Respiratory motion compensation in simultaneous PET/MR using a maximum a posteriori approach

The quantitative reliability of pulmonary PET scans is compromised by respiratory motion. The emergence of simultaneous, whole-body PET/MR imaging enables us to correct for motion artifacts in PET using motion information derived from anatomical MR images. We present here a framework for respiratory motion-compensated PET image reconstruction using simultaneous PET/MR. We have developed a radial FLASH pulse sequence for generating gated volumetric MR images at a reasonable speed without significantly sacrificing image quality. A navigator encapsulated within the pulse sequence enables us to retrospectively compute time bins corresponding to each gate. The deformation fields for each gate with respect to a reference gate are computed from the gated MR images by means of non-rigid registration. The gated MR images are also used to generate individual attenuation maps for each gate. Finally motion-compensated PET reconstruction is performed using a maximum a posteriori (MAP) approach. The complete framework was applied to a clinical study conducted on the Biograph mMR scanner (Siemens Medical Solutions), which allows simultaneous acquisition of whole-body PET/MR data. This study demonstrates the utility of our framework in generating meaningful estimates of deformation fields and correcting for motion artifacts in PET.