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Functional changes in rat kidneys during the induced ischemic injury and recovery phases were explored using multimodal autofluorescence and light scattering imaging. The aim is to evaluate the use of noncontact optical signatures for rapid assessment of tissue function and viability. Specifically, autofluorescence images were acquired in vivo under 355, 325, and 266 nm illumination while light scattering images were collected at the excitation wavelengths as well as using relatively narrowband light centered at 500 nm. The images were simultaneously recorded using a multimodal optical imaging system. The signals were analyzed to obtain time constants, which were correlated to kidney dysfunction as determined by a subsequent survival study and histopathological analysis. Analysis of both the light scattering and autofluorescence images suggests that changes in tissue microstructure, fluorophore emission, and blood absorption spectral characteristics, coupled with vascular response, contribute to the behavior of the observed signal, which may be used to obtain tissue functional information and offer the ability to predict posttransplant kidney function.

The potential of NADH autofluorescence as an in vivo intrinsic optical signature to monitor tissue metabolism is well recognized and supported by experimental results mainly in animal models. In this work, we propose a non-contact implementation of this method using large area excitation and employing a normalization method to account for non-metabolic signal changes. Proof of principle in vivo experiments were carried out using an autofluorescence imaging experimental system and a rat renal ischemia model. A hand-held fiber-optic probe was utilized to test the ability of the signal normalization method to address operational conditions associated with the translation of this method to a clinical setting. Preliminary pre-clinical in vivo test of the probe system was carried out using the same rat model.

We explore a method to quantitatively assess the ability of in vivo autofluorescence as a means to quantify the progression of longer periods of renal warm ischemia and reperfusion in a rat model. The method employs in vivo monitoring of tissue autofluorescence arising mainly from NADH as a means to probe the organ's function and response to reperfusion. Clinically relevant conditions are employed that include exposure of the kidney to ischemia on the order of tens of minutes to hours. The temporal profile during the reperfusion phase of the autofluorescence intensity averaged over an area as large as possible was modeled as the product of two independent exponential functions. Time constants were extracted from fits to the experimental data and their average values were found to increase with injury time.

Potentially transplantable kidneys experience warm ischemia, and this injury is difficult to quantify. We investigate optical spectroscopic methods for evaluating, in real time, warm ischemic kidney injury and reperfusion. Vascular pedicles of rat kidneys are clamped unilaterally for 18 or 85 min, followed by 18 or 35 min of reperfusion, respectively. Contralateral, uninjured kidneys serve as controls. Autofluorescence and cross-polarized light scattering images are acquired every 15 s using 335-nm laser excitation (autofluorescence) and 650+/-20-nm linearly polarized illumination (light scattering). We analyze changes of injured-to-normal kidney autofluorescence intensity ratios during ischemia and reperfusion phases. The effect of excitation with 260 nm is also explored. Average injured-to-normal intensity ratios under 335-nm excitation decrease from 1.0 to 0.78 at 18 min of ischemia, with a return to baseline during 18 min of reperfusion. However, during 85 min of warm ischemia, average intensity ratios level off at 0.65 after 50 min, with no significant change during 35 min of reperfusion. 260-nm excitation results in no autofluorescence changes with ischemia. Cross-polarized light scattering images at 650 nm suggest that changes in hemoglobin absorption are not related to observed temporal behavior of the autofluorescence signal. Real-time detection of kidney tissue changes associated with warm ischemia and reperfusion using laser spectroscopy is feasible. Normalizing autofluorescence changes under 335 nm using the autofluorescence measured under 260-nm excitation may eliminate the need for a control kidney.