Traumatic brain injury (TBI) is a leading cause of death and disability in people under age 45 and can lead to cognitive impairments, mood disorders, and neurodegenerative diseases. Although the precise pathophysiological mechanisms in TBI are not yet completely understood, TBI is known to cause perturbations in the energy metabolism of the brain, which may be linked to injury severity and progression. However, current tests of metabolic activity such as cerebral blood flow, arterial metabolite sampling, and microdialysis either are indirect markers of energy use or are highly invasive. Recently, magnetic resonance spectroscopy of hyperpolarized ¹³C-enriched substrates has provided a new way to measure metabolic processes in vivo. In this work, we investigate the use of ¹³C-pyruvate imaging as a direct, non-invasive method for identifying traumatic brain injury and studying its effects on energy metabolism.Traumatic brain injury was induced in the left parietal lobe of healthy adult male rats (3 Sprague Dawley and 1 Wistar, 233–256g body weight) using a controlled cortical impact (CCI) device (Pittsburgh Precision Instruments, Pittsburgh, PA). After a left-sided craniotomy was performed to expose the dura, the 5-mm round impactor tip was accelerated to 5 m/s with a vertical deformation depth of 2.0 mm and an impact duration of 50 ms. At 3.5-4 hours post injury, hyperpolarized ¹³C-pyruvate imaging was performed using a clinical GE 750w 3T MR scanner (GE Healthcare, Waukesha, WI) and a doubly tuned (¹H/¹³C) quadrature coil (∅ = 50 mm, USA Instruments Inc., Aurora, OH). The rats were anesthetized with 1.5-2% isoflurane in 1.5 L/min oxygen. They were injected in a tail vein with ~1.1 mmol/kg of ~140 mM [1-¹³C]pyruvate, which was hyperpolarized to ~50% liquid state polarization via dynamic nuclear polarization using a SpinLab polarizer (Research Circle Technology, Niskayuna, NY). Spectrally-resolved imaging of the brain was initiated 30 s after injection using a phase-encoded free-induction decay (FID) chemical shift imaging (CSI) sequence with the following parameters: single axial (animal coronal) 8 mm slice centered on the injury, 40x40 mm² field-of-view, 16x16 matrix, 5000 Hz spectral width, 19 s acquisition time. As controls, ¹³C-pyruvate imaging was performed on uninjured healthy adult male rats (4 Sprague Dawley and 2 Wistar) in the same way either 1-4 days before injury or independently. Pyruvate, lactate, alanine, and bicarbonate peaks were phased, baseline-corrected, and integrated to create a voxel-by-voxel map of relative metabolite concentrations. Comparisons were made between ROIs of the cerebral cortex ipsilateral and contralateral to the injury.We found the ratio of lactate to bicarbonate signal (R_lb = Lac/Bic) to be sensitive to traumatic brain injury. Figure 1 shows typical metabolic maps of pyruvate, lactate, and bicarbonate in the brain following CCI injury. At the site of the injury, there is a noticeable locus of relatively high lactate signal and relatively low bicarbonate signal. Figure 2 plots results calculated from signals integrated across ROIs from each side of the cerebral cortex. Following injury, R_lb = 8.3 ± 0.5 in the ipsilateral hemisphere was higher compared to 4.5 ± 1.0 in the contralateral hemisphere (p = 0.029). Prior to injury, no significant difference in R_lb existed between the two sides. The relative difference in R_lb between sides (ΔR = (R_lb,ipsi-R_lb,contra)/R_lb,contra) was -0.06 ± 0.04 prior to injury and 1.1 ± 0.3 after the injury (p = 0.046). Additionally, brain histology performed 28 days after injury demonstrated strong Fluoro Jade-B (FJB) positive staining of cortical regions of the ipsilateral hemisphere, indicative of dead and dying neurons.The primary injury in TBI is followed by a number of secondary insults that can occur over the course of minutes to days. One known effect is mitochondrial damage, which results in a disruption of oxidative phosphorylation specifically manifested as decreased activity of pyruvate dehydrogenase complex (PDH), the enzyme complex that links glycolytic with oxidative metabolism by converting pyruvate to acetyl-CoA and CO₂ [1, 2]. As a result, there is a decrease in oxidative over anaerobic metabolism such that pyruvate, the end product of glycolysis, does not enter the (oxidative) TCA cycle, but rather is converted to lactate. Our results support this finding, as the relative increase in lactate signal and decrease in bicarbonate (formed from CO₂) at the injury site suggest a transition to anaerobic respiration.For the first time, hyperpolarized metabolic imaging with ¹³C-pyruvate was applied to TBI. The presented data demonstrate a significant change in brain energy metabolism following CCI injury in rats. Future studies will apply this methodology to the noninvasive evaluation of severity and progression of traumatic brain injury.