Quantitative endovascular fluorescence-based molecular imaging through blood of arterial wall inflammation

Purpose: To evaluate an author-developed normalization algorithm for quantitative imaging of optical molecular probes through blood and to assess, in the rat aorta after focal aortic injury, the feasibility of measuring protease activity by using this method. Materials and Methods: This study was performed according to a protocol approved by the institutional animal care committee. A Monte Carlo simulation was used to determine the pair of near-infrared (NIR) dyes that was best suited for the normalization algorithm. The authors tested the correction method in vitro and in vivo by injecting free dye mixtures intramurally in the aortas of four rats. The potential clinical utility was then evaluated by applying the method to the endovascular measurement of protease activity in a rat model of focal aortic injury. Results: When the Monte Carlo simulation was used in the normalization algorithm, it was predicted that the intensities of signals from two NIR dyes would vary ±3% across 1 mm of blood compared with the intensity of the raw fluorochrome signal, which would vary ±60%. This result was validated in vitro. Endovascular imaging of free dye collections revealed that clinically relevant, uncontrollable differences in the amount of blood intervening between the imaging catheter and the dye collection precipitated dramatic variations in raw NIR fluorescence. However, use of the correction method resolved these variations such that the measured signal intensity correlated well with the different dye concentrations in the different animals. Moreover, endovascular imaging of the focal aortic injury model enabled successful measurement of enzyme activity in the walls of the rat aortas. Conclusion: The authors implemented a correction method for quantitative real-time endovascular imaging of fluorescence that enables one to resolve the attenuating effects of blood on NIR signal.