Alternative methods for attenuation correction for PET images in MR-PET scanners

This paper describes and compares procedures to obtain attenuation maps used for the absorption correction (AC) of PET brain scans if a transmission scan is not available as in the case of future MR-PET scanners. A previously reported approach called MBA (MRT-based attenuation correction) used Tl- weighted MR images which were segmented into four tissue types representing brain tissue, bone, other tissue and sinus to which appropriate attenuation coefficients were assigned. In this work a template-based attenuation correction (TBA) is presented which applies an attenuation template to single subjects. A common attenuation template was created from transmission scans of 10 normal volunteers and spatially normalized to the SPM2 standard brain shape. For each subject the Tl-MR template of SPM2 was warped onto the subject´s individual MR image. The resulting warping matrix was applied to the common attenuation template so that an attenuation map matching the subject´s brain shape was obtained. The attenuation maps of MBA and TBA were forward projected into attenuation factors which were alternatively used for AC. FDG scans of four subjects were reconstructed after AC with MBA and TBA and compared to images whose ACs were based on conventional attenuation maps (PBA=PET-based attenuation correction). Using PBA as reference in a region of interest analysis, MBA and TBA showed similar under- and overestimation of the reconstructed radioactivity up to -10% and 9%, respectively. The procedure to obtain the attenuation template needs still some improvements. Nevertheless, the TBA method of attenuation correction is a promising alternative to MBA with its still complex and not yet resolved accurate segmentation of MR images.