Acute kidney injury (AKI) is a medical problem, which are often met in the clinic, as much as 5-10% of hospitalized patients suffer from AKI and 30% of those in the critical care unit¹. Ischemia/reperfusion injury (IRI) is common cause of AKI² occurring often in relation to e.g. kidney transplantation, sepsis, heart failure, local/systemic hypoperfusion or hypovolemic shock. Currently creatinine clearance and blood urea nitrogen (BUN) is the most widely used screening method to asses kidney function. These measurements suffer from insensitivity in single kidney failure, where the residual function of the contra-lateral kidney will lead to underestimation of injury severity. Recently hyperpolarized [1,4-13C2]fumarate has shown great promise as a sensitive marker of renal tubular necrosis in a folic acid induced AKI rat model³. Here we wish to utilize [1,4-13C2]fumarate to investigate renal necrosis in a unilateral IRI rat model. Eight wistar rats were included in the study. Rats were subjected to unilateral renal ischemia for 40 min followed by reperfusion for 24 hours (n=8). A midline incision in the abdomen was made and the left renal artery was carefully dissected. I/R-I was induced by clamping the left artery with a non-traumatic clamp for 40 min. Temperature and respiration was monitored during the surgical procedure. A tail vein catheter was inserted for injection of hyperpolarized [1,4-13C2]fumarate. Temperature, arterial oxygen saturation and respiration rate were monitored throughout the experiment. Each animal received one injection of 1.1 mL hyperpolarized [1,4-13C2]fumarate over 10 s. The experiments were performed in a 3 T clinical MR system (GE Healthcare) equipped with a dual tuned 13C/1H volume rat coil. A slice-selective 13C IDEAL spiral sequence was used for hyperpolarized [1,4-13C2]fumarate imaging acquiring images every 5 s initiated 10 s after the start of injection. The acquisition was done with flip angle=10º, 11 IDEAL echoes and one initial spectrum per IDEAL encoding, TR/TE/ΔTE=100 ms/1.45 ms/1.45 ms, FOV=80x80 mm2, 5 x 5 mm real resolution and an axial slice thickness of 15 mm covering both kidneys. After MRI scanning, kidneys were stored for further biochemical analysesI/R-I resulted in statistically significantly increased plasma creatinine, BUN and decreased creatinine clearance, indicating AKI (figure 1). A clear difference between the contra-lateral control kidney (CL) and I/R-I kidney was seen in both the fumarate signal and malate signal (figure 2), indicating a healthy CL kidney and an I/R-I kidney experiencing tubular necrosis. This is supported by histological examinations showing typical signs of necrosis (figure 3). Fumarate to-malate turnover is believed to be seen because of general cellular membrane disruption during necrosis, leading to release of fumarase from the mitochondria to the renal interstitial space, circulation and urine. Therefore we measured fumarase activity in specific cellular compartments and plasma which supports the notion that [1,4-13C2]malate is only traceable under necrotic conditions. More precisely under control conditions fumarase activity is only seen in either mitochondrial purification or total tissue purification. In the I/R-I kidney, fumarase is released to the tissue and plasma, which leads to a decreased fumarase activity in mitochondria (figure 4). In conclusion we were able to measure fumarate to-malate turnover in the I/R-I kidney, which was correlated with renal necrosis and impairment in functional kidney measurements.