Urinary Oxygen Tension Measurement in Humans Using Magnetic Resonance Imaging

Rationale and Objectives: Renal medullary hypoxia is frequently implicated in renal dysfunction, and urinary oxygen tension (PO₂) in the renal pelvis can be used as a surrogate for the adjacent renal medullary oxygenation. We sought to assess the feasibility of magnetic resonance (MR) quantification of urinary PO₂ in humans. Materials and Methods: The longitudinal relaxivity (R1) of fluids is linearly related to PO₂, allowing MR quantification of urinary PO₂. We imaged urine phantoms with a range of PO₂ using a real-time saturation recovery T2-prepped single-shot fast spin-echo sequence to calibrate urine R1 values to PO₂. Following institutional review board approval, we imaged the urinary bladders of seven healthy subjects while they were breathing room air and the renal pelvis of nine healthy subjects while they were breathing room air or 100% oxygen via facemask. The renal pelvic urine PO₂ was compared before, during, and after 100% oxygen breathing. Results: Our phantom study confirmed that urine R1 is linearly related to PO₂: PO₂ (mm Hg) = (R1 - 0.2253 s^-1)/(2.61e^-4 s^-1/mm Hg). The mean bladder urine PO₂ ranged from 23 to 45 mm Hg among the seven subjects. Successful MR measurements of renal pelvic urine PO₂ were obtained in seven of nine healthy subjects. Following 100% O₂ breathing, the renal pelvic urine PO₂ showed a significant mean increase of 29 mm Hg (P < .05). Conclusions: We show that MR quantification of urinary PO₂ is feasible. Noninvasive renal pelvic urine PO₂ determinations could serve as a valuable indirect measure for renal medullary oxygenation, allowing for clinical investigations of the role of renal medullary hypoxia in renal disease.