Attenuation correction for PET/MR: Problems, novel approaches and practical solutions

Attenuation correction in PET is the primary prerequisite for quantification of the radiotracer's signal. Absolute quantification is the key to improve diagnostic performance, to enable comparisons between follow-up examinations and to perform pharmacokinetic modeling. A large fraction of the 511 keV annihilation photons from the positron emitters are scattered by the patient's body. Thus, they are discarded or do not even reach the PET detectors, while others are identified at the wrong location after being scattered. To account for these effects and thus generate quantitative PET images showing the actual activity distribution, it is necessary to determine an attenuation map with the appropriate attenuation coefficients for 511 keV photons at each voxel. In hybrid PET/CT systems, this is achieved using the information about the tissue electron density provided by the CT and adjusting it for the difference in photon energy. In PET/MR systems, there is no mechanism to directly measure the attenuation coefficients of the tissue. Determining the attenuation map in PET/MR is an important challenge involving two problems: the determination of the patient's attenuation map and the determination of the attenuation introduced by additional hardware components. We describe the approaches investigated to deal with these problems and, based on the experience with a fully integrated PET/MR system, we finally discuss potential solutions and limitations in a close to routine setting.