Coronary Plaque Microstructure and Composition Modify Optical Polarization: A New Endogenous Contrast Mechanism for Optical Frequency Domain Imaging

Abstract

Objectives: This study aimed to evaluate whether polarimetry, performed using a modified optical frequency domain imaging (OFDI) system, can improve the assessment of histological features relevant to characterizing human coronary atherosclerosis.

Background: The microscopic structure and organization of the arterial wall influence the polarization of the infrared light used by OFDI. Modification of the OFDI apparatus, along with recently developed image reconstruction methods, permits polarimetric measurements simultaneously with conventional OFDI cross-sectional imaging through standard intravascular imaging catheters.

Methods: The main coronary arteries of 5 cadaveric human hearts were imaged with an OFDI system capable of providing polarimetric assessment. Cross-sectional views of tissue birefringence, measured in refractive index units, and depolarization, expressed as the ratio of depolarized signal to total intensity, were reconstructed, together with conventional OFDI images. Following imaging, the vessels underwent histological evaluation to enable interpretation of the observed polarization features of individual tissue components.

Results: Birefringence in fibrous tissue was significantly higher than in intimal tissue with minimal abnormality (0.44 × 10^-3 vs. 0.33 × 10^-3; p < 0.0001). Birefringence was highest in the tunica media (p < 0.0001), consistent with its high smooth muscle cell content, cells known to associate with birefringence. In fibrous areas, birefringence showed fine spatial features and close correspondence with the histological appearance of collagen. In contrast, necrotic cores and regions rich in lipid elicited significant depolarization (p < 0.0001). Depolarization was also evident in locations of cholesterol crystals and macrophages.

Conclusions: Intravascular measurements of birefringence and depolarization can be obtained using conventional OFDI catheters in conjunction with a modified console and signal processing algorithms. Polarimetric measurements enhance conventional OFDI by providing additional information related to the tissue composition and offer quantitative metrics enabling characterization of plaque features.

Keywords: cholesterol crystals; collagen; optical coherence tomography; optical frequency domain imaging; polarized light.

FIGURE 1. Working Principle of PS-OFDI

Polarization-sensitive optical frequency domain imaging (PS-OFDI) is compatible with current intravascular optical frequency domain imaging catheters and enables measurement of tissue polarization properties simultaneously with the conventional reflection intensity. (A) In addition to the components of conventional optical frequency domain imaging, polarization-sensitive optical frequency domain imaging uses (B) a polarization modulator and detects the light reflected by the tissue along 2 orthogonal polarization states. The modulator alternates the polarization state of the light incident on the tissue between even and odd A-lines. Analyzing the spatial variation of the detected states allows reconstruction of birefringence (Δn) and depolarization (Dep). (C to E) Polarization-sensitive optical frequency domain imaging signals measured in vivo in the left circumflex coronary artery of a 52-year-old man. (C) Intensity (Int) of the reflection signal showing subsurface plaque morphology in a conventional logarithmic gray scale. (D) Birefringence in color hue, overlaid on the reflection signal, revealing regions and layers of distinct birefringence. Birefringence is displayed only in areas of low depolarization. (E) Depolarization in color hue, overlaid on the reflection signal, indicating zones of pronounced depolarization. Scale bar: 1 mm.

FIGURE 2. PS-OFDI of Fibroatheroma

Individual components of human atherosclerotic plaques and vessel wall ex vivo exhibit distinct polarization features in polarization-sensitive optical frequency domain imaging (PS-OFDI). (A) Reflection intensity (Int), (B) birefringence (Δn), (C) depolarization (Dep) of a fibroatheroma, and (D to F) matching histological sections, stained with trichrome (Tri), picrosirius red (PSR), and α-smooth muscle actin (SMA), respectively. (G to I) Magnified views of the regions of interest indicated in (D to F), respectively. Intimal tissue in segments of minimal pathology display low birefringence (yellow arrowhead). The media features increased birefringence (white arrowheads), caused most likely by the dense packing of smooth muscle cells. Birefringence in intimal fibrous regions corresponds closely to collagen content, assessed with picrosirius red histology. Red and yellow arrows indicate zones of higher or lower birefringence and picrosirius red signals, respectively. The necrotic core causes an abrupt depolarization (black arrows). For quantitative characterization of tissue polarization properties, segments of different tissue types were defined in each cross section, as indicated in the inset in (A) for the current example (see Figure 6 for tissue type legend). Scale bar in (A) measures 1 mm and applies to (A to F). Scale bars in (G to I) measure 250 µm.

FIGURE 3. PS-OFDI of Lipid-Rich Plaque Regions

Plaque regions characterized by increased lipid content by histological examination correspond to areas of depolarization in polarization-sensitive optical frequency domain imaging (PS-OFDI). (A) Reflection intensity (Int), (B) birefringence (Δn), (C) depolarization (Dep), and (D and E) matching histological sections, stained with trichrome (Tri) and picrosirius red (PSR), respectively. The first row displays fibrotic tissue, where the birefringence corresponds closely to picrosirius red staining (yellow and red arrows). The histological appearance of the early lesion in the second row suggests changes in the extracellular matrix organization and dispersed lipid, causing modest depolarization (black arrows). The third row demonstrates the pronounced depolarization caused by an aggregation of foam cells in a more advanced lesion. The high birefringence in the fibrous cap matches picrosirius red staining (red arrow). The last row shows depolarization caused by an advanced lesion with an extracellular lipid pool. Scale bar: 1 mm, applies to all panels.

FIGURE 4. PS-OFDI of Calcifications

Calcifications (white arrows) may exhibit low birefringence close to the lumen, but they depolarize incident light, and that prevents evaluation of birefringence at greater depths. (A) Reflection intensity (Int), (B) birefringence (Δn), (C) depolarization (Dep), and (D and E) matching histological sections, stained with trichrome (Tri) and picrosirius red (PSR), respectively. In lipid-rich plaques (first row), calcifications tend to depolarize more strongly than when surrounded by fibrous tissue (second and third rows). Scale bar: 1 mm, applies to all panels.

FIGURE 5. Polarization Signatures of Cholesterol Crystals and Macrophages

(A) Reflection intensity (Int), (B) birefringence (Δn), (C) depolarization (Dep), and (D) and (E) matching histological sections, stained with trichrome (Tri) and CD 68, respectively. (E) Magnified views of the regions of interest indicated in D. (A1 to E1) Irregularly oriented cholesterol crystals (black arrows) cause depolarization. (A2 to E2) Crystals oriented parallel to the lumen (black arrow) still depolarize, but they also create zones of high birefringence (inset). (A3 to E4) Accumulations of macrophages (black arrows) cause depolarization, without entirely randomizing the detected polarization states. Scale bar: 1 mm for (A to D); 125 µm for (E).

FIGURE 6. Quantitative Characterization of Tissue Polarization Properties

(A) Median and quartile birefringence (Δn) values in regions with depolarization ≤0.2 differed significantly across tissue types (p < 0.0001, 1-way analysis of variance). Intima with minimal abnormality had the lowest birefringence, and the tunica media had the highest. Less complicated lesions featured slightly higher birefringence than fibrous tissue (significant) and advanced lesions with defined lipid pools and necrotic cores (insignificant). (B) Median and quartile depolarization (Dep) values differed significantly from each other (p < 0.0001, Kruskal-Wallis). Lipid-rich tissues and calcifications showed significantly higher depolarization than did fibrous tissues or the layers of the normal vessel wall. Whisker lengths correspond to approximately 2.7 times the standard deviation, and data points are drawn as outliers if outside this range.