Near infrared spectroscopy system for quantitative monitoring of renal hemodynamics and oxygenation in rats

Regulation of renal hemodynamics and oxygenation is complex and its detailed understanding is crucial to improve therapeutic procedures for kidney diseases like acute kidney injury. For the challenging task of monitoring renal hemodynamics and oxygenation in rats, we designed a continuous wave (cw) multispectral near-infrared spectroscopy setup. A fiber probe with a source fiber and eight detection fibers is placed on the ventral surface of the exposed rat kidney in vivo, and an additional source fiber is positioned on the dorsal surface. Nine wavelengths from 658 nm to 1060 nm are used to have sufficient redundancy for reliable quantification of hemoglobin concentration, oxygen saturation of hemoglobin, and tissue water content. To investigate both, the surface layer and deeper tissues, the setup alternates between reflection and transmission at a rate of 10 Hz. Our system relies on spatially resolved reflection and transmission, and multispectral analysis to differentiate absorption from scattering. Monte-Carlo Simulations for a layered tissue structure are used as a model for quantitative characterization of the renal cortex and medulla. Renal parameters are monitored during baseline conditions and during dedicated pathophysiologically relevant interventions including arterial occlusion and changes of inspiratory gas mixture. Together with invasive probes, which monitor arterial blood pressure, renal perfusion and tissue oxygenation, a detailed picture of renal hemodynamics and oxygenation in several pathophysiological conditions is acquired. This detailed information can serve as a quantitative reference to other methods such as MRI.