Investigating respiratory motion induced changes on EM fields and SAR in UHF body MRI

The majority of magnetic resonance (MR) scanners in clinical MR imaging operate at a field strength of 1.5T and 3T. In research, however, so-called ultra-high field (UHF) MR scanners operating at field strengths of ≥7T are increasingly being used to achieve higher resolutions and stronger image contrasts. UHF MRI has successfully been applied to several applications targeting the human head, but its success is strongly damped for body targets due to various challenges.An essential part of the MR imaging process consists in the coherent excitation of the spins within the target volume by a time-dependent radiofrequency (RF) field that is generated by the MR coil. An increase of the main field strength, however, results in increasing spatial variations of the intensity of this RF field, which causes spatial signal and contrast modulations and, ultimately, generates images of a non-diagnostic quality. At the same time, the increased spatial variations of the RF fields lead to localized areas with high specific absorption rate (SAR) that result in localized tissue heating. To address such problems, a technique termed ‘parallel transmission’ (pTX) that makes use of an RF coil consisting of multiple RF coil elements has successfully been applied. Here, the elements are driven simultaneously but each element is driven by an independent RF waveform such that the superposing RF field generates the desired spatial signal intensity while reducing the SAR.Recently, several independent reports have demonstrated that the shape of such RF fields also strongly varies throughout the respiratory cycle. Thus, pTX excitations with a homogeneous signal during the exhale phase can lead to images during the inhale phase with local signal dropouts. This effect in particular affects UHF acquisitions performed during free-breathing, which presently is an active research field.The present work is divided into two sub-projects. The first sub-project systematically investigates the impact of respiration on the RF fields in simulations by using virtual body models that contain respiration-dependent deformations. The strength of the respiration-induced field variations will be analyzed for different physiological parameters (type of respiration, gender, body size) and technical parameters (type of RF coil, field strength). Simulations will be verified in phantoms and in-vivo at 3T and 7T. In the second sub-project novel pTX techniques will be developed that generate homogeneous and respiration-independent signal as well as low SAR throughout the entire respiratory cycle. Such pulses will be tested in phantoms and in-vivo at 7T and 10.5T. The latter scans will be performed in collaboration with researchers from the University of Minnesota, USA, that hosts a whole-body MRI system with presently the highest field strength world-wide. For the first time, we will acquire highly resolved 3D datasets of the heart using above-mentioned pTX techniques under free-breathing.