Measurement and correction of respiration-induced B₀ variations in breast ¹H MRS at 4 tesla

Respiratory motion is well known to cause artifacts in magnetic resonance spectroscopy (MRS). In MRS of the breast, the dominant artifact is not due to motion of the breast itself, but rather it is produced by B₀ field distortions associated with respiratory motion of tissues in the chest and abdomen. This susceptibility artifact has been reported to occur in the brain, but it is more apparent in the breast due to the anatomic proximity of the lungs. In the breast, these B₀ distortions cause shot-to-shot frequency shifts, which vary an average of 24 Hz during a typical ¹H MRS scan at 4 T. This variation can be corrected retrospectively by frequency shifting individual spectra prior to averaging. If not corrected, these shifts reduce spectral resolution and increase peak fitting errors. This work demonstrates the artifact, describes a method for correcting it, and evaluates its impact on quantitative spectroscopy. When the artifact is not corrected, quantification errors increase by an average of 28%, which dramatically impacts the ability to measure metabolite resonances at low signal-to-noise ratios.