Correcting image blur in spiral, retraced in/out (RIO) acquisitions using a maximized energy objective

摘要

Purpose Images acquired with spiral k‐space trajectories can suffer from off‐resonance image blur. Previous work showed that averaging 2 images acquired with a retraced, in/out (RIO) trajectory self‐corrects image blur so long as off‐resonant spins accrue less than 1 half‐cycle of relative phase over the readout. Practical scenarios frequently exceed this threshold. Here, we derive and characterize a more‐robust off‐resonance image blur correction method for RIO acquisitions. Methods Phantom and human volunteer data were acquired using a RIO trajectory with readout durations ranging from 4 to 60 ms. The resulting images were deblurred using 3 candidate methods: conventional linear correction of the component images; semiautomatic deblurring of the component images using an established minimized phase objective function; and semiautomatic deblurring of the average of the component images using a maximized energy objective function, derived below. Deblurring errors were estimated relative to images acquired with 4 ms readouts. Results All 3 methods converged to similar solutions in cases where less than 2 and 4 cycles of phase accrued over the readout in in vivo and phantom images, respectively (13 ms readout at 3T). Above this threshold, the linear and minimized phase methods introduced several errors. The maximized energy function provided accurate deblurring so long as less than 6 and 10 cycles of phase accrued over the readout in in vivo and phantom images, respectively (34 ms readout at 3T). Conclusion The maximized energy objective function can accurately deblur RIO acquisitions over a wide spectrum of off resonance frequencies.