A field-mapping scan enables offline distortion correction of your BOLD images during post-processing with software such as SPM or FSL. The field-map scan acquires two simple T2*-weighted images using the gradient echo method. T2*-weighting is similar to the T2 weighting described above, except that the signal is more heavily weighted by effects from susceptibility-induced gradients (drop-out as well as distortion). The two images are acquired with different TE times, to produce different weightings. A simple formula relates the phase-difference of the signal in each voxel to the 3D field variation in that voxel. The offline processing software can then figure out how much to “un-distort” each voxel based on the field variation in that voxel.
Talk with Ross (rmair@fas.harvard.edu) if you want to add this feature to your scanning protocol. The scan must be set up with the same field-of-view, spatial resolution and number of slices as your BOLD scan, so if you vary these parameters, a field-map scan will need to be set up for each one. For most common BOLD scan parameters, the field-map scan takes about a minute. As with other methods described above that rely on pre-scans of any sort, subject motion works to invalidate any correction, so these methods are really only useful for very compliant subjects that can remain still for extended periods. The reference for this method is:
P. Jezzard, R. Balaban, Correction for geometric distortion in echo planar imagesfrom B0 field variations. Magn. Reson. Med. 34, 65–73 (1995).
Talk with Ross (rmair@fas.harvard.edu) if you want to add this feature to your scanning protocol. The scan must be set up with the same field-of-view, spatial resolution and number of slices as your BOLD scan, so if you vary these parameters, a field-map scan will need to be set up for each one. For most common BOLD scan parameters, the field-map scan takes about a minute. As with other methods described above that rely on pre-scans of any sort, subject motion works to invalidate any correction, so these methods are really only useful for very compliant subjects that can remain still for extended periods. The reference for this method is:
P. Jezzard, R. Balaban, Correction for geometric distortion in echo planar imagesfrom B0 field variations. Magn. Reson. Med. 34, 65–73 (1995).
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