Edited¹H magnetic resonance spectroscopy in vivo: Methods and metabolites

Abstract

The 1H-MRS spectrum contains information about the concentration of tissue metabolites within a predefined region of interest (a voxel). The conventional spectrum in some cases obscures information about less abundant metabolites due to limited separation and complex splitting of the metabolite peaks. One method to detect these metabolites is to reduce the complexity of the spectrum using editing. This review provides an overview of the one-dimensional editing methods available to interrogate these obscured metabolite peaks. These methods include: sequence optimizations, echo-time averaging, J-difference editing methods (single BASING, dual BASING and MEGA-PRESS), constant-time PRESS and quantum filtering. It then overviews the brain metabolites whose detection can benefit from one or more of these editing approaches, including ascorbic acid, γ-aminobutyric acid (GABA), lactate, aspartate, N-acetyl aspartyl glutamate, 2-hydroxyglutarate, glutathione, glutamate, glycine, and serine.

Big GABA: Edited MR spectroscopy at 24 research sites

“…In contrast, associations between metabolite measurements, or their uncertainty, and SNR and/or linewidth are widely observed in investigations of linear combination modeling of unedited spectra (Bartha et al, 2007; Kanowski et al, 2004; Near et al, 2013a). With spectral editing, the goal is to attain an unambiguously resolved signal that allows for simple peak fitting and integration (Bogner et al, 2016; Harris et al, 2017), but with (short-TE) unedited spectra quantification is based on linear-combination fitting, the outcome of which depends on the degree of orthogonality of the basis-set, which itself depends on data quality (Graveron-Demilly, 2014). Although edited MRS of lower-concentration metabolites typically necessitates comparatively longer scan durations or larger voxels to achieve reasonable SNR, the advent of multiplexed editing (Chan et al, 2016, 2017a, 2017b; Oeltzschner et al, 2017; Saleh et al, 2016) and development of edited MRSI (Bogner et al, 2014; Hnilicová et al, 2016; Zhu et al, 2011) continues to improve the efficiency of spectral editing approaches.…”

“…For lower-concentration metabolites such as GABA, spectral editing differentiates the weak signals of interest from the stronger, overlapping signals of higher-concentration metabolites. Difference editing techniques in particular use frequency-selective inversion pulses to achieve this (for methodological reviews, see Harris et al, 2017; Puts and Edden, 2012). The popularity of MEGA-PRESS is attributed to a number of factors, including the wide availability of the basic PRESS sequence across scanner platforms, its relatively straightforward implementation (Mullins et al, 2014), its reproducibility (Bogner et al, 2010; Brix et al, 2017; Geramita et al, 2011; Mikkelsen et al, 2016a; Near et al, 2014; O’Gorman et al, 2011; Shungu et al, 2016) and continued development of acquisition methodology and data processing tools (Chan et al, 2016; Edden et al, 2014).…”

Impact of tissue correction strategy on GABA-edited MRS findings

“…A potential limitation of the current study is the inability to assess changes in relaxation with aging, though a recent magnetic resonance spectroscopic imaging (MRSI) study (37) did not show T2 or T2′ changes in the frontal and parietal regions studied here. A second limitation of the current study is that the GABA+ measurement here is contaminated with macromolecules (MM) due to the bandwidth of the editing pulses (1). 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).…”

“…Anatomical images were collected for MRS voxel placement and segmentation using a 3D, T1-weighted, magnetization-prepared rapid gradient-echo (MP-RAGE) acquisition, prescribed in the sagittal plane, repetition time = 8 ms, echo time = 3.7 ms, 1-mm 3 isotropic voxels. A standard GABA+-edited MEGA-PRESS acquisition (1) was used (TR/TE = 2s/68 ms, 14 ms editing pulses placed at 1.9 ppm in the “ON” condition and at 7.46 ppm in the “OFF” condition, 320 averages, 2048 data points at 2 kHz sampling rate, 3×3×3 cm 3 voxels, VAPOR water suppression and 8 transients of unsuppressed water data for water-referenced GABA+ quantification. Measurements were acquired in mid-sagittal frontal and posterior voxels (locations shown in Figure 1).…”

“…Measurement of GABA in vivo , in humans is possible using specialized magnetic resonance spectroscopy (MRS) techniques; in this study we apply GABA-edited MEGA-PRESS (1). Due to limitations with the method, measurements of GABA are contaminated with macromolecules.…”