In vivo B 0 field shimming methods for MRI at 7 T

Abstract

Functional MRI (fMRI) at 7 T and above provides improved Signal-to-Noise Ratio and Contrast-to-Noise Ratio compared to 3 T acquisitions. In addition to the beneficial effects on spin polarization and magnetization of deoxyhemoglobin, the increased applied field also further magnetizes air and tissue. While the magnets themselves typically provide a static B0 field with sufficient spatial homogeneity, the diamagnetism of tissue and the paramagnetism of air causes local field deviations inside the human head. These spatially-varying field offsets (ΔB0) cause image artifacts, especially in single shot EPI, including geometric distortion, signal dropout, and blurring. These effects are particularly strong near air-tissue interfaces such as the frontal sinus, and ear canals. Furthermore, if the field offsets are dynamically modulated through physiological processes such as respiration or motion, then the effect on the image time-series can be even more problematic. While post-processing methods have been developed to mitigate these effects, the ideal solution would be to reduce the ΔB0 variations at their source. Typically 7 T scanners contain 2nd and some 3rd order spherical harmonic shim coil terms to cancel static ΔB0 variations of low spatial order. In this article, we will motivate the need for improved, higher-order compensation for B0 inhomogeneity and potentially add dynamic control of these fields. We discuss and compare several promising hardware approaches for static and dynamic B0 shimming using either higher-order spherical harmonic shim coils or multi-coil shim arrays as well as passive shimming approaches, and active variants such and adaptive current networks.

Dynamic B0 shimming of the human brain at 9.4 T with a 16‐channel multi‐coil shim setup

“…In addition, a small number of large coils requires a more sophisticated amplifier design, because gradient switching induces in this case higher voltages that need to be counteracted. However, an increase of the coil count does not linearly improve shimming performance (e.g., increasing the number of coils from 8 to 32 results in an improvement of only 5% for global and 23% for dynamic shimming) . The number and size of the coils as well as the number of wire turns used for this setup reduced the average required current to 1.5 A.…”

Methodological consensus on clinical proton MRS of the brain: Review and recommendations

“…Figure illustrates how poor B 0 homogeneity results in unusable spectra in the anterior cingulate cortex when using 3D EPSI, therefore restricting the use of MRS in this region to 2D MRSI or SVS techniques. Homogeneous B 0 across the whole brain is also desirable for gradient‐echo EPI, and particularly challenging at ultrahigh field (7 T and above), resulting in the development of a number of novel shimming approaches . Although these methods show great potential for improving MRS, at both low and high field, it is currently unclear which will provide the best balance among patient comfort, cost, and efficacy.…”

Rapid acquisition of the 3D MRI gradient impulse response function using a simple phantom measurement

“…In addition to its high sensitivity, the phantom-based method offers the option of expanding the signal into larger sets of spatial basis functions (e.g., 4th and partially 5th order spherical harmonics) that may be useful for characterizing the dynamic field responses generated by dynamic high order shim systems. 26,27 In principle, the sequence applied here with ~40 usable voxels delivers enough information for expansion into more than the 16 components of 3rd order spherical harmonics and could be further extended to use more slices and smaller voxels for finer spatial resolution, however, at the price of a longer scan time. As discussed above, the flexibility in changing size, number, and position of voxels can be used to optimize noise propagation and minimize noise in the harmonic order of interest.…”