J‐difference editing of gamma‐aminobutyric acid (GABA): Simulated and experimental multiplet patterns

Abstract

Purpose To investigate factors that influence the multiplet pattern observed in J-difference editing of GABA. Theory and Methods Density matrix simulations were applied to investigate the shape of the 3 ppm GABA multiplet as a function of the editing sequence’s slice selective refocusing pulse properties, in particular bandwidth, transition width, and flip angle. For comparison to the calculations, experimental measurements were also made at 3T on a 10 mM GABA solution using the MEGA-PRESS sequence at various refocusing pulse flip angles. Results Good agreement was found between experiments and simulations. The edited multiplet consists of 2 outer lines of slightly unequal intensity due to strong coupling, and a smaller central line, the result of the unequal J-couplings between the C4 and C3 protons. The size of the center peak increases with increasing slice selective refocusing pulse transition width, and deviation of the flip angle from 180°. Conclusion The 3 ppm GABA multiplet pattern observed in the MEGA-PRESS experiment depends quite strongly on the properties of the slice selective refocusing pulses used. Under some circumstance the central peak can be quite large; this does not necessarily indicate inefficient editing, or a subtraction artifact, but should be recognized as a property of the pulse sequence itself.

Big GABA: Edited MR spectroscopy at 24 research sites

“…S1. Distinctive edited GABA peak lineshapes were seen for each vendor, likely a consequence of the different implementations of the MEGA-PRESS sequences between each vendor (Near et al, 2013b). GABA+/Cr and MM-suppressed GABA/Cr values, broken down by site and by vendor, are shown in Fig.…”

“…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).…”

Normalizing data from GABA-edited MEGA-PRESS implementations at 3 Tesla

“…This results in an ON signal that is less positive than it might be, and therefore losses in the difference (ON-OFF) signal. Losses in the OFF experiment arise from spatial heterogeneity of the coupling evolution [8,9,21], and due to the fact that the four couplings between spins at 1.9 ppm and 3.0 ppm are not equal so a single TE cannot capture all four fully antiphase. This results in an OFF signal that is less negative than it might be, and therefore, losses in the difference (ON-OFF) signal.…”

“…Additionally, the duration, shape, bandwidth and timing of editing pulses differs between implementations. These various parameters have a significant impact on the intensity and multiplet pattern of the edited 3.0 ppm GABA signal [6,8] as well as co-editing of other compounds (in particular, macromolecule resonances (MM) which also resonate at 3.0 ppm). Overall, the differences between GABA-editing sequences can be summarized by two metrics: (a) how much GABA signal is produced ( i.e.…”

“…Losses in editing efficiency can occur due to imperfect refocusing of coupling in the ON experiment, e.g. , if the editing pulses have reduced flip angle, are not separated by TE/2, or because of imperfect evolution of coupling in the OFF experiment, as, for instance, can be caused by chemical shift displacement effects [8,9]. …”