Correction of physiologically induced global off‐resonance effects in dynamic echo‐planar and spiral functional imaging

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Abstract
In functional magnetic resonance imaging, a rapid method such as echo‐planar (EPI) or spiral is used to collect a dynamic series of images. These techniques are sensitive to changes in resonance frequency which can arise from respiration and are more significant at high magnetic fields. To decrease the noise from respiration‐induced phase and frequency fluctuations, a simple correction of the “dynamic off‐resonance ink‐space” (DORK) was developed. The correction uses phase information from the center ofk‐space and a navigator echo and is illustrated with dynamic scans of single‐shot and segmented EPI and, for the first time, spiral imaging of the human brain at 7 T. Image noise in the respiratory spectrum was measured with an edge operator. The DORK correction significantly reduced respiration‐induced noise (image shift for EPI, blurring for spiral, ghosting for segmented acquisition). While spiral imaging was found to exhibit less noise than EPI before correction, the residual noise after the DORK correction was comparable. The correction is simple to apply and can correct for other sources of frequency drift and fluctuations in dynamic imaging. Magn Reson Med 47:344–353, 2002.