FUNCTIONAL MR-IMAGING USING GRADIENT-ECHO ECHO-PLANAR IMAGING IN THE PRESENCE OF LARGE STATIC-FIELD INHOMOGENEITIES

At 1.5 T, the held 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 oxygenatio n or T2 effects. Unfortunately, gradient-echo sequences are also extr emely sensitive to magnetic held inhomogeneities, and this sensitivity has precluded examination of regions of cortex near field inhomogenei ties with functional MR imaging. This article presents a gradient-echo echo-planar imaging method that uses variable amplitude scaling on th e slice-select refocusing lobe to generate images compensated for stat ic held inhomogeneities. a technique for constructing composite images to he used In statistical tests for activation is also presented, Thc method is shown to produce clean activation maps in the presence of l arge static field inhomogeneities, The technique retains the sensitivi ty of gradient-echo imaging to changes in blood oxygenation while remo ving the sensitivity to large static held inhomogeneities.