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Abstract:
Purpose of the software: B0 shimming is a vital pre-scan adjustment that directly affects the quality of the data. However, vendor implemented B0 shimming can be time-consuming and sub-optimal. For smaller regions-of-interest (ROI), such as single voxel spectroscopy,
the B0 shim can be especially poor and exceed the hardware
constraints. We present a software application that can calculate the optimal B0 shims for ROIs of any size, for arbitrary ROIs and also single-voxels. The process is fast, non-iterative and the shim values are always within the hardware constraints. The software package can easily be
installed and used on any MR and shim system, and has the ability to easily be calibrated to account for any imperfections of the system.
Methods and Implementation: The B0 shim values are calculated from B0 maps using an optimized and robust algorithm. We use the ConsTru algorithm1 that has previously been shown to give the optimal shim values for any application: whole brain, single slice, single voxel. The algorithm considers the hardware constraints of the
system and regularizes the problem accordingly so that the shim values are the best for the given shim system.
The software uses B0 maps in DICOM format. The region of interest can be defined by the user or copied from the scan protocol and the B0 shim values are then calculated.
Figure 1 shows the superiority of the ConsTru shimming algorithm over a variety of other B0 shimming algorithms currently used in the field in the case of a challenging shimming application (single voxel located in the pre-frontal cortex at 7 T). The resulting shimmed B0
maps and the standard deviation of the frequency shifts are shown for each algorithm. Figure 2 shows the resulting shim quality using ConsTru at 7 and 9.4 T over 66 volunteers for different ROIs. The software can also be modified to account for non-ideal spherical harmonic fields. Real fields models and reference fields can be used to calculate the optimal shim values2.
Features illustrated at the exhibit: The software is a simple, standalone executable. It can run on Windows 7 and upwards (and Linux binaries can also be compiled).
There is a single-voxel version and an arbitrary ROI version available. The arbitrary ROI version also comes with an autosegmentation feature. The two versions are shown in Figure 3. After the ROI is defined, the shim values are calculated immediately and no further iterations are required. The shim values as well as the defined ROIs
can be saved to file.
