Cross-Polarization Optical Coherence Tomography in Evaluation of Atherosclerotic Plaque Structure

The cross-polarization optical coherence tomography (CP OCT) technique allows for tissue structure imaging by registration of backscattered radiation in initial and orthogonal polarizations and further comparison of the obtained images. Spatial structure of collagen fibers gives rise not only to backscattering of probing radiation, but also to evolution of its polarization state during propagation through the tissue (depolarization). Collagen fibers of the fibrous cap play a key role in determining the stability of atherosclerotic plaques. Inflammation observed in atherosclerosis is the principal mechanism of collagen fibers disorganization, therefore, the assessment of the depolarizing properties of the fibrous cap can characterize an atherosclerotic plaque as being “vulnerable” to rupture.

The aim of the study was to evaluate CP OCT efficiency to determine the condition of collagen fibers of an atherosclerotic plaque fibrous cap, which characterize its “vulnerability”.

Materials and Methods. 54 post mortem samples of intact human aorta and aorta with atherosclerotic plaques at different stages were studied. The study involved 150 CP OCT-images in which the value of OCT signal in orthogonal polarization was used to evaluate the ratio of organized and disorganized by inflammation collagen fibers within the fibrous cap. For histological imaging comparison we used hematoxylin-eosine and picrosirius red staining with evaluation in polarized light. Numerical analysis of CP OCT-images was used as a complementary tool for visual assessment.

Results. We showed CP OCT to have significant advantages over the traditional OCT in the assessment of atherosclerotic plaque. In orthogonal CP OCT-image one can differentiate the main structural components of a plaque: a fibrous cap and a lipid core. The thickness of the fibrous cap in the orthogonal polarization image correlates with the thickness of the fibrous cap measured from histological preparations (correlation coefficient r=0.991, p<0.0001). The integral depolarization factor which characterizes the functional status of collagen fibers of fibrous cap has been used to differentiate the “vulnerable” atherosclerotic plaque. Its value within the range of 0.08–0.12 with 95% probability indicates low content of highly organized collagen in the fibrous cap, and hence, its tendency to rupture.

Conclusion. CP OCT is capable of assessing the functional state of collagen fibers of fibrous cap of an atherosclerotic plaque with high probability. Numerical analysis of CP-OCT images provides identification of “vulnerable” atherosclerotic plaques.

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