Dynamic contrast-enhanced optical imaging of in vivo organ function

Abstract

Conventional approaches to optical small animal molecular imaging suffer from poor resolution, limited sensitivity, and unreliable quantitation, often reducing their utility in practice. We previously demonstrated that the in vivo dynamics of an injected contrast agent could be exploited to provide high-contrast anatomical registration, owing to the temporal differences in each organ's response to the circulating fluorophore. This study extends this approach to explore whether dynamic contrast-enhanced optical imaging (DyCE) can allow noninvasive, in vivo assessment of organ function by quantifying the differing cellular uptake or wash-out dynamics of an agent in healthy and damaged organs. Specifically, we used DyCE to visualize and measure the organ-specific uptake dynamics of indocyanine green before and after induction of transient liver damage. DyCE imaging was performed longitudinally over nine days, and blood samples collected at each imaging session were analyzed for alanine aminotransferase (ALT), a liver enzyme assessed clinically as a measure of liver damage. We show that changes in DyCE-derived dynamics of liver and kidney dye uptake caused by liver damage correlate linearly with ALT concentrations, with an r2 value of 0.91. Our results demonstrate that DyCE can provide quantitative, in vivo, longitudinal measures of organ function with inexpensive and simple data acquisition.

Fig. 1

DyCE Imaging System. System configuration for dynamic ICG imaging.

Fig. 2

DyCE analysis in mice before and after induced liver damage. (a) Pseudo-colored simple anatomical map images generated using merged frames at 2.4 s (green), 5.2 s (blue), and 35 s (red) after ICG injection with manually selected ROIs overlaid. (b) Time courses of selected regions in healthy mice showing good repeatability. Plot to right shows averages with standard error, normalized to the maximum kidney intensity. (c) Liver and kidney time courses 28 h after mice 1 and 2 received an IP injection of ${\mathrm{CCl}}_4$ to induce acute liver damage. Right: averages of treated mice with standard error, normalized to the maximum kidney intensity. Arrows indicate crossover point of liver and kidney signal. Traces in treated animals no longer cross within 40 s.

Fig. 3

Quantification of the changes seen after ${\mathrm{CCl}}_4$ injection. (a) DyCE data following ICG bolus injection in a mouse before (left) and 54 h after ${\mathrm{CCl}}_4$ injection (right), after removal of breathing artifacts. The post-${\mathrm{CCl}}_4$ mouse has decreased signal in the liver and increased signal in the kidney after 180 s (Video 1). (b) The average time courses for the liver and kidney (using organ-specific ROIs) before and 54 h after ${\mathrm{CCl}}_4$ injection in the same mouse (normalized to maximum kidney intensity in each case). Dotted line shows 180 s time point. (c) The liver signal to kidney signal ratio before and 54 h after ${\mathrm{CCl}}_4$ injection. (Video 1, MPG, 1.8 MB) [URL: http://dx.doi.org/10.1117/1.JBO.17.9.XXXXXX.1.]

Fig. 4

Comparison of DyCE measures to liver enzyme concentrations during transient liver damage. (a) DyCE-derived dynamic coefficient (Dc). (b) Alanine aminotransferase (ALT) measurements from blood samples taken during each imaging session. IP injections of ${\mathrm{CCl}}_4$ to induce liver damage were given immediately following the first imaging session. Mice were then imaged 3 more times each over the following 217 h. (c) Larger Dc values correlate with higher levels of plasma ALT. The third time point for mouse 4 was not obtained owing to failed injection. A linear fit applied to the Dc and ALT data gives an $r^2$ value of 0.91.