Respiration-Induced B0 Fluctuations in the Human Brain at 7 Tesla and its Spatial Distribution

In functional magnetic resonance imaging, it is known that physiological influences such as cardiac pulsation, respiration or brain motion can induce signal intensity and phase fluctuations. Some of the mechanisms potentially involved in those phenomena are expected to be amplified at higher magnetic fields. This study specifically addresses the issue of B0 fluctuations induced by susceptibility changes during respiration attributed to movements of chest and diaphragm and variations in the oxygen concentration. It is demonstrated that respiration induced resonance offsets (RIRO) at 7 T are significant and were found in data acquired with a RF pulse without gradients, multi-slice echo-planar imaging (EPI), and dynamic 3D-FLASH imaging. The experimental findings can be summarized with three main observations. First, in FIDs measured after a single RF pulse, a RIRO with large amplitude was consistently detected although the average amplitude varied from subject to subject, ranging from 1.45Hz to 4Hz. Second, in transverse EPI images obtained in the occipital lobe, the RIRO amplitude showed a monotonic increase along Z axis toward the lungs. Third, a more detailed spatial analysis with 3D FLASH phase maps revealed that a previously published analytical model can describe the spatial distribution of RIRO accurately. Subsequent apparent motions in the EPI series as well as implication of slice orientation on correction strategies are discussed