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 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. K E Y W O R D SB1 mapping, Bloch-Siegert shift, navigator echo, physiological artifacts, respiration artifacts | INTRODUCTIONHuman tissues are predominantly diamagnetic, whereas the ambient air and air cavities in the body are paramagnetic due to oxygen molecules. This leads to a nonuniform bulk susceptibility distribution within the body and its surroundings. This distribution varies during respiration due to factors such as the movement of the chest wall and internal organs, and variations in volume and oxygen concentration of the air inside the chest cavity. During an MRI scan, these respiration-related susceptibility variations give rise to temporal B 0 variations extending throughout the brain. 1, 2 These temporal variations have weak spatial dependence in a transverse slice in the brain with their amplitude inversely proportional to the distance of the slice from the chest cavity. 3 They scale linearly with the field strength and can cause artifacts such as ghosting, image shifts, signal drop, and blurring depending on the MRI pulse sequence. These artifacts have been thoroughly investigated in functional MRI studies since