Development of in vivo Raman spectroscopy of atherosclerosis

Abstract

Cardiovascular disease is the primary cause of mortality in developed nations. Current diagnostic techniques are not able to provide the information that is related to the majority of associated complications. Raman spectroscopy, which is capable of providing a detailed chemical analysis of biological tissue, has previously been shown to be a useful method for diagnosing atherosclerosis. However, widespread clinical implementation has been prohibited by the lack of optical fiber probes which can provide remote access to small diameter vessels. To this end, we have developed a new spectroscopic model and a novel Raman probe. The model interprets Raman spectra of intact tissue in terms of its morphology. The spectrally identifiable morphological structures include collagen and elastin fibers, cholesterol crystals, calcium mineralizations, adipocytes, crystals containing 3-carotene, foam cells, necrotic core, and smooth muscle cells. This model prospectively classifies atherosclerotic tissue into three categories: non-atherosclerotic, non-calcified plaque, and calcified plaque, with >94% accuracy. Furthermore, this model has the potential ability to identify the vulnerable atherosclerotic plaques whose rupture accounts for the majority of myocardial infarctions. By studying the distribution of Raman light in tissue, we have designed, constructed, and tested small diameter, high throughput optical fiber Raman probes which employ a modular micro-optical filter configuration to remove the intense background generated in the fibers. The probes have been extensively tested during peripheral vascular surgery, providing real-time disease diagnosis for the first time. These in vivo investigations demonstrate the clinical applicability of Raman spectroscopy and have also provided the first identification of vulnerable plaques with this technique.

(cont.) We have shown that plaque vulnerability is spectrally identified by determining the collagen, foam cell/necrotic core, calcification, cholesterol, and hemoglobin content of the lesion. This type of analysis may eventually lead to a diagnostic technique capable of staging atherosclerotic lesions, thereby providing a method to optimize therapeutic measures. In addition, the small diameter Raman probes have applicability for studying other diseases such as breast, oral, and gastrointestinal cancer.