Abstract
In functional MRI, magnetic field inhomogeneities due to air-tissue susceptibility differences may lead to severe signal dropouts and geometric distortions in echo-planar images. Therefore, the inhomogeneities in the field are routinely minimized by shimming prior to imaging. However in fMRI, the Blood Oxygen Level Dependent (BOLD) effect is the measure of interest, so the BOLD sensitivity (BS) should be optimized rather than the magnetic field homogeneity. The analytical expression for an estimate of the BOLD sensitivity has been recently developed, allowing for the computation of BOLD sensitivity maps from echo-planar images and field maps. This report describes a novel shimming procedure that optimizes the local BOLD sensitivity over a region of interest. The method is applied in vivo and compared to a standard global shimming procedure. A breath-holding experiment was carried out and demonstrated that the BS-based shimming significantly improved the detection of activation in a target region of interest, the medial orbitofrontal cortex.
Highlights
In functional magnetic resonance imaging, magnetic field inhomogeneities due to air-tissue susceptibility differences can lead to severe signal dropouts and geometric distortions in echo-planar images (EPI)
The BS can be expressed as a product of three factors corresponding to the effect of field gradients in the three orthogonal imaging directions: BS = BS0 · αPE · αRO · αSS where BS0 is the Blood Oxygen Level Dependent (BOLD) sensitivity in the absence of field gradients (BS0 = 100%), αPE and αRO are the factors corresponding to the in-plane field gradients GPE and GRO in the phase encoding and readout directions respectively, and αSS is the through-plane factor corresponding to field gradients in the slice selection direction GSS
An increase between 8.1% and 10.8% was observed in the region of interest (ROI) and a global increase between 5.4% and 7.2% was observed in the WSA
Summary
In functional magnetic resonance imaging (fMRI), magnetic field inhomogeneities due to air-tissue susceptibility differences can lead to severe signal dropouts and geometric distortions in echo-planar images (EPI). The optimization of the field homogeneity, socalled shimming, is an important step preceding the imaging process, and various shimming techniques using linear and higher-order resistive shim coils have been developed (Blamire et al, 1996; de Graaf et al, 2003; Gruetter, 1993; Kim et al, 2002; Poole and Bowtell, 2008; Prammer et al, 1988; Webb and Macovski, 1991; Wilson et al, 2002). Based on the same approach, the automated shimming technique by Wilson et al (2002) goes towards a more localized strategy by preferentially optimizing the field homogeneity in specified areas according to the experimental hypothesis. A variant has been proposed recently that divides the optimization problem into a
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