Demonstration and suppression of respiration-related artifacts in Bloch-Siegert shift-based B1+ maps of the human brain

Abstract

Respiration-induced movement of the chest wall and internal organs causes temporal B₀ 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 B₀ 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.

Keywords: B1 mapping; Bloch-Siegert shift; navigator echo; physiological artifacts; respiration artifacts.