A robust automated shimming procedure for breast MR spectroscopy

Introduction Proton magnetic resonance spectroscopy (H-MRS) offers attractive possibilities for the non-invasive in vivo assessment of breast lesion biochemistry, as an aid for diagnosis [1] and an early indicator of disease response to treatment [2-4]. Effective shimming to achieve a high magnetic field homogeniety over the volume of interest (VOI) is paramount for MRS to achieve sufficient spectral resolution and avoid positional errors. This is often challenging in vivo, but particularly so in the breast where the off-center position of the VOI leads to an increased breakdown of the orthogonality of the shim gradients assumed when describing the spatial variation of shim fields with spherical harmonics. Furthermore, breast tumours are often marked with a radiographic tissue marker or “clip” consisting of a small metallic wire bent into a particular shape visible on X-ray mammography. This produces strong field distortions in the immediate vicinity of the marker. Strategies for shimming the breast have been described and investigated by MARIL et al. [5], combining shimming and spatial saturation pulses. However, the authors do not develop strategies for the calculation of the shim currents. To our knowledge, no adequate automated shimming procedure for breast MRS has been described so far. In this study, the regularized automated procedure described by KIM et al. [6] for shimming of the brain has been implemented and adapted, and is tested under breast MRS conditions. Results were compared to the manual adjustment of the linear X, Y and Z shims and to the manufacturer’s automated shimming procedure which makes use of spherical harmonics to model the effect of the shim gradients.