Assessing Metabolism and Injury in Acute Human Traumatic Brain Injury with Magnetic Resonance Spectroscopy: Current and Future Applications

Abstract

Traumatic brain injury (TBI) triggers a series of complex pathophysiological processes. These include abnormalities in brain energy metabolism; consequent to reduced tissue pO₂ arising from ischemia or abnormal tissue oxygen diffusion, or due to a failure of mitochondrial function. In vivo magnetic resonance spectroscopy (MRS) allows non-invasive interrogation of brain tissue metabolism in patients with acute brain injury. Nuclei with "spin," e.g., ¹H, ³¹P, and ¹³C, are detectable using MRS and are found in metabolites at various stages of energy metabolism, possessing unique signatures due to their chemical shift or spin-spin interactions (J-coupling). The most commonly used clinical MRS technique, ¹H MRS, uses the great abundance of hydrogen atoms within molecules in brain tissue. Spectra acquired with longer echo-times include N-acetylaspartate (NAA), creatine, and choline. NAA, a marker of neuronal mitochondrial activity related to adenosine triphosphate (ATP), is reported to be lower in patients with TBI than healthy controls, and the ratio of NAA/creatine at early time points may correlate with clinical outcome. ¹H MRS acquired with shorter echo times produces a more complex spectrum, allowing detection of a wider range of metabolites.³¹ P MRS detects high-energy phosphate species, which are the end products of cellular respiration: ATP and phosphocreatine (PCr). ATP is the principal form of chemical energy in living organisms, and PCr is regarded as a readily mobilized reserve for its replenishment during periods of high utilization. The ratios of high-energy phosphates are thought to represent a balance between energy generation, reserve and use in the brain. In addition, the chemical shift difference between inorganic phosphate and PCr enables calculation of intracellular pH.¹³ C MRS detects the ¹³C isotope of carbon in brain metabolites. As the natural abundance of ¹³C is low (1.1%), ¹³C MRS is typically performed following administration of ¹³C-enriched substrates, which permits tracking of the metabolic fate of the infused ¹³C in the brain over time, and calculation of metabolic rates in a range of biochemical pathways, including glycolysis, the tricarboxylic acid cycle, and glutamate-glutamine cycling. The advent of new hyperpolarization techniques to transiently boost signal in ¹³C-enriched MRS in vivo studies shows promise in this field, and further developments are expected.

Keywords: 13C MRS; 1H MRS; 31P MRS; biomarker; energy metabolism; trauma; traumatic brain injury.

Figure 1

Simplified schematic of major energy pathways in the brain includes glycolysis, which takes place in the cytosol and produces pyruvate, which enters mitochondria and is converted into acetyl-CoA that enters the tricarboxylic acid (TCA) cycle. Alternatively, pyruvate can stay in the cytosol and is converted into lactate that is exported out of the cell. The pentose phosphate pathway takes place in the cytosol and is an alternative pathway that can be upregulated after injury; it is an important source of NADPH used to produce the reduced form of glutathione (GSH) for preventing oxidative stress. This figure was originally published by Carpenter et al. (4). © 2014 The Authors. Published by Elsevier B.V. Open Access under a CC–BY license.

Figure 2

¹H MRI (T2W axial slice) and ¹H magnetic resonance spectroscopy chemical shift imaging (CSI) (echo time 135 ms, TR 2,200 ms, 3 averages, 4:41 min acquisition time, 200 ms Hanning filter) of healthy control (A), and patient with acute severe traumatic brain injury after craniectomy (B) acquired with a 12 channel ¹H volume coil on a Siemens 3 T scanner, data analysis performed with Siemens Syngo software. Panel (A) demonstrates the position of the selected voxel (blue square, ^†), represented in panel (C), within the CSI grid (hidden). (C) ¹H spectrum of 10 mm × 10 mm × 15 mm voxel ^† from healthy volunteer (A). (D) ¹H spectrum of 8 mm × 8 mm × 15 mm blue square voxel ^‡ of patient (B), within the CSI grid (hidden). Metabolite peaks are annotated in panels (C,D): Cr, creatine; Cho, choline; NAA, N-acetylaspartate, chemical shift on the x-axis in parts per million, signal intensity on y-axis using arbitrary units. Unpublished images by Tonny V. Veenith, courtesy of the Wolfson Brain Imaging Centre, Cambridge, UK.

Figure 3

¹H magnetic resonance spectroscopy (MRI) and ³¹P MRS chemical shift imaging (CSI) (echo time 2.30 ms, TR 4,000 ms, 25 mm voxels, 30 averages, 18 min acquisition time, 200 ms Hanning filter) of patient with acute severe traumatic brain injury acquired with a ³¹P birdcage volume coil (PulseTeq, Chobham, Surrey, UK) on a Siemens 3 T scanner, data analysis performed with Siemens Syngo software. (A) Axial FLAIR image demonstrating decompressive craniectomy on patient’s right side with associated regions of high signal in that hemisphere. (B) Axial T2 HASTE acquired with ¹H channel on a ³¹P coil overlaid with ³¹P MRS CSI grid of 8 × 8, 25 mm cubed voxels. Each voxel contains the spectrum from its volume. (C,D) ³¹P spectrum from voxel ^† (D) and ^‡ (C) of image 5A with phosphorus peaks annotated. Species can be identified by their chemical shift on the x-axis in parts per million. PE, phosphomonoesters; Pi, inorganic phosphate; GPE, glycerol 3-phosphorylethanolamine; GPC, glycerol 3-phosphorylcholine; PCr, phosphocreatine; ATP, adenosine triphosphate. Signal intensity on y-axis using arbitrary units. Unpublished images by the authors, courtesy of the Wolfson Brain Imaging Centre, Cambridge, UK.

Figure 4

(A) Example of a ¹³C surface coil (Rapid Biomedical GmBH, Rimpar, Germany) with flexible design, allowing it to come in closer contact to the patients’ head. Here, it is positioned to sample the occipital lobe. The coil contains a ¹³C channel and ¹H channel within its housing. (B) Example of a ³¹P birdcage volume coil (PulseTeq Ltd., Chobham, Surrey, UK), which can be opened, allowing to access a patient’s head. The coil also contains a ¹H channel for imaging to allow spectral localization.

Figure 5

Simplified schematic of different metabolites and processes in the brain that can be interrogated using ¹H MRS, ³¹P MRS, ¹³C MRS, and DNP ¹³C MRS. ¹H and ³¹P MRS show endogenous metabolites; ¹³C MRS requires exogenous ¹³C-enriched substrate, while for DNP ¹³C MRS the exogenous ¹³C-enriched substrate is hyperpolarized before administration, transiently boosting ¹³C signal. Pathways include uptake of glucose that is metabolized via glycolysis in the cytosol [with a low yield of ATP per mole of glucose consumed] producing pyruvate. Pyruvate can enter mitochondria where it is converted into acetyl-CoA that enters the TCA cycle. Pyruvate remaining in the cytosol can be converted into lactate, simultaneously recycling NADH into NAD⁺ allowing glycolysis to continue. The rate of glucose uptake and glycolysis can be interrogated with ¹³C MRS (glucose and lactate appearance) whereas the relative flux of “anaerobic” metabolism vs. aerobic mitochondrial metabolism can be measured with DNP ¹³C MRS (lactate vs. ${\text{HCO}}_{\text{3}}^{-}$) and ¹H MRS (lactate). The TCA cycle drives the mitochondrial electron transport chain for high-yield ATP synthesis. The rate of the TCA cycle can be calculated by the rate of appearance of ¹³C labeled glutamate (Glu) (¹³C MRS) and ATP produced measured with ³¹P MRS (γ-ATP, β-ATP, and Pi). Neuronal integrity and mitochondrial function can be measured indirectly by detection of NAA with ¹H MRS (and ¹³C MRS). Neuronal–glial coupling is represented by Glu–glutamine (Gln) cycling detected by ¹³C MRS, whereas total combined Glu and Gln that may be raised in pathological excitotoxicity can be measured with ¹H MRS. Cell membrane integrity and damage and turnover may be represented by ¹H MRS (choline and lipid) and ³¹P MRS (PME/PDE ratio), which also can detect the balance and consumption of high-energy phosphates (ATP, PCr, and Pi). Further details of the above, and other MRS-detectable molecules (including creatine, myoinositol, glycogen, and nicotinamide-adenine dinucleotides), can be found in the text. Abbreviations: ADP, adenosine diphosphate; ATP, adenosine triphosphate; Cr, creatine; DNP, dissolution dynamic nuclear polarization; GABA, gamma-aminobutyric acid; NAA, N-acetylaspartate; MRS, magnetic resonance spectroscopy; NAD⁺, nicotinamide adenine dinucleotide oxidized form; NADH, nicotinamide adenine dinucleotide reduced form; PCr, phosphocreatine; PDE, phosphodiester; PME, phosphomonoester; Pi, inorganic phosphate; PPP, pentose phosphate pathway; TCA, tricarboxylic acid.