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 B 1 + maps obtained at 7 T with the Bloch–Siegert shift (BSS) B 1 + 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, B 1 + 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 B 1 + map. Due to the periodicity of respiration‐related phase shifts, their effect on the reference B 1 + map was assumed to be negligible through averaging. The individual B 1 + maps of the 16 repetitions were calculated with and without using the proposed navigator scheme. These maps were compared with the B 1 + 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 B 1 + 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 B 1 + mapping at high magnetic fields.