Advanced shimming matched to the human head anatomy

DFG Project

Magnetic field inhomogeneities cause several types of severe imaging artifacts such as geometric distortions, signal voids, banding artifacts or other issues related to frequency-selective excitation or reception. Susceptibility-related imaging artifacts can usually be reduced by applying additional shim fields, or by increasing imaging speed and/or read-out bandwidth with stronger gradient performance. However, commercially available gradient and shim systems in clinical MR systems are often insufficient for cutting-edge application. Furthermore, the performance of these gradient and shim coils are basically identical for 3T and 7T or 9.4T 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 produced by additional magnetic fields represented by 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 have a large distance to the target object. This increases the required shim currents to compensate local field deviations. Both high shim currents and the possibility of dynamic changes of shim currents are usually not possible in clinical systems. 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 as they were not specifically designed to compensate for the typical strong field drops at frontal air-tissue and lateral bone-tissue interfaces.

This application aims to design dedicated shim arrays adapted to the requirements of brain imaging based on acquired sets of representative human brain field maps, rapid methods to measure and dynamically update field inhomogeneities including fast algorithms to calculate required shim currents, and cutting-edge applications at 3T and 9.4T.The main objective of this application is to investigate field inhomogeneities created by anatomic susceptibility variations in the human head as a basis for the design and construction of dedicated shim arrays adapted to the requirements of human brain imaging based on acquired sets of representative human brain field maps, and to develop rapid methods to measure and dynamically update field inhomogeneities including fast algorithms to calculate required shim currents. Finally, the rapid and advanced B0 shim will by applied for BOLD bSSFP imaging, multislice EPI and 3D-CSI at 9.4T as well as MREG-applications at 3T.

DFG Project
SCHE 658/13
Advanced shimming matched to the human head anatomy PI: K. Scheffler, M. Zaitsev
Funding Scheffler: EUR 83.050 (+166.100) for 1(+2) years

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