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Abstract:
Introduction:
Proton Magnetic Resonance Spectroscopic Imaging based on Free Induction Decay acquisition (MRSI-FID) is highly promising at ultra-high magnetic field (>3 T) as it avoids in-plane chemical shift displacement and allows short echo time [1]. However, the necessity to use fat and water saturation results in an excessive amount of time needed for signal preparation. The aim of this project was to improve the time efficiency of the MRSI-FID sequence to obtain high spatial resolution spectra within reasonable time.
Methods and results:
In-vivo measurements were performed at 9.4 T whole body MR scanner (Siemens, Erlangen, Germany) equipped with a custom-build head coil consisting of 16 transmit and 31 receive channels [2]. Study was conducted with the approval of the local ethics board. The developed MRSI-FID sequence was preceded by non-localized fat saturation gauss radio-frequency (RF) pulse, allowing reduction of the fat contaminations by approximately 40. The flip angles (FA) of three water suppression pulses were numerically optimized for static (B0) and transmit (B1 +) magnetic field inhomogeneities. The use of an asymmetric RF excitation pulse shortened the acquisition delay to 1.6 ms. The slice-selection and refocusing gradient shape was optimized to minimize the influence of frequency sidebands; whose presence hinder spectral quantification [3]. Specifically, the gradient frequency spectrum was numerically optimized to minimize mechanical resonances (at 550 and 1100 Hz). Additionally, acquisition duration, repetition time (TR) and excitation FA were set to achieve optimal signal-to-noise ratio (SNR) [4]. Sequence optimization resulted in total acquisition time (TA) of 2 min 8 sec for a 32×32 MRSI matrix (voxel size: 6×6×10 mm3, 2 weighted averages, TR 138 ms), and 8 min 43 sec for a 64×64 matrix (voxel size 3×3×10 mm3 and otherwise identical parameters). Further shortening of the TA was possible with the use of generalized autocalibrating partially parallel acquisition (GRAPPA) [5]. Together with further optimization of acquisition parameters it enabled shortening the TA to 38 sec for low-resolution and to 2 min 27 sec for high-resolution MRSI. High temporal resolution allowed examination of stimulus evoked changes in human brain biochemistry. Functional experiments were conducted in human visual cortex, using a flickering (7Hz) radial checkerboard as a stimulus. A clear correlation between the changes in GABA/tCr and Glu/tCr concentration ratios and the
stimulation periods were observed. The average differences between the stimulus on- and off-set were ~13 and ~11, for GABA/tCr and Glu/tCr, respectively.
Discussion:
The proposed MRSI-FID sequence enables fast and reliable acquisition of proton spectra at 9.4T. The high SNR makes it possible to reduce the acquisition time even further by utilizing parallel imaging techniques and high temporal resolution enabled the assessment of functional related changes in metabolite concentrations during visual stimulation. Our observations of functional MRSI are in accordance with the literature [6-8]. However, strong
contaminations with lipid signal currently still hinders the analysis of the spectra from the regions close to the
scalp. Nevertheless, further studies, with a large number of participants will be necessary to elucidate the
observed changes in concentrations of Glu and GABA in the regions associated with positive BOLD response.
