Gold nanorods can be internalized by macrophages (an important early cellular marker in atherosclerosis and cancer) and used as an imaging contrast agent for macrophage targeting. Objective of this study is to compare two-photon luminescence (TPL) properties of four aspect ratios of gold nanorods with surface plasmon resonance at 700, 756, 844 and 1060 nm respectively. TPL from single nanorods and Rhodamine 6G particles was measured using a laser-scanning TPL microscope. Nanorod TPL emission spectrum was recorded by a spectrometer. Quadratic dependence of luminescence intensity on excitation power (confirming a TPL process) was observed below a threshold (e.g., <1.6 mW), followed by photobleaching at higher power levels. Dependence of nanorod TPL intensity on excitation wavelength indicated that the two-photon action cross section (TPACS) is plasmon-enhanced. Largest TPACS of a single nanorod (12271 GM) was substantially larger than a single Rhodamine 6G particle (25 GM) at 760 nm excitation. Characteristics of nanorod TPL emission spectrum can be explained by plasmon-enhanced interband transition of gold. Comparison results of TPL brightness, TPACS and emission spectrum of nanorods can guide selection of optimal contrast agent for selected imaging applications.

Objectives We hypothesized that bright spots in intravascular optical coherence tomography (IVOCT) images may originate by co-localization of plaque materials of differing indices of refraction (IR). To quantitatively identify bright spots, we developed an algorithm that accounts for factors including tissue depth, distance from light source, and signal-to-noise ratio. We used this algorithm to perform a bright spot analysis of IVOCT images, and compared these results with histologic examination of matching tissue sections. Background Although bright spots are thought to represent macrophages in IVOCT images, studies of alternative etiologies have not been reported. Methods Fresh human coronary arteries (n=14 from 10 hearts) were imaged with IVOCT in a mock catheterization laboratory then processed for histologic analysis. The quantitative bright spot algorithm was applied to all images. Results Results are reported for 1599 IVOCT images co-registered with histology. Macrophages alone were responsible for only 23% of the bright-spot positive regions, though they were present in 57% of bright-spot positive regions. Additional etiologies for bright spots included: cellular fibrous tissue (8%), interfaces between calcium and fibrous tissue (10%), calcium and lipid (5%), and fibrous cap and lipid pool (3%). Additionally, we showed that large pools of macrophages in CD68 histology sections correspond to dark regions in comparative IVOCT images; this is due to the fact that a pool of lipid-rich macrophages will have the same index of refraction as a pool of lipid and thus will not cause bright spots. Conclusions Bright spots in IVOCT images are correlated to a variety of plaque components that cause sharp changes in the index of refraction. Algorithms that incorporate these correlations may be developed to improve the identification of some types of vulnerable plaque and allow standardization of IVOCT image interpretation.

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