MR-based Attenuation Correction for PET/MR

There has recently been a growing interest in combining PET and MR. While promising hardware prototypes have been presented [1], the software for combined imaging is in its infancy. PET attenuation correction (AC), which accounts for radiation attenuation properties of the tissue, is mandatory for quantitative PET. Usually the attenuation map in stand-alone PET or PET/CT is obtained from a transmission scan or from a CT image. In the case of PET/MR, there is insufficient space for the rotating source so that the tissue attenuation information needs to be determined from the MR image. This is inherently difficult: The PET attenuation value is not related to the proton density and tissue relaxation properties expressed in the MR intensities. For example bone compacta and air both typically do not contribute a MR signal, whereas their attenuation values are maximally distinct. Here, we report our progress on evaluating suitable MR sequences and methodology for calculating the attenuation images (“Pseudo-CT”) based on the MR image. We present results for head and whole body imaging. To our knowledge, our contribution is the first to show the use of ultra short echo time (UTE) sequences for AC in clinical examples. It is also among the first to show results for human whole body AC.