Comprehensive intravascular imaging of atherosclerotic plaque in vivo using optical coherence tomography and fluorescence lifetime imaging

Abstract

Comprehensive imaging of both the structural and biochemical characteristics of atherosclerotic plaque is essential for the diagnosis and study of coronary artery disease because both a plaque's morphology and its biochemical composition affect the level of risk it poses. Optical coherence tomography (OCT) and fluorescence lifetime imaging (FLIm) are promising optical imaging methods for characterizing coronary artery plaques morphologically and biochemically, respectively. In this study, we present a hybrid intravascular imaging device, including a custom-built OCT/FLIm system, a hybrid optical rotary joint, and an imaging catheter, to visualize the structure and biochemical composition of the plaque in an atherosclerotic rabbit artery in vivo. Especially, the autofluorescence lifetime of the endogenous tissue molecules can be used to characterize the biochemical composition; thus no exogenous contrast agent is required. Also, the physical properties of the imaging catheter and the imaging procedures are similar to those already used clinically, facilitating rapid translation into clinical use. This new intravascular imaging catheter can open up new opportunities for clinicians and researchers to investigate and diagnose coronary artery disease by simultaneously providing tissue microstructure and biochemical composition data in vivo without the use of exogenous contrast agent.

Figure 1

Schematic diagram of the fully integrated IV-OCT/FLIm system for structural and biochemical imaging simultaneously. IV-OCT/FLIm system consisted of OCT subsystem (yellow), FLIm subsystem (red), an intravascular imaging part (green) and data acquisition and display part (light orange). Black, purple, blue, green and red solid lines: optical fiber; gray solid lines: electric wire for transferring data; gray dashed lines: electrical wire for triggering; IRF: instrument response function; Ch1: channel 1; Ch2: channel 2; Ch3: channel 3.

Figure 2

Optical rotary joint (ORJ) and multimodal imaging catheter. (a) Schematic diagram and photograph of ORJ. L, lens; DM, dichroic mirror; SMF, single-mode fiber; MMF, multi-mode fiber; DCF, double-clad fiber. (b) Photograph of multimodal imaging catheter with a 1 cent coin. The outer diameter of the imaging sheath is 1.04 mm. (c) Intensity plot without (dark gray) and with (gray) imaging window and transmittance plot (black) of imaging window. Transmittance of imaging window is more than 75% at OCT and FLIm light.

Figure 3

Phantom experiment validating the co-registration between OCT and FLIm. (a) Schematic diagram of the phantom consisting of glass tubes filled with milk and standard fluorophores. (b) Representative cross-sectional image of phantom. Grayscale and pseudo color represent OCT image and fluorescence lifetime, respectively. (c) Intensity graph of OCT and fluorescence along the rotational angle. (d) OCT enface map of maximum projection. The two-dimensional fluorescence lifetime maps obtained from channel (e) 1, (f) 2, and (g) 3. Scale bar: 1 mm.

Figure 4

In vivo OCT/FLIm imaging results in a rabbit aorta. (a) Longitudinal OCT/FLIm images of a rabbit aorta. Grayscale and pseudo color represent the OCT image and fluorescence lifetime, respectively. (b) Representative cross-sectional image of normal aorta. Structures of normal aorta, including media (M) and adventitia (A) are clearly visible. (c) Representative cross-sectional image of atherosclerotic plaque. It shows lipid-rich plaque with thickened media and diffuse borer (yellow arrowheads) suggesting presence of lipid in OCT, and it also contains normal area (red arrowheads). (d) 3D rendering longitudinal cut-view image. The fluorescence lifetime is overlaid on the luminal surface of the OCT 3D rendering. The fluorescence lifetime in (a–d) was measured on channel 2. The 2D fluorescence lifetime maps measured on channel (e) 1, (f) 2, and (g) 3. The horizontal and vertical directions are the pullback direction and rotational direction, respectively. (h) Comparison of the averaged fluorescence lifetime between the normal aorta and atherosclerotic plaque. Results are presented as means ± standard deviation. *P < 0.001, by unpaired t-test. Scale bars: 1 mm. M, media; A, adventitia.

Figure 5

Histopathological staining and cross-sectional OCT/FLIm images of (a–d) normal aorta and (e–h) atherosclerotic plaque. (a, e) Oil red O (ORO) stained tissue that represents lipids (red color). (b,f) Immunohistochemistry with RAM 11 antibody (brown color) that represents macrophages. (c,g) Picrosirius red (PSR) stained tissue that represents collagen types I (yellow-orange birefringence) and III (green birefringence). (d,h) Cross-sectional OCT/FLIm images of rabbit aorta. Intensity-weighted fluorescence lifetime is expressed as rings from channels 1, 2 and 3 (in turn from inner to outer) outside the OCT images. Morphological landmarks, such as side branch (red arrows) and plaque shape (red asterisks), show the match between the histopathological sections and the in vivo OCT/FLIm cross-sectional images. Fluorescence lifetime colormap is the same as given in Fig. 4. Scale bars: 1 mm.