Optimized EPI for fMRI studies of the orbitofrontal cortex.

A common problem in gradient-echo echo planar imaging (EPI) is the occurrence of image distortions and signal losses caused by susceptibility gradients near air/tissue interfaces. Since EPI is frequently used for functional magnetic resonance imaging experiments based on the blood oxygenation level-dependent effect, functional studies of certain brain regions affected by susceptibility gradients, such as the temporal lobes and the orbitofrontal cortex, may be compromised. In this work a method for signal recovery in certain regions of the orbitofrontal cortex is presented. The influence of in-plane susceptibility gradients is reduced by optimization of the imaging slice orientation. Through-plane susceptibility gradients are partly compensated by means of a moderate preparation gradient pulse similar to z-shimming. In contrast to several other techniques proposed in the literature for reducing susceptibility effects, this method does not compromise the temporal resolution and is therefore applicable to event-related studies.