A method for unwrapping highly wrapped multi‐echo phase images at very high field: UMPIRE

Abstract

To develop a method of unwrapping phase images from multi-echo scans that works even where there are several wraps between echoes, and which generates unwrapped phase images in addition to phase difference (PD) images. The difference between the echo spacings in an acquisition with three unevenly spaced echoes (an imposed delay) can be selected such that the phase evolution in that time is in the range -π to +π in all voxels of interest. Under this condition, an image of the difference between the phase evolutions in the two inter-echo periods, an estimate of ΔB0 , is free of wraps. This ΔB0 estimate can be used to identify and remove receiver phase offsets and wraps in phase images. The approach was tested on simulated data and high-resolution in vivo brain data acquired from six subjects at 7 Tesla. The method generated wrap-free phase images. It was able to remove more wraps than is possible with PD imaging and was faster and more reliable than spatial unwrapping. Unwrapping Multi-echo Phase Images with iRregular Echo spacings (UMPIRE) is conceptually simple, fast, reliable, and requires no fitting, thresholds, or operator intervention.