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

• Published in: NMR in biomedicine Vol. 31; no. 4; pp. e3890 - n/a
• Main Authors: Hendriks, Arjan D., Fracasso, Alessio, Arteaga de Castro, Catalina S., Gosselink, Mark W.J.M., Luijten, Peter R., Petridou, Natalia, Klomp, Dennis W.J.
• Format: Journal Article
• Language: English
• Published: England Wiley Subscription Services, Inc 01.04.2018; John Wiley and Sons Inc
• Subjects: Activation; Biological products; Cortex (temporal); Creatine - metabolism; fast edited spectroscopy; Field strength; fMRS; Functional magnetic resonance imaging; gamma-Aminobutyric Acid - metabolism; half volume coil; Humans; In vivo methods and tests; Macromolecules; Magnetic Resonance Imaging; Magnetic Resonance Spectroscopy - instrumentation; MEGA‐sLASER; Metabolism; Neuroimaging; Phantoms, Imaging; Sensitivity; Signal-To-Noise Ratio; Spectroscopy; Spectrum analysis; Stability analysis; Temporal resolution; Visual cortex; Visual Cortex - metabolism; Visual stimuli; γ-Aminobutyric acid; γ‐aminobutyric acid (GABA); γ-aminobutyric acid (GABA); fast edited spectroscopy; half volume coil; MEGA-sLASER; visual cortex; fMRS
• ISSN: 0952-3480; 1099-1492
• DOI: 10.1002/nbm.3890

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. The acquisition time of GABA edited MRS in the visual cortex is reduced using a specially built half volume coil setup (a). With this setup the activated cortical volume is increased, as is demonstrated with fMRI (b). This enables a large voxel size for spectroscopy, resulting in a 2.2‐fold GABA SNR gain. The GABA spectra over time (c) would take 1:23 minutes to acquire individually. The results bring GABA detections at a temporal resolution closer to the physiological time scale of fMRI.