A Synergistic Passive and Active Shimming System to Optimize B0 Field Homogeneity in Micro MR Spectroscopy
Open Access
- Author:
- Dewal, Rahul Pradeep
- Graduate Program:
- Bioengineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- March 15, 2016
- Committee Members:
- Qing X Yang, Dissertation Advisor/Co-Advisor
- Keywords:
- passive shimming
magnetic susceptibility
B0 correction
MR artifact - Abstract:
- This research presents a targeted, synergistically combined passive and active shimming system to correct for susceptibility-induced B0 inhomogeneity artifacts in magnetic resonance (MR) imaging and spectroscopy. This system would be of great significance to researchers using MR imaging at ultra-high fields (7–14 T), where B0 inhomogeneity artifacts are a major limiting factor for many in vivo applications. At boundaries between materials of differing magnetic susceptibility, such as the air-tissue boundaries in the sinus cavities or ear canals, the differing susceptibilities of the imaged tissues can distort the applied magnetic field. These inhomogeneities can lead to local distortion and signal loss artifacts in MR images and line broadening in MR spectra. Active shimming, involving the adjustment of magnetic shim coils, is used to correct first-order inhomogeneities, but higher-order active shimming can be difficult and expensive. Instead, passive shimming may be used to correct such higher-order inhomogeneities. Passive shimming involves placing pieces of material that have high magnetic susceptibility values compared to body tissues in the vicinity of said tissues in order to perturb the magnetic field such that the inhomogeneities are corrected. Previous experiments have been performed with several passive shimming methodologies that show promising results. However, they have various drawbacks, with several lacking precision shim placement and another requiring excessive manual measurements for best results. This research pursues precision correction of inhomogeneities via two specific aims: Specific Aim 1: Develop numerical synergistic shim simulation and optimization software, which simultaneously optimizes the linear active shim gradient settings and passive shim element configuration for B0 inhomogeneity correction of a given volume of interest (VOI). Specific Aim 2: Develop passive shim frame hardware that implements the synergistic shimming solution. The technique was validated in a 7T Bruker MRI system in a customized susceptibility phantom and in in vivo mice by using it to improve the B0 magnetic field homogeneity and spectral water line widths from predetermined regions of interest within the mouse brain. These pilot data demonstrate the feasibility of using such a system to improve MR spectroscopy data quality. Future work could extend the technique to human applications, which would be of great benefit to human neurological studies of regions near the sinuses.