A correction to the bandwidth difference method for assessment of magnetic resonance static field homogeneity

Published guidance offers four methods of static field inhomogeneity $\left(\Delta B_0\right)$ measurement, including a bandwidth difference method. However, an error exists in the implementation of this method, which we demonstrate analytically and experimentally.

To propose and validate a correction to the bandwidth difference method for quantifying $\Delta B_0$ .

We outline a correction in the method for quantifying local $\Delta B_0$ by measuring spatial translations rather than diameter measurements between MRI images with different receive bandwidths. To validate the approach, we acquired images of a cylindrical phantom containing a fixed array of fluid-filled pins. We adjusted static field shims to create approximately linear and quadratic field perturbations and acquired high- and low-bandwidth spoiled gradient echo sequences with anterior-posterior (AP) and left-right (LR) frequency-encoding. We then measured translations of centroid pin locations between low- and high-bandwidth images. Finally, we propose and validate an implementation strategy using a spherical phantom. In both cases, a vendor-provided field mapping sequence provided reference homogeneity measurements.

Under manually adjusted shim settings, root-mean-square (RMS) and peak-to-peak (PP) $\Delta B_0$ were 2.46 and 6.29 parts per million (PPM) respectively, measured with the field mapping technique. Results of the proposed bandwidth difference method showed strong agreement; RMS and PP $\Delta B_0$ were 2.38 and 6.22 PPM respectively for AP frequency-encoding and 2.38 and 6.08 PPM respectively for LR frequency-encoding. In a spherical phantom, the proposed method agreed with reference measurements to within 0.25 PPM (< 7%). However, PP $\Delta B_0$ was underestimated using the diameter-based method-in a spherical phantom, measured $\Delta B_0$ was 0.74 PPM compared to 3.52 and 3.76 PPM using the reference and revised methods, respectively.

This work experimentally validates a revised bandwidth difference approach for $\Delta B_0$ quantification, showing that it remains sensitive under spatially nonlinear $\Delta B_0$ ; the diameter-based method is shown to be inaccurate for such cases.