A robust coregistration method for in vivo studies using a first generation simultaneous PET/MR scanner.

PURPOSE Hybrid positron emission tomography (PET)/magnetic resonance (MR) imaging systems have recently been built that allow functional and anatomical information obtained from PET and MR to be acquired simultaneously. The authors have developed a robust coregistration scheme for a first generation small animal PET/MR imaging system and illustrated the potential of this system to study intratumoral heterogeneity in a mouse model. METHODS An alignment strategy to fuse simultaneously acquired PET and MR data, using the MR imaging gradient coordinate system as the reference basis, was developed. The fidelity of the alignment was evaluated over multiple study sessions. In order to explore its robustness in vivo, the alignment strategy was applied to explore the heterogeneity of glucose metabolism in a xenograft tumor model, using 18F-FDG-PET to guide the acquisition of localized 1H MR spectra within a single imaging session. RESULTS The alignment method consistently fused the PET/MR data sets with subvoxel accuracy (registration error mean = 0.55 voxels, < 0.28 mm); this was independent of location within the field of view. When the system was used to study intratumoral heterogeneity within xenograft tumors, a correlation of high 18F-FDG-PET signal with high choline/creatine ratio was observed. CONCLUSIONS The authors present an implementation of an efficient and robust coregistration scheme for multimodal noninvasive imaging using PET and MR. This setup allows time-sensitive, multimodal studies of physiology to be conducted in an efficient manner.