Detrending phase drift: A preprocessing step to improve the effectiveness of the UNFOLD technique

Abstract

Purpose: To improve the effectiveness of the UNFOLD technique in removing aliasing artifacts by detrending temporal phase drift in each voxel before the implementation of the UNFOLD technique. This is because linear and quadratic phase trends in temporal signal can shift and broaden aliasing peaks in the spectrum and cause the removal of aliasing artifacts by spectral filtering to be insufficient or even fail when minimal filtering of the spectrum is used. Materials and Methods: A functional magnetic resonance imaging (fMRI) study with a visual stimulus was performed on normal subjects to test the hypothesis. A 2D spiral-in/out sequence was used to acquire k-space data. Undersampled k-space trajectories were used to improve temporal resolution. Aliasing artifacts were removed by the UNFOLD technique. For comparison, two image sets, with and without phase trends removal, were obtained from each set of functional data. Results: After detrending temporal phase drift, residual aliasing artifacts that were not suppressed by the standard UNFOLD technique could be successfully removed. Better image quality, temporal stability, and activation maps could be achieved by the proposed method. Conclusion: The proposed method can improve the effectiveness of the UNFOLD technique in removing aliasing artifacts when spectral filtering is kept minimal to preserve temporal resolution. © 2011 Wiley-Liss, Inc.