Maximizing sensitivity for fast GABA edited spectroscopy in the visual cortex at 7 T

Abstract

The combination of functional MRI (fMRI) and MRS is a promising approach to relate BOLD imaging to neuronal metabolism, especially at high field strength. However, typical scan times for GABA edited spectroscopy are of the order of 6‐30 min, which is long compared with functional changes observed with fMRI.The aim of this study is to reduce scan time and increase GABA sensitivity for edited spectroscopy in the human visual cortex, by enlarging the volume of activated tissue in the primary visual cortex. A dedicated setup at 7 T for combined fMRI and GABA MRS is developed. This setup consists of a half volume multi‐transmit coil with a large screen for visual cortex activation, two high density receive arrays and an optimized single‐voxel MEGA‐sLASER sequence with macromolecular suppression for signal acquisition.The coil setup performance as well as the GABA measurement speed, SNR, and stability were evaluated. A 2.2‐fold gain of the average SNR for GABA detection was obtained, as compared with a conventional 7 T setup. This was achieved by increasing the viewing angle of the participant with respect to the visual stimulus, thereby activating almost the entire primary visual cortex, allowing larger spectroscopy measurement volumes and resulting in an improved GABA SNR. Fewer than 16 signal averages, lasting 1 min 23 s in total, were needed for the GABA fit method to become stable, as demonstrated in three participants. The stability of the measurement setup was sufficient to detect GABA with an accuracy of 5%, as determined with a GABA phantom. In vivo, larger variations in GABA concentration are found: 14‐25%. Overall, the results bring functional GABA detections at a temporal resolution closer to the physiological time scale of BOLD cortex activation.

Influence of Repetitive Transcranial Magnetic Stimulation on Human Neurochemistry and Functional Connectivity: A Pilot MRI/MRS Study at 7 T

“…Of particular note is the quantification of GABA. The average CRLB (13%) and across-subjects coefficient of variation (15%) obtained with semi-LASER were similar to those observed with edited-MRS sequences at 7 T (Chen et al, 2017; Hendriks et al, 2018). A direct comparison of acquisition strategies in same subjects is still warranted to ultimately identify the most accurate and sensitive approach for GABA detection in functional MRS designs.…”

“…The capability to measure these functional changes provides unique insights into how brain metabolism is associated with neural activity (Mangia et al, 2009). Importantly, extensive efforts are currently focused on optimizing methodologies of edited MRS at 7 T for further enhancing the reliability of detection of the inhibitory neurotransmitter GABA (Chen et al, 2017; Hendriks et al, 2018).…”

SNR optimized 31P functional MRS to detect mitochondrial and extracellular pH change during visual stimulation

“…First, the volume of activated tissue can be enlarged. A large volume of activated tissue would enable the use of a large spectroscopy voxel size, resulting in an additional increase in SNR similar as recently observed for enlarged voxel measurements of γ‐aminobutyric acid (GABA) using a setup with a large visual angle . Secondly, the RF coil can be optimized for higher SNR.…”

“…A large volume of activated tissue would enable the use of a large spectroscopy voxel size, resulting in an additional increase in SNR similar as recently observed for enlarged voxel measurements of γ-aminobutyric acid (GABA) using a setup with a large visual angle. 27 Secondly, the RF coil can be optimized for higher SNR. Using a tight-fit volume resonator, a uniform excitation can be obtained while assuring highest SNR in the center of the brain.…”

A vision of 14 T MR for fundamental and clinical science

“…For example, it may be possible to directly measure GABA and glutamate concentrations in anatomically homogeneous cortical volumes and in a relatively short time, and thus to index changes in excitation/inhibition balance as a function of a participant’s cognitive state (e.g., during learning). The expected increase in spectral resolution and sensitivity at 14 T will both be crucial for improved detectability of these metabolites, particularly GABA, enabling measurements on the order of ~ 0.3 cc in few minutes (as compared to much larger volumes possible at 7 T, [ 53 ]). The increased chemical shift dispersion will mean that it may no longer be necessary to use editing experiments to identify GABA, and a short TE could be used to further increase sensitivity.…”