods of processing transmission data, SSRB or FORE followed by two-dimensional (2D) reconstruction and an iterative algorithm for transmission data that uses the cone-beam LORs directly (TR-OSEM-CB). All three algorithms produce acceptable transmission images when the source is inside the scanner. For the case when the transmission source is outside the scanner, TR-OSEM-CB produces images with only minor artifacts near the ends of the axial FOV. SSRB and FORE images have significant axial blurring, although the central region of the FORE image is artifact-free and potentially useful for generating attenuation correction factors. Moving the source closer to the detector ring reduces the residual artifacts seen with TR-OSEM-CB.