Dynamic field map estimation using a spiral‐in/spiral‐out acquisition

The long readout times of single‐shot acquisitions and the high field strengths desired for functional MRI (fMRI) using blood oxygenation level‐dependent (BOLD) contrast make functional scans sensitive to magnetic field inhomogeneity. If it is not corrected during image reconstruction, field inhomogeneity can cause geometric distortions in the images when Cartesian k‐space trajectories are used or blurring with spiral acquisitions. Many traditional methods to correct for field inhomogeneity distortions rely on a static field map measured with the use of images that are themselves distorted. In this work, we employ a regularized least‐squares approach to jointly estimate both the undistorted image and field map at each acquisition using a spiral‐in/spiral‐out pulse sequence. Simulation and phantom studies show that this method is accurate and stable over a time series. Human functional studies show that the jointly estimated field map may be more accurate than standard field map estimates in the presence of respiration‐induced phase oscillations, leading to better detection of functional activation. The proposed method measures a dynamic field map that accurately tracks magnetic field drift and respiration‐induced phase oscillations during the course of a functional study. Magn Reson Med 51:1194–1204, 2004. © 2004 Wiley‐Liss, Inc.