Exploring the limits of RF shimming for high-field MRI of the human head

Abstract

Several methods have been proposed for overcoming the effects of radiofrequency (RF) magnetic field inhomogeneity in high-field MRI. Some of these methods rely at least in part on the ability to independently control magnitude and phase of different drives in either one multielement RF coil or in different RF coils in a transmit array. The adjustment of these drive magnitudes and phases alone to create uniform RF magnetic (B(1)) fields has been called RF shimming, and has certain limits at every frequency as dictated by possible solutions to Maxwell's equations. Here we use numerical calculations to explore the limits of RF shimming in the human head. We found that a 16-element array can effectively shim a single slice at frequencies up to 600 MHz and the whole brain at up to 300 MHz, while an 80-element array can shim the whole brain at up to 600 MHz.

FIG. 1

Shaded-surface rendition of the 3D digital head model and 80-element coil array.

FIG. 2

The 16-element optimization of gradient echo signal intensity maps of single slices and whole brain at 300 MHz. From top to bottom: standard drive before optimization, optimization on an axial slice, optimization on a sagittal slice, optimization on a coronal slice, optimization on the whole brain. Three orthogonal views (from left to right: axial slice, sagittal slice, and coronal slice) are shown for each driving condition.

FIG. 3

The 16-element optimization of gradient echo signal intensity maps of single slices and whole brain at 600 MHz. From top to bottom: standard drive before optimization, optimization on an axial slice, optimization on a sagittal slice, optimization on a coronal slice, optimization on the whole brain. Three orthogonal views (from left to right: axial slice, sagittal slice, and coronal slice) are shown for each driving condition.

FIG 4

The 80-element optimization of gradient echo signal intensity maps of single slices and whole brain at 600 MHz. From top to bottom: standard drive before optimization; optimization on an axial slice, optimization on a sagittal slice, optimization on a coronal slice, optimization on the whole brain. Three orthogonal views (from left to right: axial slice, sagittal slice, and coronal slice) are shown for each driving condition.