Deuterium metabolic imaging (DMI) for MRI-based 3D mapping of metabolism in vivo

Abstract

Currently, the only widely available metabolic imaging technique in the clinic is positron emission tomography (PET) detection of the radioactive glucose analog 2-¹⁸F-fluoro-2-deoxy-d-glucose (¹⁸FDG). However, ¹⁸FDG-PET does not inform on metabolism downstream of glucose uptake and often provides ambiguous results in organs with intrinsic high glucose uptake, such as the brain. Deuterium metabolic imaging (DMI) is a novel, noninvasive approach that combines deuterium magnetic resonance spectroscopic imaging with oral intake or intravenous infusion of nonradioactive ²H-labeled substrates to generate three-dimensional metabolic maps. DMI can reveal glucose metabolism beyond mere uptake and can be used with other ²H-labeled substrates as well. We demonstrate DMI by mapping metabolism in the brain and liver of animal models and human subjects using [6,6'-²H₂]glucose or [²H₃]acetate. In a rat glioma model, DMI revealed pronounced metabolic differences between normal brain and tumor tissue, with high-contrast metabolic maps depicting the Warburg effect. We observed similar metabolic patterns and image contrast in two patients with a high-grade brain tumor after oral intake of ²H-labeled glucose. Further, DMI used in rat and human livers showed [6,6'-²H₂]glucose stored as labeled glycogen. DMI is a versatile, robust, and easy-to-implement technique that requires minimal modifications to existing clinical magnetic resonance imaging scanners. DMI has great potential to become a widespread method for metabolic imaging in both (pre)clinical research and the clinic.

Fig. 1. In vivo ²H NMR.

Deuterium NMR spectra acquired without localization from (A) rat brain in vivo at 11.7 T before infusion of any ²H-labeled substrate, (B) rat brain after infusion of [6,6′-²H₂]glucose in vivo, (C) rat brain after infusion of [6,6′-²H₂]glucose postmortem, and (D) rat brain after infusion of [²H₃]acetate in vivo. (E) ²H NMR spectrum acquired from human brain at 4 T, 60 min after oral administration of [6,6′-²H₂]glucose. (F) ²H NMR spectra from rat (top, black) and human (bottom, gray) liver after intravenous and oral administration of ²H-labeled glucose, respectively. Each spectrum shown represents 1 min of signal averaging. While the lower magnetic field strength used for human studies leads to a proportionally lower spectral resolution [compare (B) and (E)], the sparsity of the ²H NMR spectra in combination with the absence of large water or lipid signals still allows robust signal identification and quantification (see also Figs. 2 to 5).

Fig. 2. DMI in rat brain after [6,6′-²H₂]glucose infusion.

(A) MRI-based 3D rendering of a RG2 glioma rat with radiofrequency (RF) coil positioning for MRI, shimming (green loops), and DMI (yellow loop). The color-coded map overlaid onto the 3D MR image shows the lactate/Glx ratio. (B) Contrast-enhanced T₁-weighted MRI in which the tumor appears hyperintense relative to normal brain tissue. The yellow circle shows the DMI surface RF coil position. As RF transmission and reception sensitivity profiles are highest close to the surface coil, the DMI field of view is effectively limited to the RF coil diameter. Localized ²H NMR spectra extracted from an 11 × 11 × 11 MRSI grid with 2 × 2 × 2 mm³ voxels are overlaid onto the MR image. The data shown were acquired between 60 and 90 min after starting the [6,6′-²H₂]glucose infusion. Spectra shown in red had sufficient signal for further processing, whereas those in blue were discarded. Experimental ²H NMR spectra from (C) normal-appearing brain and (D) tumor lesion, with the spectral fitting results for the total spectrum, individual peaks, and fitting residue shown below. Note that localized spectra have higher spectral resolution than nonlocalized spectra (Fig. 1), a consequence of the smaller 8-μl volumes’ inherent increased magnetic field homogeneity. (E to H) DMI maps of ²H-labeled glucose and metabolites, based on spectral fitting of individual peaks, expressed in millimolar (E to G). The ratio of lactate/Glx (H) represents the Warburg effect, characterized by high glycolysis and low oxidative metabolism. Scale bars, 5 mm (B and E).

Fig. 3. DMI in human brain after oral [6,6′-²H₂]glucose intake.

(A) 3D MRI overlaid with ²H MR spectra from a 3D MRSI data set (9 × 13 × 11 matrix) with 20 × 20 × 20 mm³ nominal spatial resolution, acquired between 65 and 90 min after oral [6,6′-²H₂]glucose administration. (B) Typical ²H NMR spectrum from a single MRSI voxel overlaid with a spectral fit (red line) indicating the peaks from water, glucose, Glx, and lactate. (C) Grid (2 × 3) extracted from the MRSI with data color-coded by the Glx intensity. (D) 3D maps of ²H-labeled cerebral glucose and (E) Glx levels in millimolar, extrapolated from the 3D MRSI to the 3D MRI grid. Note the seemingly lower level of Glx in areas corresponding to the ventricles.

Fig. 4. DMI visualizes the Warburg effect in a patient with GBM after oral [6,6′-²H₂]glucose intake.

(A) Clinical MR images acquired as standard-of-care in a patient diagnosed with GBM in the right frontal lobe. MR images include (from left to right) T₂-weighted fluid-attenuated inversion recovery (T₂W FLAIR), T₁-weighted contrast-enhanced imaging (T₁W CE), susceptibility-weighted imaging (SWI), and diffusion-weighted imaging (DWI). The patient (man, 63 years old) had undergone subtotal resection of the lesion 9 months before the DMI study. He was undergoing treatment on an experimental protocol involving nivolumab or placebo combined with the standard-of-care chemoradiation with temozolomide, followed by adjuvant temozolomide. (B and C) T₂W MRI and overlaid DMI maps in two slices that contain the tumor lesion. The MRI and DMI data shown in (C) correspond to the slice position of the clinical MRI scans shown in (A). DMI maps show a homogeneous distribution of ²H-glucose across the slices but lower levels of ²H-labeled Glx and a higher concentration of ²H-labeled lactate in the tumor lesion compared to normal-appearing brain. (D) ²H NMR spectra from selected locations depicted in the T₂W MR image, including tissue (1 and 3) within the lesion as seen on T₁W CE, (2) from normal-appearing occipital lobe, and (4) containing cerebrospinal fluid from the left lateral ventricle. (E) 3D illustration of combined MRI and DMI of the lactate/Glx ratio representing the spatial distribution of the Warburg effect. The DMI maps show glucose metabolism patterns similar to the RG2 glioma model (Fig. 2)

Fig. 5. DMI of liver glycogen.

(A) Respiratory-gated, axial MR image of rat abdomen with the liver delineated in red. Scale bar, 10 mm. (B) ²H NMR spectra extracted from a 3D ²H MRSI grid with 4 × 4 × 4 mm³ resolution. Spectra in red have sufficient signal for DMI quantification, whereas those in blue were not considered further. (C) ²H MR spectrum from the voxel indicated in green in (B). (D) Color-coded map of ²H-labeled glucosyl units (in millimolar) after 120 min of [6,6′-²H₂]glucose infusion. (E) Axial MR image of the liver acquired with the RF coil placed on the lateral side of the abdomen. RF coils’ position and size indicated in gray (¹H) and white (²H). Dashed yellow lines emphasize one dimension of the 3D oblique MRSI grid parallel to the ²H surface coil. The red line delineates the liver. (F) MR image acquired parallel to the RF coils, with the ²H RF coil (white) superimposed. Scale bar, 50 mm. As with the rat brain data, the ²H surface coil’s sensitivity limits DMI data acquisition to positions close to and within the diameter of the coil. (G) ²H NMR spectra extracted from a 3D ²H MRSI grid with 25 × 25 × 25 mm³ resolution, 60 min after oral [6,6′-²H₂]glucose intake. Spectra in red have sufficient signal for quantification, whereas those in blue were not considered further. (H) ²H MR spectrum from the voxel indicated in green in (G), with the fitted peaks of ²H-labeled glycogen + glucose and water.