Prospective frequency correction for macromolecule-suppressed GABA editing at 3T

Purpose: To investigate the effects of B₀ field offsets and drift on macromolecule (MM)-suppressed GABA-editing experiments, and to implement and test a prospective correction scheme. “Symmetric” editing schemes are proposed to suppress unwanted coedited MM signals in GABA editing. Materials and Methods: Full density-matrix simulations of both conventional (nonsymmetric) and symmetric MM-suppressed editing schemes were performed for the GABA spin system to evaluate their offset-dependence. Phantom and in vivo (15 subjects at 3T) GABA-edited experiments with symmetrical suppression of MM signals were performed to quantify the effects of field offsets on the total GABA+MM signal (designated GABA+). A prospective frequency correction method based on interleaved water referencing (IWR) acquisitions was implemented and its experimental performance evaluated during positive and negative drift. Results: Simulations show that the signal from MM-suppressed symmetrical editing schemes is an order of magnitude more susceptible to field offsets than the signal from nonsymmetric editing schemes. The MM-suppressed GABA signal changes by 8.6% per Hz for small field offsets. IWR significantly reduces variance in the field offset and measured GABA levels (both P < 0.001 by F-tests), maintaining symmetric suppression of MM signal. Conclusion: Symmetrical editing schemes substantially increase the dependence of measurements on B₀ field offsets, which can arise due to patient movement and/or scanner instability. It is recommended that symmetrical editing should be used in combination with effective B₀ stabilization, such as that provided by IWR. J. Magn. Reson. Imaging 2016;44:1474–1482.