Dynamic shimming of the human brain with a 32-channel integrated parallel reception, excitation, and shimming (iPRES) head coil array
Integrated parallel reception, excitation, and shimming (iPRES) is a novel MRI coil design, in which radiofrequency (RF) currents and direct currents (DC) flow in the same coil elements to perform MR image acquisition and localized magnetic field shimming, respectively, with a single coil array. The purpose of this study was to implement dynamic shimming with a 32-channel iPRES head coil array, by dynamically updating the DC currents to shim individual slices within a single scan. Dynamic shimming is more effective than global static shimming, in which the same currents are used to shim the whole brain, and more efficient than slice-optimized static shimming, in which different currents are used to shim different slices, but in separate scans.
To implement dynamic shimming, a Python script was written to send new DC current amplitudes and polarities to a DC power supply and a switch box connected to the iPRES head coil array, respectively, for each slice acquisition. This current-update process was optimized by performing timing measurements with an oscilloscope and by modifying the Python script to ensure that the DC currents to shim the ith slice were updated as efficiently as possible after the data acquisition of the (i-1)th slice and before the excitation of the ith slice.
Magnetic field maps were acquired in a phantom and in a human brain with either dynamic shimming or slice-optimized static shimming, and the root-mean-square-error was calculated to evaluate the shimming performance of the dynamic shimming relative to slice-optimized static shimming. The results show that dynamic shimming with the iPRES head coil array was successfully implemented and that it was as effective as, but much more efficient than, slice-optimized static shimming.
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