Assessing metabolism and injury in acute human traumatic brain injury with magnetic resonance spectroscopy: current and future applications

Abstract

Traumatic brain injury triggers a series of complex pathophysiological processes. These include abnormalities in brain energy metabolism; consequent to reduced tissue pO₂ arising from ischaemia 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, creatine and choline. N-acetylaspartate, a marker of neuronal mitochondrial activity related to ATP, is reported to be lower in patients with TBI than healthy controls, and the ratio of N-acetylaspartate/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. ³¹P MRS detects high energy phosphate species, which are the end-products of cellular respiration: adenosine triphosphate (ATP) and phosphocreatine. ATP is the principal form of chemical energy in living organisms, and phosphocreatine (PCr) is regarded as a readily mobilised reserve for its replenishment during periods of high utilisation. The ratios of high energy phosphates are thought to represent a balance between energy generation, reserve and use in the brain Additionally, the chemical shift difference between Pi and PCr enables calculation of intracellular pH. ¹³C 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 (TCA) 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.