Compensation of susceptibility-induced signal loss in echo-planar imaging for functional applications

Functional magnetic resonance imaging favors the use of multi-slice gradien t-recalled echo-planar imaging due to its short image acquisition times, wh ole brain coverage and sensitivity to BOLD contrast. However, despite its a dvantages, gradient-recalled echo-planar imaging also is sensitive to stati c magnetic field gradients arising primarily from air-tissue interfaces. Th is can lead to image artifacts such as voxel shifts and complete signal los s. A method to recover signal loss by adjusting the refocusing gradient amp litude in the slice-select direction, preferably axially, is proposed. This method is implemented as an automated computer algorithm that partitions e cho-planar images into regions of recoverable signal intensities using a hi stogram analysis and determines each region's proper refocusing gradient am plitude. As an example, different refocusing gradient amplitudes are interl eaved in a fMRI acquisition to maximize the signal to noise ratio and obtai n functional activation in normal and dropout regions. The effectiveness of this method is demonstrated by recovering signal voids in the orbitofronta l cortex, parahippocampal/amygdala region, and inferior visual association cortex near the cerebellum. (C) 2000 Elsevier Science Inc. All rights reser ved.