Abstract
In functional MRI (fMRI) examinations using Echo-planar Imaging (EPI) sequences, brain displacement in the reconstructed images occurs due to physiological motion artifacts and static magnetic field drift. We have developed a new reordering technique to correct for such displacement. This reordering technique is based on the principle that the displacement can be reduced when using an EPI sequence by removing first-order phase shift along the k-space axes; that is, by using a phase-mapping technique. We applied the reordering technique to an fMRI data set to confirm its effectiveness. The fMRI data set was acquired with a 1.5-T MRI system (MRH1500AD, Hitachi Medial Corporation) using a multislice interleaved EPI sequence (4 slices, 8 shots per slice, FOV=240×240mm, slice thickness 6mm, matrix size=128×128, TR/TE=1250/10ms, FA=60°). We used the first image of each slice as the reference image. The inclination angles along the k-space axes were calculated with the least-squares method. The signal-to-noise ratio will affect the precision of the inclination angles, so we tried to optimize the data-selection parameters by varying the number of data points from the origin in k-space (±10 to 30 points), and by varying the threshold value (1% to 5% of the signal intensity of the origin in k-space). After correcting for displacement, we made error images of each imaging plane to estimate the accuracy of the selection methods. All error images before the correction had high error value regions. We compared the error values in the regions for various parameter values and found that setting the threshold value at 1% could achieve the most accurate correction. We conclude that this reordering technique using phase maps can be used to accurately correct an fMRI data set taken from an EPI sequence.
