Correction for eddy current induced Bo shifts in diffusion-weighted echo-planar imaging

The importance of diffusion‐weighted MRI in the assessment of acute stroke is well‐recognized, and quantitative maps of the apparent diffusion coefficient (ADC) are now widely used. Echo‐planar imaging provides a robust method of acquiring diffusion‐weighted images free of motion artifact. However, initial experience with clinical MRI systems indicates that calculation of artifact‐free ADC maps from a series of echo‐planar diffusion‐weighted images is not necessarily straight‐forward. One of the problems is that frequency shifts resulting from eddy currents can cause misregistration of base diffusion‐weighted images. In this study, an on‐line correction method that overcomes this problem is described, and phantom and human images that demonstrate the validity of the technique are presented. The method uses a non‐phase‐encoded reference scan to correct the phase of each echo in the echo train, and can provide ADC maps that are free of misregistration artifacts, without the need for off‐line postprocessing