Optical molecular imaging of atherosclerosis using nanoparticles: shedding new light on the darkness.

The application of optical nanoparticles in cardiovascular research is increasing because of the high spatiotemporal resolution and high sensitivity of optical techniques as compared with other imaging platforms. The major cause of cardiovascular events is atherosclerosis, which is a chronic inflammation of the arterial wall. Interestingly, the composition rather than the size of nonstenotic atherosclerotic plaques and severe plaques with >90% stenosis are indicators for high-risk vulnerability to rupture and acute cardiovascular events. Optical techniques may be highly suitable for discriminating, at subcellular resolution, the different stages of plaque progression by targeting bright and nontoxic optical nanoparticles toward distinct molecular epitopes in order to distinguish vulnerable from stable atherosclerotic plaques. Several optical techniques including two-photon laser scanning microscopy (TPLSM), optical coherence tomography (OCT), and photoacoustic imaging (PAI) have been applied for (in vivo) characterization of atherosclerotic plaques, in addition to investigate their feasibility in the clinical setting. Optical nanoparticles, however, have predominantly been used in optical molecular imaging of tumors, but their application in cardiovascular research is increasing. In this review, we first describe shortly the basics of the mentioned optical techniques. Then, we detail on the most-extensively studied optical nanoparticles and relatively new optical nanoparticles that hold promise for in vivo applications in atherosclerosis research.