Attenuation correction of human soft tissue is necessary to correct for the individual patient anatomy. Since no linear attenuation coefficient-based CT information is available in integrated PET/MRI, tissue-specific AC has to be based on MR information which is based on proton density and relaxation properties (e.g., T1 and T2 relaxation times), rather than on the attenuation of X-rays in tissue. Both air and solid bones lack signal in MRI; thus these fundamentally different tissue classes are difficult to separate. In the current implementation of integrated PET/MRI systems, tissue attenuation and scatter correction is performed using a three-dimensional (3D) Dixon-based MR imaging technique, providing two sets of images where water and fat are “in phase” and “out of phase” (Fig. 1.5) [8]. This allows reconstruction of fat-only, water-only, and fat-water images and results in tissue segmentation of air, fat, muscle, and lungs in the reconstructed and displayed μ-maps (Fig. 1.5) [8]. The cortical bone is currently not being accounted for in the Dixon-based AC approach. The bone is here classified as soft tissue, and thus the exact magnitude of PET signal attenuation of the bone might be underestimated [9].

Radiofrequency surface receiver coils are a technical precondition for high-resolution MR imaging and are well established in clinical MRI. In a standard setup of integrated PET/MR cardiac imaging, the patient is placed on top of a rigid phased-array spine RF coil. For anterior signal detection, a second multichannel phased-array RF surface coil is placed on the thorax of the patient during simultaneous MR and PET data acquisition (Fig. 1.6). Thus, all RF surface coils used in the PET field of view during simultaneous PET data acquisition have to be optimized for PET transparency, i.e., such coils should attenuate the photons only minimally [15–18].

The PET signal attenuation of rigid and stationary equipment such as the RF spine array and the RF head/neck coil can be compensated for by straightforward AC methods. After scanning this equipment by using CT, a 3D map of attenuation values can be generated. This data can then be converted into a 3D representation of the 511 keV attenuation values, the so-called μ-map. By linking the RF spine or RF head coil’s position to the patient’s table position, the corresponding AC μ-map for each table position is automatically selected by the system for PET image reconstruction [17].

For flexible surface RF coils like the 6-channel RF body phased array (Fig. 1.6) which may be used in the context of cardiac PET/MR, the AC is performed differently. Because the design of such surface RF coils is flexible, the individual position and shape of this RF coil during a patient examination is not known. Thus a pre-acquired rigid 3D CT template cannot directly be co-registered [17]. As alternative strategy for AC of flexible RF coils, visible markers have been proposed to perform an automatic nonrigid co-registration of the pre-acquired 3D CT attenuation template to the individual position and shape of the flexible RF coil during the cardiac PET/MR examination (Fig. 1.6) [17, 19].