Macromolecule-suppressed GABA-edited magnetic resonance spectroscopy at 3T

Abstract

Edited magnetic resonance spectroscopy makes possible noninvasive studies of the role of the inhibitory neurotransmitter GABA in the healthy brain and in disease processes. A major limitation of the methodology is coediting of macromolecular signals. Although it has previously been shown that macromolecular signal can be suppressed using a symmetrical editing scheme, this approach is rarely applied at field strength of 3T as insufficiently selective pulses result in loss of GABA signal (in addition to the intended suppression of macromolecular signal). In this article, the authors show that increasing the echo time to 80 ms lets more selective editing pulses be used, allowing for symmetric editing-based suppression of coedited macromolecular signal without loss of GABA signal. The method is applied to acquire macromolecule-suppressed GABA-edited spectra in 10 healthy participants.

Figure 1

Pulse sequences: a) MEGA-PRESS acquired at TE = 68 ms incorporating 14 ms editing pulses; b) MEGA-PRESS acquired at TE = 80 ms incorporating 20 ms editing pulses; c) editing pulse amplitude waveform (highlighted in gray in (a) and (b)) noting that the amplitude is above 50% of maximum for 55% of the pulse duration.

Figure 2

Frequency-dependence of editing in a 10mM GABA phantom: a) 14 ms editing pulses; and b) 20 ms editing pulses. Editing pulse frequency during ‘ON’ scans is expressed as an offset in ppm from 1.9 ppm. Editing pulses are maintained at 7.5 ppm during all ‘OFF’ scans. The integrals of these scans are plotted in c) against offset (left) and against offset normalized to pulse length (right).

Figure 3

Spectra recorded as a function of TE in a 10 mM GABA phantom: a) log integral plot of ‘ON’ spectra acquired at a range of TEs to determine the T_2,phantom = 204 ms; b) integral plot of ‘DIFF’ (i.e. edited) spectra acquired at TE from 60 ms to 80 ms, showing signal integrals in gray (with T2 weighting) and with T2 relaxation removed in black.

Figure 4

In vivo GABA-edited spectra from the medial parietal lobe with TE = 80 ms and 20 ms editing pulses applied at 1.9 and 1.5 ppm ((3.5 cm)³, scan time = 8.5 min) from ten subjects a) overlaid, and b) plotted individually.