Author: Christina Brunnquell, Daniel Vergara, Joseph Everett Wishart 👨🔬
Affiliation: University of Minnesota, University of Washington 🌍
Purpose: To propose and validate a correction to the bandwidth difference method presented in Task Group Report No. 325 for quantifying static magnetic field homogeneity (ΔB0).
Methods: A clarification of the mathematics for quantifying local ΔB0 using pixel shifts rather than diameter measurements between MRI images with different bandwidths is outlined. For validation, a 3.0 T scanner (MR 7700; Philips, Best, The Netherlands) was used to acquire magnitude and phase images of a 40 cm cylindrical phantom in a homogeneous plane and through a fixed array of fluid-filled pins. Manual shim settings were applied to simulate linear and quadratic field perturbations across anterior-posterior (AP) and left-right (LR) dimensions. High- and low-bandwidth spoiled gradient echo sequences were acquired and repeated for both AP and LR frequency-encoding. Pixel shifts of centroid pin locations between low- and high-bandwidth images were measured using Matlab (Natick, MA). A vendor-generated field mapping sequence provided reference standard homogeneity measurements at each pin location.
Results: Comparing measured phantom diameters is not always an accurate method to quantify ΔB0. Even- and odd-valued terms of field inhomogeneity about the isocenter produce translation and distortion of imaged objects along the read-direction, respectively. Field perturbations following even-valued functions may occur more frequently in non-spherical phantoms. The technique recommended in TG 325 is insensitive to translation effects. Measurements of ΔB0 under exaggerated shim settings using the revised approach agree with field mapping techniques; differences in peak-to-peak and root-mean-square ΔB0 were 0.070 (6.29 vs. 6.22 PPM) and 0.074 (2.46 vs. 2.38 PPM) PPM respectively.
Conclusion: The proposed clarification for ΔB0 quantification has been experimentally validated and is robust under nonlinear ΔB0; current methods are shown to be inaccurate for such cases. This is an important consideration for clinical physicists, as homogeneity testing is required by accreditation and regulatory bodies for routine MRI evaluation.