Method for correcting magnetic field inhomogeneity distortion in mri

Abstract

Disclosed is a method to correct the geometric distortion caused by field inhomogeneity in MR images such as images of patients wearing MR-compatible stereotaxic frames. A previous distortion correction method derives patient-dependent error maps by computing the phase-difference of 3D images acquired at different T.sub.E 's. The time difference (.DELTA.T.sub.E =4.9 ms at 1.5 T) is chosen such that the water and fat signals are in phase. However, .DELTA.T.sub.E is long enough to permit phase wraps in the difference images for frequency offsets greater than 205 Hz. Phase unwrapping techniques resolve these only for connected structures; therefore the phase difference for fiducial rods may be off by multiples of 2 .pi. relative to the head. This uncertainty is removed by using an additional single 2D phase-difference image with .DELTA.T.sub.E =1 ms (during which time no phase-wraps are typically expected) to determine the correct multiple of 2 .pi. for each rod. The method was tested in a cadaver and in a patient using CT as a reference. Targets in the frame coordinates were chosen from CT and compared to their locations in MR. Localizing errors were as large as 3.7 mm before correction and were reduced to less than 1.11 mm after correction.