Magnetic resonance imaging of glutamate

Abstract

Glutamate, a major neurotransmitter in the brain, shows a pH- and concentration-dependent chemical exchange saturation transfer effect (GluCEST) between its amine group and bulk water, with potential for in vivo imaging by nuclear magnetic resonance. GluCEST asymmetry is observed ∼3 p.p.m. downfield from bulk water. Middle cerebral artery occlusion in the rat brain resulted in an ∼100% elevation of GluCEST in the ipsilateral side compared with the contralateral side, predominantly owing to pH changes. In a rat brain tumor model with blood-brain barrier disruption, intravenous glutamate injection resulted in a clear elevation of GluCEST and a similar increase in the proton magnetic resonance spectroscopy signal of glutamate. GluCEST maps from healthy human brain were also obtained. These results demonstrate the feasibility of using GluCEST for mapping relative changes in glutamate concentration, as well as pH, in vivo. Contributions from other brain metabolites to the GluCEST effect are also discussed.

Figure 1

pH dependence and sensitivity advantage of GluCEST. (a) CEST z-spectra of 10 mM Glu at varying pH and 37°C at 7T show the CEST effect at 3 p.p.m. downfield to bulk water resonance. (b) GluCEST asymmetry curves from different pH solutions corresponding to (a). The overlay of simulated GluCEST (dotted line) on the curve corresponding to pH 7 is also shown. (c) This plot shows dependence of GluCEST on pH (3–8). The fitted line shows linear dependence of GluCEST in physiological pH range (6.0–7.4). (d) Image from a phantom of 10 mM glutamate (pH = 7.0, 37°C) prepared in PBS with the localization voxel indicated. (e) ¹HMRS PRESS water suppressed spectrum (TR = 20 s, TE = 16 ms, 4 averages) from the single voxel shown in the image (d). The three resonances correspond to two –CH₂ groups and one –CH group of glutamate. (f) Water ¹H resonance spectra obtained while saturating at ± 3 p.p.m. as well as their difference spectrum. (g) The rescaled difference spectrum from (f). The difference spectrum represents the 3 p.p.m. point on the CEST asymmetry spectrum. The ratio of the CEST difference spectrum to Glu –CH₂ resonance at 2.3 p.p.m. is ~700.

Figure 2

GluCEST images at 7T of a phantom consisting of test tubes with different concentrations of Glu solutions (pH 7.0) immersed in a beaker containing PBS. All the experiments were performed at 37 °C (a) Shows the GluCEST contrast color-coded on the original CEST image (3 p.p.m.), acquired with application of saturation pulse train with B_1rms = 155 Hz (3.6 μT) for 2 s. Color bar represents GluCEST contrast in percentage. (b) Approximately linear dependence of GluCEST effect on Glu concentration with a slope of 0.6% mM⁻¹ Glu. (c) GluCEST dependence on B_1rms and duration of the saturation pulse. (d,e) CEST images of a phantom consisting of test tubes with solution of different metabolites at their physiological concentrations and pH 7 [Glu (10 mM), GABA (2 mM), NAA (10 mM), Gln (2 mM), Asp (2 mM), taurine (2 mM), Cr (6 mM) and MI (10 mM)] immersed in a beaker containing PBS. The CEST contrast color-coded on the original CEST images (3 p.p.m.), which show the substantial CEST contrast from Glu (~6%), and ~1% from GABA, < 0.5% from Cr and no contrast from other metabolites. Color bar represents CEST contrast in percentage.

Figure 3

GluCEST maps of healthy and ischemic rat brain. (a) Rat brain anatomic images, (b) GluCEST maps, (c) and (d) show B₀, and B₁ maps of the corresponding brain slices, respectively. Clear differences of GluCEST contrast in GM and WM regions can be seen. (e) Rat brain anatomic proton image. (f,g) The GluCEST maps of the rat brain acquired at 1 h and 4.5 h following the induction of stroke. (h) The GluCEST contrast vs. time after MCAO at regions of interest within the rectangular areas shown in (g). In the ipsilateral side GluCEST is almost doubled at 4.5 h after occlusion. (i) The GluCEST asymmetry plots from the contralateral side (blue curve) and ipsilateral side (red curve) at 4.5 h after occlusion. At 3.00 p.p.m., it is evident that there is almost 100% increase in GluCEST. (j) The contralateral and ipsilateral sides of MCAO rat brain data acquired at 4.7T after 4 h post occlusion and 1 h post mortem (Reprinted by permission from Macmillan Publishers Ltd: Nature Medicine , Figure 4, copyright (2003)).

Figure 4

GluCEST images and ¹HMRS of rat brain with tumor before and after injection of Glu. (a) Rat brain anatomic proton image demonstrating the tumor and a rectangular region of interest. (b) B₀ map of the rat brain, which shows about ± 0.5 p.p.m. variation. (c) B₁ map of the rat brain showing fairly uniform B₁ field. (d) GluCEST maps (color bar represents GluCEST contrast in percentage) of the rat brain acquired pre-injection (BSL) and at 1, 1.5 and 2.0 h following the injection of Glu solution. Gradual elevation in the GluCEST is evident over a period of 2 h. (e) Stacked stimulated echo acquisition mode (STEAM) localized ¹HMRS data acquired at baseline and at 2 h following the injection. Clear elevation of Glu –CH₂ (2.3 p.p.m.) and –CH peak (3.75 p.p.m.) amplitude can be seen in the spectra. (f). Time course of GluCEST and the 2.3 p.p.m. Glu peak integral normalized with the values of pre-injection from regions of interest within the rectangular areas shown in (a). A strong correlation between GluCEST and ¹HMRS data can be seen.

Figure 5

GluCEST imaging and ¹HMRS from a healthy human brain acquired at 7T. (a) Anatomic proton image of the axial slice. (b) B₁ and B₀ corrected GluCEST contrast map (Color bar represents GluCEST contrast in percentage). (c) Map of distribution volumes (DVs) of metabotropic Glu receptor subtype 5 from a PET image (Reprinted by permission of the Society of Nuclear Medicine: J Nucl Med , Figure 2, copyright (2007)). The GluCEST map and the PET image show similar distribution pattern of Glu in brain, which is higher in GM compared to WM. (d) B₀ map and (e) B₁ map corresponding to the slice in (a). (f) ¹HMRS spectra obtained from regions of interest of GM and WM as shown in (a). These spectra show higher amplitude of Glu –CH₂ resonance (2.3 p.p.m.) and –CH resonance (3.75 p.p.m.) in GM than that in WM. (g) Saturation pulse duration dependence of GluCEST data from human brain. The GluCEST reaches a maximum at a saturation duration of ~1 s and decreases with further increase in duration. (h,i) z-spectra and corresponding asymmetry plots from human GM and WM regions. The GluCEST at 3 p.p.m. (dotted line in figure i) is ~11% from GM and ~7% from WM.