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

Abstract

Purpose ‘Symmetric’ editing schemes have been proposed to suppress unwanted co-edited MM signals in GABA editing. 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. Methods Full density-matrix simulations of both conventional (non-symmetric) 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 macromolecular (MM) signals were performed to quantify the effects of field offsets on the total GABA+MM signal (designated as 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 non-symmetric 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) the scanner offset, maintaining symmetric suppression of MM signal. Conclusions 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 interleaved water referencing.

Big GABA: Edited MR spectroscopy at 24 research sites

“…One major contributor of variance that has been identified is frequency offset, with the data reproducing the approximately linear relationship observed by Edden et al (2016). The ratio between MM-suppressed GABA and GABA+ measurements (0.38) is lower than expected.…”

“…For the sequences used in this study, GE and Philips editing pulse offsets are calculated assuming a water frequency of 4.68 ppm and Siemens assumes 4.7 ppm. Given that GABA editing involves the use of frequency-selective editing pulses, their inversion frequency bandwidth has a significant impact on editing efficiency, determining the extent of MM co-editing in GABA+ acquisitions and the extent of GABA nulling in symmetric MM suppression (see Edden et al, 2016; Harris et al, 2014; Terpstra et al, 2002). For GE and Philips implementations where editing pulse duration is specified, editing pulse duration was set to 15 ms for the GABA+ acquisition and 20 ms for the MM-suppressed GABA acquisition.…”

“…Parameters common between the two acquisitions included: TR = 2000 ms; 320 averages (i.e., 160 ON and 160 OFF transients); ~10 min scan time. Although the spectral width and number of discrete data points differed from site to site (see Table 2), in all cases the aim was to achieve a data acquisition time of ~1 s. All Philips sites except P8 addressed B 0 field offsets with prospective frequency correction based on interleaved water referencing (Edden et al, 2016). Specifically, for every 40 water-suppressed acquisitions, a water-unsuppressed acquisition was performed and used to correct the center frequency in real-time.…”

“…For instance, pulse sequence parameters, and in particular pulse timings, differ between vendor-specific PRESS sequences and lead to subtle but important differences in the resolved GABA signal (Near et al, 2013b). Moreover, spectral editing of GABA is associated with a number of complexities, including TE-dependent J -evolution of the GABA spin system (Edden et al, 2012b), frequency and spatial effects of volume localization (Edden and Barker, 2007; Kaiser et al, 2008), sensitivity to B 0 field frequency offsets (Edden et al, 2016; Harris et al, 2014) and contamination from co-edited macromolecules (MM) (Henry et al, 2001; Rothman et al, 1993). It is generally assumed that these factors limit the degree to which a GABA-edited measurement from one site can be compared to another at a different site.…”

Impact of tissue correction strategy on GABA-edited MRS findings

“…In a study of aging of a younger population (ages 21–52 y) GABA+MM was seen to increase with age while GABA measures with MM suppression appear much more stable (39). Interpretation of these results is complicated by the difficulties of MM suppression (40), but the potential influence of MM cannot be discounted, although the mechanism mediating a relationship between MM and MoCA is unclear. A limitation of applying the α-correction is that, in addition to gross tissue changes, α itself may change with age or pathology.…”