Intravascular near-infrared fluorescence molecular imaging of atherosclerosis: toward coronary arterial visualization of biologically high-risk plaques

Abstract

New imaging methods are urgently needed to identify high-risk atherosclerotic lesions prior to the onset of myocardial infarction, stroke, and ischemic limbs. Molecular imaging offers a new approach to visualize key biological features that characterize high-risk plaques associated with cardiovascular events. While substantial progress has been realized in clinical molecular imaging of plaques in larger arterial vessels (carotid, aorta, iliac), there remains a compelling, unmet need to develop molecular imaging strategies targeted to high-risk plaques in human coronary arteries. We present recent developments in intravascular near-IR fluorescence catheter-based strategies for in vivo detection of plaque inflammation in coronary-sized arteries. In particular, the biological, light transmission, imaging agent, and engineering principles that underlie a new intravascular near-IR fluorescence sensing method are discussed. Intravascular near-IR fluorescence catheters appear highly translatable to the cardiac catheterization laboratory, and thus may offer a new in vivo method to detect high-risk coronary plaques and to assess novel atherosclerosis biologics.

Figure 1

Catheter prototype for intravascular sensing of NIR fluorescence signals. (a) The NIRF catheter consists of a 0.36-mm∕0.014-in. floppy radiopaque tip with a maximum outer diameter of 0.48 mm∕0.019 in.. The arrow highlights the focal spot (40±15 μm) for the 90-deg arc-sensing catheter at a distance of 2±1 mm (arrow). (b) Phantom experiment to measure NIR light attenuation in the presence of whole blood. Plaque (P) consists of 1% Intralipid plus India ink 50 ppm plus AF750 (an NIR fluorochrome, concentration 300 nmol∕L); tissue (T: fibrous cap) consists of polyester casting resin plus titanium dioxide plus India ink; a container (gray shaded area) was filled with fresh rabbit blood or saline. The catheter was immersed in fresh rabbit blood and positioned at variable distance (D) from a fluorescent phantom representing the plaque (P). To mimic the presence of a fibrous cap, a solid tissue phantom of thickness T was interposed between the plaque and the lumen. (c) Plot of detected NIRF signal as a function of distance D in presence of blood compared to saline, showing only modest attenuation by blood. Inset, fluorescence signal decay in saline at distance of up to 10 mm. (d) Plot of the detected NIRF signal in blood in the presence of a tissue phantom (T) of thickness 500 μm shows modest NIRF signal attenuation (<35%) vs the case in (c) where T=0. Reproduced by permission from Ref. .

Figure 2

Real-time, in vivo fluorescence sensing of inflammation in atherosclerotic vessels through blood: (a) repeated real-time manual pullback of the catheter performed in each iliac artery over 20 s (dotted arrow); (b) in rabbits that received a first-generation protease-activatable NIRF agent 24 h beforehand, strong NIRF signal was detected on pullback in iliac artery lesions [target-to-background ratio (TBR)] average of 6.8±1.9 (vs 1.3±0.3 in saline-injected control animals, p<0.05) and (c) and (d) ex vivo paired white light and NIRF images of atherosclerotic arteries. Augmented NIRF signal was evident in plaques from rabbits injected with the cysteine protease activatable agent. Minimal autofluorescence was noted in saline-injected control animals (data not shown). RIA, right iliac artery; LIA, left iliac artery; Ao, aorta. Modified by permission from Ref. .

Figure 3

Correlative histopathology and fluorescence microscopy of atheroma sections of a rabbit injected with the cathepsin-activatable NIRF agent and following intravascular NIRF sensing. (a) left, hematoxylin and eosin (H&E); middle, NIRF microscopy (pseudocolored red); right, merged NIRF and 500-nm autofluorescence (pseudocolored green) image. Images acquired at ×100 magnification. (b) Left, abundant NIRF signal from activation of the protease-activatable agent (red) overlying diffuse autofluorescence (green); middle and right, immunoreactive macrophages and cathepsin B respectively colocalize with the NIRF signal. Images acquired at ×200 magnification. Modified by permission from Ref. . (Color online only.)