MR-based attenuation correction for torso-PET/MR imaging: pitfalls in mapping MR to CT data.

Abstract

MR-based attenuation correction (AC) will become an integral part of combined PET/MR systems. Here, we propose a toolbox to validate MR-AC of clinical PET/MRI data sets. Torso scans of ten patients were acquired on a combined PET/CT and on a 1.5-T MRI system. MR-based attenuation data were derived from the CT following MR-CT image co-registration and subsequent histogram matching. PET images were reconstructed after CT- (PET(CT)) and MR-based AC (PET(MRI)). Lesion-to-background (L/B) ratios were estimated on PET(CT) and PET(MRI). MR-CT histogram matching leads to a mean voxel intensity difference in the CT- and MR-based attenuation images of 12% (max). Mean differences between PET(MRI) and PET(CT) were 19% (max). L/B ratios were similar except for the lung where local misregistration and intensity transformation leads to a biased PET(MRI). Our toolbox can be used to study pitfalls in MR-AC. We found that co-registration accuracy and pixel value transformation determine the accuracy of PET(MRI).