DYNAMITE Shimming

Introduction

Excellent magnetic field homogeneity is an essential prerequisite for most MRI and MRS applications to guarantee the correct spatial registration, the best signal strength and optimal spectral resolution. Varying magnetic susceptibility conditions around the human brain, however, cause strong and complex magnetic field distortions inside the brain and reduce the informative value of the results.

The ongoing trend towards higher B0 fields for increased signal strength, spectral dispersion and blood oxygen level dependence (BOLD) sensitivity further magnifies the problem as the induced magnetic field distortions scale linearly with the scanner B0 field strength. The potential of ultra-high field MRI/MRS has not been fully realized at least in part due to methodological and technical challenges related to imperfections of the employed static B0 magnetic fields. These limitations represent an important hurdle for in vivo MR investigations of the human brain.

Goal

The goal of this methodological research is to enable optimal information content of MR investigations through mitigation of imperfections of the static B0 magnetic field with dedicated engineering and software solutions.

Left: a figure of a head surrounded by multi-colored circular coil rings Right: Three images of a brain scan labeled "Human Brain Map at 7T Before Shimming", "Dynamic Multi-Coil Magnetic Field Modeling", and "Dynamic Multi-Coil Shimming Result"

Current Work

Our current research focuses on the development of advanced B0 magnetic field modeling techniques for magnetic field shimming in the human. Optimal magnetic field homogeneity is essential for meaningful functional MR imaging and spectroscopy and will open up a large range of clinical and neuroscience applications.

Conventional B0 correction fields resemble the shapes of low-order spherical harmonic functions. They are able to compensate ('shim') large-scale, shallow field variations, but fail to resemble and compensate more complex and localized field foci. We developed a novel, generalized method for the synthesis of magnetic field shapes with generic basis fields from a matrix of localized coils. The combination of simple, unspecific (i.e. not spherical harmonic shaped) basis fields allows the flexible synthesis of complex and high amplitude shim fields. We achieved unrivaled levels of magnetic field homogeneity with multiple techniques in the human brain in a safe way which is a prerequisite for meaningful MRI, MRS and MRSI.

Recent publications:

Dynamic Multi-Coil Technique (DYNAMITE) Shimming of the Human Brain at 7 Tesla (J Magn Reson 212:280-288, 2011)

Dynamic multi-coil technique (DYNAMITE) shimming for echo-planar imaging of the human brain at 7 Tesla (Neuroimage 105:462-72, 2015)