A technique for accurate magnetic resonance imaging in the presence of field inhomogeneities

Abstract

A technique for producing geometrically accurate magnetic resonance images (MRIs) with undistorted intensity in the face of high levels of static field inhomogeneity arising from either source is presented. The technique requires the acquisition of two images of the same object with altered gradients. On the basis of a knowledge of these gradients it employs an automatic postprocessing step that exploits some invariant characteristics of the distortions to produce a rectified image from the two acquired images. No phantom imaging is involved and no operator interaction is required. The technique is theoretically justified and compared to other techniques, and experimental results that show that the technique works are presented. The improved accuracy in geometry and intensity may improve reliability of stereotactic surgery, may enhance the feasibility of both clinical and industrial imaging via external fields, and may increase the resolution of microscopic imaging.

Correction of spatial distortion in EPI due to inhomogeneous static magnetic fields using the reversed gradient method

“…The effects of an unknown static magnetic field inhomogeneity on a two-dimensional MR image acquired in its presence have been described mathematically (4,9). The addition of a spatially varying static magnetic field inhomogeneity, ⌬B(x, y, z), results in the spins experiencing a modified local magnetic field that changes their frequency of precession by ␥⌬B(x, y, z), where ␥ is the gyromagnetic ratio.…”

“…Hence, the spatial distortion along the slice-selective axis will be significantly smaller than that occurring in-plane. Its effect is therefore generally ignored in the reversed gradient correction method for single slices (9), as it is in phase map-based correction methods (7).…”

“…Corresponding values of x 1 and x 2 are identified as those that equalize the two integrals (9). In practice, instead of calculating x-values from pairs of x 1 and x 2 coordinates, it is simpler to work backwards by stepping through pixel locations in the corrected image matrix (x) for which the computer then searches for corresponding values of x 1 and x 2 that have equal line integrals and satisfy Eq.…”

“…[7]), Eqs. [8] and [9] may be stated as f Ͻ 1 and f Ͼ -1, respectively. Equations [8] and [9] set limits on the maximum ⌬B that may be corrected using this method, for a particular gradient strength.…”

“…The field map also may be derived directly from the spatial distortion it produces. This method, which was proposed by Chang and Fitzpatrick (9), is termed the reversed gradient correction method in the present study (9).…”