Cost function guided 3order B0 shimming for MR spectroscopic imaging at 7T

Introduction At high field strengths, shimming becomes increasingly more difficult and important since susceptibility induced B0 inhomogeneities increase. Exploiting up to 3 rd order shim coils on the system, a high number of degrees of freedom are available to generate a more homogeneous B0 field. In MR spectroscopy excellent shimming is an important factor for acquiring high quality data since it dictates not only spectral line width, but in many MRS methods also the quality of the spectral baseline. Especially for non-localized chemical shift imaging (CSI) where the signal is coming from a complete slice instead of a small volume, the method and quality of the shimming procedure determines the region where good spectra can be obtained. Using up to 3 order shim coils, an improved B0 homogeneity can be achieved in a small region of interest (ROI), generally at the expense of uncontrollable frequency offsets outside this ROI. Traditionally, high bandwidth suppression bands are put over these badly shimmed regions. However, at 7T, the limited B1 field restricts the effective bandwidth of these pulses, and the number of these pulses is limited due to SAR restrictions. When suppression is not used (or fails) outside the shimmed ROI, water and lipid resonances can cause baseline distortions over the whole slice, a phenomenon known as signal bleeding. In particular, if frequency offsets of more than approx. 0.6 ppm (180 Hz at 7T) arise, these artifacts appear in the region where most resonances of metabolites are located (4.1–1.9 ppm). In this work a novel shimming method is presented which uses a cost function for finding optimal shim fields that minimize B0 inhomogeneities in a ROI while confining the frequency offsets in regions outside this ROI. Experiments show that by employing the cost function guided shimming approach, high quality CSI data can be obtained without outer volume suppression (OVS) or conventional volume selection.