MR-based attenuation correction for a whole-body sequential PET/MR system

PET/MR has started gaining more interest recently because of its obvious advantages over PET/CT: firstly MRI offers much better soft-tissue contrast; secondly it does not cause patients' exposure to extra radiation dose. However, it is technically very challenging to bring an MRI system together with a PET system. Interference between the two systems has to be removed. Additionally, MR image intensity is a function of proton density and other MR tissue properties, including T1 and T2 relaxation rates, which is not directly related to the attenuation of the photons generated in PET. Thus, deriving the photon attenuation map using MRI is not as straight-forward as the CT-based attenuation correction (CT-AC) which directly measures the attenuation property of the object.