1H magnetic resonance spectroscopic imaging of deuterated glucose and of neurotransmitter metabolism at 7 T in the human brain

Abstract

Impaired glucose metabolism in the brain has been linked to several neurological disorders. Positron emission tomography and carbon-13 magnetic resonance spectroscopic imaging (MRSI) can be used to quantify the metabolism of glucose, but these methods involve exposure to radiation, cannot quantify downstream metabolism, or have poor spatial resolution. Deuterium MRSI (²H-MRSI) is a non-invasive and safe alternative for the quantification of the metabolism of ²H-labelled substrates such as glucose and their downstream metabolic products, yet it can only measure a limited number of deuterated compounds and requires specialized hardware. Here we show that proton MRSI (¹H-MRSI) at 7 T has higher sensitivity, chemical specificity and spatiotemporal resolution than ²H-MRSI. We used ¹H-MRSI in five volunteers to differentiate glutamate, glutamine, γ-aminobutyric acid and glucose deuterated at specific molecular positions, and to simultaneously map deuterated and non-deuterated metabolites. ¹H-MRSI, which is amenable to clinically available magnetic-resonance hardware, may facilitate the study of glucose metabolism in the brain and its potential roles in neurological disorders.

Fig.1 |. Quantification of SV-MR spectra obtained in posterior cingulum with single-voxel proton-MRS.

Bar diagrams demonstrate concentration comparisons quantified from the first and last time-point spectra following ²H-Glc and ¹H-Glc administration for 5 healthy volunteers and reflect continuously decaying metabolite signals through the acquisition period. Peaks that originated from the specific carbon position undergoing deuteration, namely, the 6^th carbon position for Glc (Glc₆) the 4^th carbon position for Glu (Glu₄) and Gln (Gln₄) and the 2^nd carbon position for GABA (GABA₂), were separated. Concentrations were compared with a standard, two-tailed, paired t-test between the first and last time-point within the ²H-Glc and ¹H-Glc sessions. The metabolites that appeared significant (p < 0.05) are displayed here. The whiskers represent the minimal and maximal values, the boxes first and third quartile and the dashed lines are medians. P-values were not corrected for multiple comparisons. The full analyzed neurochemical profile is shown at Suppl. Fig.1.

Fig. 2 |. Example of MR spectra obtained from one voxel in the posterior cingulum with single voxel 1H-MRS (panel A) and with 2H-MRSI (panel B) in one participant at 7 Tesla.

The displayed spectra were obtained after peroral administration of ²H-Glc. A. The summed spectra show decreasing signal at the resonance frequency of -C4¹H₂- in the glutamate molecule (2.34 ppm) due to enrichment of the glutamate pool with ²H (either -C4²H²H- or -C4²H¹H). The robust change at the 4^th carbon position is documented by the increasing signal amplitude at 2.34 ppm in the difference spectra calculated by subtraction of the respective spectrum from the last session minus the first session. B. ²H-MR spectra reflect an increase in the deuterated fraction of Glc and Glx (Glu+Gln). The insets with anatomical images show location of voxels in the posterior cingulate gyrus. The voxel dimensions were 22×20×20 mm for single-voxel proton MRS, whereas voxel selected from ²H-MRSI data had volume of (12.5 mm)³.

Fig.3 |. ¹H-MRS difference spectra and their quantification.

Summed spectra from all subjects (N=5) represent the first and last time-points after ²H-Glc ingestion. The spectra (A and B) were linewidth-matched with exponential line-broadening and subtracted. The resulting difference spectrum in panel C represents the effect of metabolite ²H enrichment. The metabolite components (D) were obtained via LCModel analysis using a basis set containing simulated spectra of neurochemicals that were undergoing deuteration (i.e., Glu, Gln, and GABA and Glc). The proton signals that originated from different carbon groups were separated. The summed 2H-MRS spectrum (N=5) from the last acquired time-point resembles the ¹H-MR spectrum except for the spectral resolution (E).

Fig. 4 |. Fitting of time courses obtained by quantification of ¹H (left column) and ²H single-voxel MR spectra (right column).

The spectra were acquired in a single PCC voxel and concentrations were obtained by LCmodel. Decay in the concentrations of glutamate (Glu₄) and glutamine (Gln₄), glucose (Glc₆), Glx (Glu+Gln) were fitted using the exponential function Y=Y₀^(−t/tau)+c (Glu₄), and linear regression Y = a + bX (Gln₄,Glc₆, ²H-Glc, and ²H-Glx). The ¹H-MRS time-courses and their fits following ²H-Glc ingestion are in line with those obtained by quantification of ²H-MR spectra. Two-tailed test was used to calculate significance of the slopes (p-values).

Fig. 5 |. Effect of Deu-Glc on the spectra obtained from the gray and white matter with 3D multi-voxel ¹H-MRSI and ²H-MRSI data.

Difference spectra (red) in the panel A were calculated by subtraction of spectra obtained from the first (purple) and last (green) time-point after deuterium ingestion in one healthy volunteer. The spectra were selected using a quality control mask and segmented gray or white matter masks within the 40 mm-thick volume-of-interest (180×180×40 mm) (white box). The signal loss at 2.34 ppm reflects the exchange of protons and deuterons at the 4^th carbon position in the glutamate molecule and is displayed as a positive peak in the difference spectra. The signal decay is convincingly found in both gray and white matter. Lack of unwanted signals in the quantified range 1.9–4.2 ppm verifies good spectral quality and stability during the acquisition. The ²H MRS spectra in panel B were acquired in the last time point in the whole brain (the white box, 200×200×175 mm) and averaged from the voxels with most gray and white matter content. While the ¹H-MRSI data were acquired with spatial resolution of ~(5 mm)³, the nominal voxel dimension for ²H-MRSI were ~(12.5 mm)³.

Fig. 6 |. Fitting of time courses from averaged regional MRSI maps after ²H-Glc ingestion:

While the exponential fits of ¹H-MRS data (panel A) showed 18% faster decay (smaller constant of the decay – tau, M=M0^(−t/tau)+c) in the gray (GM, blue) than in the white matter (WM, red), the linear fits of ²H-MRS data (panel B) yielded only minimal differences between GM and WM.

Fig. 7 |. Voxel-wise fitting of Glu₄ and Glx₄ time-courses obtained with high time resolution.

MRSI data were acquired in one participant with a time-resolution of five (¹H-MRSI, panel A.) and 6 minutes (²H-MRSI, panel B.) after peroral administration of ²H-Glc. Maps demonstrate the signal decay of protons on the 4^th carbon position in the glutamate molecule (Glu₄, ¹H-MRSI) and in Glx₄ (²H-MRSI). Voxel-wise linear regression was applied to Glu₄ and Glx₄ time-courses. Respective slopes tend to be higher in the gray matter, which suggests a higher glutamate turnover in the gray than in the white matter. The p-values refer to the significance of the slopes. Spectral postprocessing of ¹H-MRSI included channel-wise L2-regularization for suppression of unwanted lipid signals.