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Abstract:
Purpose/Introduction: Several studies utilize the advantages of 7T to investigate the dynamic changes of brain metabolites (fMRS) under different types of functional stimulation.1–5 The purpose of this work is to exploit the even higher sensitivity of a metabolite cycled (MC)
semi-LASER sequence6,7 applied at 9.4 T for fMRS under visual stimulation to verify previous results with regard to these additional metabolic adaptations. The MC technique also allows for simultaneous acquisition of water and metabolite spectra enabling the synchronous investigation of dynamic alternations of BOLD effect and metabolite levels.
Subjects and Methods: All measurements were performed on a 9.4 T whole-body MRI scanner (Siemens Healthcare, Germany) using a home-built coil.8 11 volunteers participated in this study. A visual stimulus consisted of a radial red–black checkerboard that flickered with 10 Hz (ON), alternating with a black screen (OFF; Fig. 2) was applied. During the stimulation, fMRS data (NEX: 224) were acquired from a voxel (2 9 2 9 2 cm3) in the occipital lobe using MC semi-LASER sequence7 (TE/TR: 24/6000 ms). Neither water suppression nor outer volume suppression was implemented. fMRS data were processed9 and analyzed. FIDs from all volunteer were
averaged and smoothed using a moving average over 16 acquisition (94 timepoints). All the acquisitions of OFF and ON periods across volunteers were summed together forming two blocks of 610 averages each. Next, the OFF block was multiplied with correction factors for linewidth using a lorentzian function, frequency shift and zero order phase.
Results: The pattern of SNRwater acquired using the MC technique follows the stimulation paradigm and reflects the BOLD effect equivalent to fMRI studies (Fig. 1). High correlation coefficients between the changes of SNRwater and FWHMwater (R = -0.99), between FHWMNAA and SNRwater (R = -0.87), as well as, between FHWMtCr and FWHMwater (R = -0.92) are found (Fig. 1). Changes in Glu and GABA are reported (Fig. 2). Quantification of macromolecular
baseline included the basis-set varies during different timepoints. However, these alterations are not correlated with Glu timecourse pattern indicating real changes in Glu (Fig. 3). SNR of water signal acquired during visual stimulation MC follows the paradigm pattern (red line), while SNR of water signal acquired without stimulation does not show any correlation with the paradigm blocks (blue line). In addition, changes in FHWM of water, NAA and
tCr are highly correlated with the changes of water SNR indicating the BOLD effect
Discussion/Conclusion: This is the first fMRS study at 9.4 T and it indicates Glu and GABA alterations in the visual cortex. For the first time, water and metabolite alterations are detected simultaneously. Correlation of FWHMwater and FWHMNAA with SNRwater shows that linewidth changes are the result of the BOLD effect10. The influence of the variations of LCModel macromolecular baseline in metabolites time-courses is evaluated. More datasets will be acquired to fully verify and confirm these results.
