Advanced shimming matched to the human head anatomy

• Applicants: Professor Klaus Scheffler, Ph.D.; Professor Dr. Maxim Zaitsev
• Subject Area: Medical Physics, Biomedical Technology
• Term: from 2017 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 340690730

Field inhomogeneities cause in magnetic resonance imaging (MRI) several types of severe artifacts such as geometric distortions, signal voids or banding artifacts, especially at very high magnetic field strengths. These inhomogeneities are produced by differences in magnetic properties (so-called magnetic susceptibility) and shapes of the tissues. Susceptibility-induced field variations increase linearly with increasing strength of the applied main magnetic field. Typically, field inhomogeneities induced by the human body are in part compensated for by so-called shimming fields produced by specialized shim coils. Commercially available gradient and shim systems in clinical MR scanners are often insufficient for compensating the effects of field inhomogeneities. Furthermore, the performance of the available gradient and shim coils is identical at 7T and 9.4T and is similar to that of 3T scanners, which results in a more than three times worse field homogeneity at 9.4T compared to 3T.In clinical MR systems shim fields are typically based on global spherical harmonics, usually up to the second order and sometimes with additional channels of third order. These shim coils are integrated into the gradient coil and thus located at a large distance to the target object. Therefore, such shim systems turn out to be insufficient to compensate local field deviations in the human head. Higher shim currents or a possibility of dynamic changes of shim currents are usually not available in clinical systems. To overcome these limitations several types of local shim coils have been proposed, either by a set of shim coils that produce higher spherical harmonics or by a set of loop coils placed on a cylinder close to the object. One further approach is based on distributing small pieces of magnetic materials around the object to passively compensate field inhomogeneities.Electrically-controlled local shimming approaches show a great promise for in vivo applications in the human brain for their versatility and compatibility with dynamic shimming. However, none of these systems were carefully optimized towards the shape and large range of field inhomogeneities observed in the human brain.This application aims to design and implement dedicated shim arrays adapted to the requirements of brain imaging. They will be based on a large dataset of magnetic field maps, acquired in different subjects in varying head positions, and used to find an optimal shape and placement of the shim coils. Preliminary evaluations show a great potential of the proposed approach. Furthermore, rapid methods to measure and dynamically update field inhomogeneities including fast algorithms to calculate required shim fields or currents will be developed. We expect dynamic correction of physiologic B0 deviations, e.g. due to breathing or unintended movements, to bring in substantial benefits for cutting-edge applications at 3T and 9.4T.

• DFG Programme: Research Grants