Fig 1.
DDE RE-MRS sequence incorporating LASER localization and CPMG modules that mitigate the cross-terms arising from internal gradients.
The 8 ms spectrally-selective excitation pulse comprised two narrow bands targeting the NAA singlet resonance at 2.02 ppm and the mI multiplets at 3.51 and 3.61 ppm. This DDE-CPMG module was followed by refocusing and adiabatic LASER pulses. All spectrally-selective pulses were designed using the SLR algorithm (Pauli et al, 1991). The correlated DDE gradients are shown in blue and green. Crusher gradients are shown in grey and slice-selective gradients in black. Acq = acquisition.
Fig 2.
RF pulse shape used to selectively address mI and NAA (left) and its frequency spectrum (right).
The bandwidths of the 8 ms pulse are sufficiently narrow to efficiently convert all magnetization aligned initially along Mz into Mxy magnetization within the desired bands, but not elsewhere. This results in a cellular-specific mode of excitation.
Fig 3.
Direct comparison between RE MRS and STEAM spectra collected for different echo times, performed at 21.1T using identical RF and spectral width components.
All spectra are shown in magnitude mode; notice the strong water-induced distortion in the mI region arising from the STEAM sequence at short TE, as well as the small SNR for the targeted metabolites for STEAM at TE = 160ms (a similar effective TE as used for the RE MRS experiment), where it had to be multiplied by 3 to be visible on this graph. Experiments were acquired under identical conditions in terms of voxel sizes (100 μL) and positions, over 10 min (256 scans) acquisition times. Similar results were observed over N = 6 independent tests. We ascribe the poor water suppression performance of the STEAM sequence to limitations in the sensitivity-optimized surface coil 900 MHz setup used, to accurately impart the large-angle nutations needed over the relevant voxel.
Fig 4.
Analyzing the coupling between the DDE gradient waveform and potentially deleterious internal gradients modulations brought about by RF spin echoes.
(a) Temporal evolution of the gradient waveform for the DDE filter (blue) and for the internal gradient waveform modulated by N = 2 CPMG (red) and N = 6 CPMG modulation. These waveforms account for the effects of refocusing pulses. (b) Corresponding dephasing spectra for the three waveforms. Notice that, for N = 6 CPMG, the internal gradient and the DDE spectra are minimally overlapping, suggesting a strong suppression of their cross-term. By contrast, there is significant overlap between the N = 2 CPMG and DDE spectra, explaining the potential vulnerability of the latter sequence towards susceptibility-induced cross-terms which may corrupt the desired information.
Fig 5.
Examples of the voxel localization and of spectra obtained in representative DDE RE-MRS in vivo experiments.
(a) Coronal (upper panel) and sagittal (lower panels) views of the brain via a low-resolution T2-weighted water-based imaging experiment, applied without (left most images) and with (middle panels) a LASER localization module identical to the one that was subsequently applied in DDE RE-MRS. A clearer view of the voxel is presented in the right-hand panel of (a), where the voxel image is overlaid on a darkened image of the brain. (b) Representative spectrum arising from the 125 μL voxel shown in (a), collected in ~6.5 min and showing the targeted resonances, together with small residuals from water and from a nearby taurine resonance.
Fig 6.
Fractional anisotropies (FAs) derived from diffusion tensor spectroscopy analyses at 21.1T.
As a result of the large voxel targeted in the rat brain, the averaging of diffusion directors over many orientations results in a negligible macroscopic anisotropy for both metabolites. The lower variability measured for NAA predominantly reflects the higher effective SNR for this singlet resonance (compared to the multiplet mI). Results reflect measurements on n = 4 animals.
Fig 7.
Probing micro-architectural features of neurons and astrocytes by in vivo DDE RE-MRS on the cell-specific metabolites NAA and mI.
(a) Illustrative spectral set collected at 21.1 T in 160 scans (~6.5 min) as a function of the angle ψ between the diffusion-sensitizing gradients. Further details on these experiments are given in Methods. (b,c) Peak oscillations observed for the two metabolites in a series of experiments collected from N = 6 animals, together with the mean oscillations fitted from these experiments.
Fig 8.
Fitting the experimental data to the randomly oriented infinite cylinder model.
(a,b) Fitting landscape for NAA and mI, respectively, showing the residuals as function of different intrinsic diffusivity and diameter, respectively. (c,d) Plots of the best fit model parameters alongside the experimental data for NAA and mI, respectively. Notice that the astrocytic processes appear to be somewhat larger than the neurites.