On the UNFOLD method

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Abstract
A graphical formalism is presented, showing that “UNaliasing by Fourier-encoding the Overlaps Using the temporaL Dimension” (UNFOLD) is equivalent to sampling k-t-space in a sheared grid pattern. Discrete regular sampling in k-t-space leads to periodic replication of the support region in x-f-space. Thus, the maximum acceleration achievable by UNFOLD is equivalent to the maximum packing of support regions in x-f-space. When the support region is separable along the x and f axes, the reconstruction can be performed separately for each k. UNFOLD can be combined with SiMultaneous Acquisition of Spatial Harmonics (SMASH) to further accelerate acquisition. However, a straightforward combination of the methods has been shown to result in a size restriction, which limits the portion of the field of view (FOV) with a larger temporal bandwidth to only a quarter of the FOV. Two solutions are presented to overcome this restriction. Magn Reson Med 47:202–207, 2002.