Modeling real shim fields for very high degree (and order) B0 shimming of the human brain at 9.4 T

Paul Chang, Sahar Nassirpour, Anke Henning

Research output: Contribution to journalArticlepeer-review

16 Scopus citations

Abstract

Purpose: To describe the process of calibrating a B0 shim system using high-degree (or high order) spherical harmonic models of the measured shim fields, to provide a method that considers amplitude dependency of these models, and to show the advantage of very high-degree B0 shimming for whole-brain and single-slice applications at 9.4 Tesla (T). Methods: An insert shim with up to fourth and partial fifth/sixth degree (order) spherical harmonics was used with a Siemens 9.4T scanner. Each shim field was measured and modeled as input for the shimming algorithm. Optimal shim currents can therefore be calculated in a single iteration. A range of shim currents was used in the modeling to account for possible amplitude nonlinearities. The modeled shim fields were used to compare different degrees of whole-brain B0 shimming on healthy subjects. Results: The ideal shim fields did not correctly shim the subject brains. However, using the modeled shim fields improved the B0 homogeneity from 55.1 (second degree) to 44.68 Hz (partial fifth/sixth degree) on the whole brains of 9 healthy volunteers, with a total applied current of 0.77 and 6.8 A, respectively. Conclusions: The necessity of calibrating the shim system was shown. Better B0 homogeneity drastically reduces signal dropout and distortions for echo-planar imaging, and significantly improves the linewidths of MR spectroscopy imaging. Magn Reson Med 79:529–540, 2018.

Original languageEnglish (US)
Pages (from-to)529-540
Number of pages12
JournalMagnetic resonance in medicine
Volume79
Issue number1
DOIs
StatePublished - Jan 2018

Keywords

  • B shimming
  • field modeling
  • high-order shimming
  • ultrahigh field strengths

ASJC Scopus subject areas

  • Radiology Nuclear Medicine and imaging

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