I. Introduction
From the very beginning of positron emission tomography (PET) accurate correction for tissue attenuation is an essential prerequisite for quantitative PET imaging in vivo [1]. In stand-alone PET scanners this attenuation correction was achieved by directly measuring the attenuation with blank and transmission scans using positron emitting sources which was regarded to be the gold standard because spatial resolution and energy matched the corresponding emission data. Since the introduction of PET/CT tissue-based attenuation has been calculated by use of CT-derived data [2]. To date, transformation of the resulting Hounsfield units into -maps for attenuation correction is widely accepted. However, in PET/MR hybrid scanners none of these methods are applicable any longer which presents a major challenge for absolute PET signal quantification.