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Abstract:
Purpose/Introduction: The field inhomogeneities produced by different susceptibilities in biological tissue and air cavities increase linearly with the applied static magnetic field. The most recent approach demonstrated advantage of using irregularly shaped coil elements in an array configuration [1, 2] instead of loop elements [3, 4] in close fitting B0 matrix shim. We introduce independent
parameters describing a geometry and position of every single coil which are then optimized jointly.
Subjects and Methods: For the simulations, a set of coils were placed on a cylinder (ø 360 mm), which is large enough to accommodate inside additional RF transmit and receive arrays. The optimization procedure was based on a field or frequency map that was acquired within the brain of a volunteer. One approach was to design individual setups, each one containing 6 coils optimized on a single slice. After having, for example, 2 setups tuned on 2 different
slices we could accommodate them all on a cylinder having in total 12 channels. The second approach was to take 16 channels and configure them so as to optimize an accuracy over different slices simultaneously. The comparison of different setups was based on the standard deviation (in Hz) of the resulting magnetic field distributions.
Two-slice optimization is just for proof of concept, in the future optimization will be performed on several slices or on the specified volume.
Results: Figure 1 shows the slices after 2nd SH shim and after shimming with 6 channel optimized setups for a particular slice. Figure 2 shows the resulting shim field after simultaneous optimization of the setup on 2 slices. The final standard deviation after optimization on a single slice and after simultaneous optimization was very similar. For comparison, standard deviation after shimming with
a 32 channel setup containing only loop elements on slice 1 was 13.4 and on slice 2 was 11.1 Hz. Thus, the resulting standard deviation was better with the proposed irregular coils on asymmetric positions.
Discussion/Conclusion: It is shown by numerical simulations that there is a significant advantage of using coils with irregular geometries over traditional arrays containing only circular elements. Future work will include optimization performed on a given volume and practical realization of the proposed structures.
