Effect of respiration on the B0 field in the human spinal cord at 3T

Respiration affects the bulk magnetic susceptibility, causing frequency shifts as shown in the brain at 7T. Due to the close proximity of the spine to the lungs, this effect is expected to be even larger in the spinal cord, resulting in detrimental B0 offset. The goal of this study was to quantify the effect of respiration on B0 variation in the spinal cord.

Seven healthy subjects were scanned at 3T. Field maps were acquired during inspired and expired conditions. Frequency shift was quantified in the brain, brainstem, and cervico-thoracic spinal cord. A skewed Gaussian function with linear term was fitted to the frequency shift as a function of z-location along the spine.

Large frequency shifts were measured along the cord, with a maximum of 74 Hz at C7 (P < 0.05), corresponding to 0.58 ppm. The proposed model was adequately fitted to the respiratory-induced frequency-shifts (adjusted R(2)  = 0.9954). The morphology of subjects (weight and height) seemed to have an impact on the amplitude of frequency shift, although correlations were not significant.

This study provides a deeper understanding of the contribution of respiration to B0 shift in the spinal cord. The proposed model can be useful for designing future hardware or software strategies to compensate for these B0 variations dynamically.