Correction of Intensity Nonuniformity in MR Images of Any Orientation

The specialised radiofrequency (RF) coils used in MRI such as head or surface coils can give rise to marked image intensity nonuniformities. There are two situations in which it is essential to correct this: (1) When a global intensity threshold is used to segment particular tissues; and (2) in proton density images, from which the proton concentration can be measured provided that the system gain is uniform or known over the whole image. We describe experiments to determine the magnitude and sources of nonuniformity in a 0.5-T system, and assess methods devised to correct for them in scans of any orientation, including oblique scans at arbitrary angles to the magnet axis and arbitrary offsets from the magnet iso-centre. A correction based on the response of a system to a uniform phantom was implemented. Tests of the correction with orthogonal views demonstrate that the uniformity of images of any orientation can be improved significantly with a correction matrix from just one orientation and still further with two matrices, one axial and the other either coronal or sagittal. We expect further improvements to be possible if gradient coil eddy current effects can be reduced.