Functional MR imaging using gradient-echo echo-planar imaging in the presence of large static field inhomogeneities.

At 1.5 T, the field strength of most clinical MR imagers, gradient-echo imaging is the primary imaging method for measuring brain activation, as such sequences are highly sensitive to changes in blood oxygenation or T2* effects. Unfortunately, gradient-echo sequences are also extremely sensitive to magnetic field inhomogeneities, and this sensitivity has precluded examination of regions of cortex near field inhomogeneities with functional MR imaging. This article presents a gradient-echo echo-planar imaging method that uses variable amplitude scaling on the slice-select refocusing lobe to generate images compensated for static field inhomogeneities. A technique for constructing composite images to be used in statistical tests for activation is also presented. The method is shown to produce clean activation maps in the presence of large static field inhomogeneities. The technique retains the sensitivity of gradient-echo imaging to changes in blood oxygenation while removing the sensitivity to large static field inhomogeneities.