Demonstration and suppression of respiration-related artifacts in Bloch-Siegert shift-based B-1(+) maps of the human brain

Respiration-induced movement of the chest wall and internal organs causes temporal B-0 variations extending throughout the brain. This study demonstrates that these variations can cause significant artifacts in B1+ maps obtained at 7 T with the Bloch-Siegert shift (BSS) B1+ mapping technique. To suppress these artifacts, a navigator correction scheme was proposed. Two sets of experiments were performed. In the first set of experiments, phase shifts induced by respiration-related B-0 variations were assessed for five subjects at 7 T by using a gradient echo (GRE) sequence without phase-encoding. In the second set of experiments, B1+ maps were acquired using a GRE-based BSS pulse sequence with navigator echoes. For this set, the measurements were consecutively repeated 16 times for the same imaging slice. These measurements were averaged to obtain the reference B1+ map. Due to the periodicity of respiration-related phase shifts, their effect on the reference B1+ map was assumed to be negligible through averaging. The individual B1+ maps of the 16 repetitions were calculated with and without using the proposed navigator scheme. These maps were compared with the B1+ reference map. The peak-to-peak value of respiration-related phase shifts varied between subjects. Without navigator correction, the interquartile range of percentage error in B1+ varied between 4.0% and 8.3% among subjects. When the proposed navigator scheme was used, these numbers were reduced to 2.5% and 2.9%, indicating an improvement in the precision of GRE-based BSS B1+ mapping at high magnetic fields.