Advances in Lasers and Electro Optics Part 16 pptx

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Applied Optics, 42 , 4023-4030. 31 Polarization-Sensitive Optical Coherence Tomography in Cardiology Wen-Chuan Kuo Institute of Electro-optical Science and Technology, National Taiwan Normal University Taiwan 1. Introduction Atherosclerotic vascular disease is a common cause of morbidity and mortality in developed countries (Arroyo & Lee, 1999). In particular, the rupture of atherosclerotic plaques is the most common event initiating acute ischemic heart disease (Shah, 2003). Therefore, it is crucial to detect vulnerable coronary atheromatous plaques prior to their rupture or erosion to prevent irreversible myocardial damage. Autopsy studies have identified several histological characteristics of these vulnerable plaques, such as a large lipid pool, thin fibrous cap (<65 μm), and activated macrophages near the fibrous cap (Falk et al., 1995). Therefore, modalities capable of visualizing the vessel wall might help in detecting lesions with high risks for acute events (Pasterkamp et al., 2000; Peters et al., 1994). There are several plaque imaging modalities. The oldest and most widely used technology is X-ray angiography, which can detect narrowing of the coronary blood vessels. The first imaging technique to demonstrate the benefits of imaging inside the arterial wall is intravascular ultrasound (IVUS). However, the current resolution is not sufficient to visualize the thin fibrous caps and small disruptions within the intimal and medial dissections. In the 1980s, coronary angioscopy, which allows direct visualization of the surface color and superficial morphology of atherosclerotic plaque, thrombus, neointima, and stent struts, was introduced. However, it cannot help in the assessment of subsurface lesions. Other proposed techniques include electron beam computed tomography (EBCT), magnetic resonance imaging (MRI), or positron emission tomography (PET); these are noninvasive screening tools that do not subject the patient to catheterization. In addition to the aforementioned techniques, which are merely a selection of the imaging modalities currently used in vivo or that are in the validation stage, the use of optical techniques for biomedical imaging is gaining considerable attention. This is largely due to the potential of optical techniques to provide high-resolution imaging without the need for ionizing radiation and associated risks. Optical coherence tomography (OCT), which is based on a low-coherence interferometer, has emerged as a rapid, non-contact and noninvasive, high-resolution imaging tool (Huang et al., 1991). From the mid-1990s, the ability of intravascular OCT to provide high-resolution (10–20 μm) cross-sectional images of both in vitro human aorta and coronary arteries has been demonstrated (Brezinski et al., 1996; Fujimoto et al., 1995). The resolution of OCT images was up to 10 times better than that of conventional ultrasound, MRI, and computed tomography (CT) (Jang et al., 2002; Yabushita et al., 2002). Therefore, using OCT, small Advances in Lasers and Electro Optics 736 structural details (such as the width of intimal caps and the presence of fissures in atherosclerotic plaques (Bresinski et al., 1997) could be resolved and intramural collections of lipid within the intima of a vessel wall could be detected (Brezinski et al., 1996; Fujimoto et al., 1995). Furthermore, the objective OCT image criterion for risk-stratifying plaque characterization has been established on the basis of the intrinsic optical properties of a typical plaque, whose constituents are lipid, calcium, and fibrous tissue (Bresinski et al., 1997; Jang et al., 2002; Stamper et al., 2006; Tearney et al., 2006; Yabushita et al., 2002). On this basis, OCT has a detection sensitivity and specificity of 71%–79% and 97%–98% for fibrous plaques, 95%–96% and 97% for fibrocalcific plaques, and 90%–94% and 90%–92% for lipid-rich plaques, respectively (Tearney et al., 2006; Yabushita et al., 2002). Moreover, OCT has also been shown to quantify plaque macrophage content (Tearney et al., 2003) in lipid- rich plaques and to assess the success of intracoronary stent implantation in patients with coronary artery disease during percutaneous intra-arterial procedures (Bouma et al., 2003). At present, a company, LightLab Imaging, is targeting the cardiovascular market using commercializing intravascular OCT technology by providing dedicated imaging wires and occlusion balloon catheters. In general, OCT images are obtained from measurements of the echo time delay and the intensity of the backscattered light from a specimen. Further, OCT employs the inherent differences in the index of refraction, rather than enhancement with dyes, to differentiate tissue types. However, since the plaque components are heterogeneous, they may sometimes generate reflected signals that confuse or obscure the identity of these components; multiple scattering by the cap also creates difficulties in identifying the plaque due to the diffuse nature of the plaque border (Stamper et al., 2006). Polarization-sensitive OCT (PS-OCT), a functional mode of OCT, combines the advantages of OCT with additional image contrasts obtained by using the birefringence of the specimen as a contrast agent. Many biological tissues have a microscopic fibrous structure and so exhibit intrinsic birefringence. Moreover, changes in birefringence may indicate changes in functionality, structure, or viability of tissues in the early stages of the disease (de Boer et al., 1997). From 2004, we have been presenting the application of PS-OCT in human atherosclerosis, and have proposed approaches to characterize a plaque lesion on the basis of its birefringence property (Kuo et al., 2004; 2005; 2007). Moreover, in a recent study, our laboratory has assessed the arterial characteristics in human atherosclerosis by quantitatively determining both scattering and birefringence properties of vessel tissue from PS-OCT images (Kuo et al., 2007; 2008). Based on our findings, a quantitative PS-OCT image criterion for plaque characterization was constructed. In the remainder of this chapter, the results that we obtained using the PS-OCT system for imaging human atherosclerosis in vitro are summarized. We hope that our results, along with the results from other investigators, will construe a step forward in the application of PS-OCT imaging technology for clinically diagnosing atherosclerosis in the near future. 2. Principle of polarization-sensitive optical coherence tomography (PS-OCT) system The optical setup of the PS-OCT system used in this study is shown in Fig. 1. A collimated beam from a superluminescent diode (SLD) centered at a wavelength of 837 nm with a spectral bandwidth of 17.5 nm was used as a low-coherence light source in a Michelson interferometer. The axial resolution, which depends on the temporal coherence properties of Polarization-Sensitive Optical Coherence Tomography in Cardiology 737 the SLD), was 17 μm, while the lateral resolution (determined by the numerical aperture of the objective) was 10 μm. The incident beam was vertically polarized by a polarizer placed in the interferometer. A nonpolarization beam splitter (BS) was used to split the light wave into signal and reference beams. In the Michelson interferometer, a quarter-wave plate (QWP) with an azimuth angle set at 45° to the horizontal was used to focus the circular polarized light onto the examined specimen. On the other hand, the reference beam light was directed to a plane mirror mounted on a linear translator, which repetitively scanned the reference arm optical path length at a constant speed (1 mm/s). Another QWP (set at 22.5° to the horizontal) in the reference beam path rotated the polarization of the incident laser beam by 45°, thereby becoming the reflected reference beam. Fig. 1. Schematic of the conventional PS-OCT system: SLD, superluminescent diode; QWP, quarter wave plate; M, reference mirror; BS: beam slitter; PBS, polarized beam splitter; Dp and Ds, photo-detectors; PC, personal computer. The laser beam was reflected from the specimen and recombined with the reflected reference beam, and then both the horizontal (P wave) and vertical components (S wave) were independently directed toward two photodetectors Dp and Ds, respectively, using a polarized BS (PBS). From the ac coupling of the detector signals, the full interferometric signals were recorded. The amplitudes A i (z) and phases φ i (z) of the interference signals at different depths (z) were determined using the Hilbert transform; i = P and S represent the P and S polarization states, respectively. Three parameters—the backscatter intensity R(z), phase retardation )(zΦ , and fast-axis angle β(z) of a specimen—were calculated using the amplitude and phase of the interference signal (Hitzenberger et al., 2001): 22 )(A)(A~)( zzzR SP + (1) Advances in Lasers and Electro Optics 738 () )(/)(tan)( 1 zAzAz PS − =Φ (2) )180(2/1)( φ β Δ−°×=z (3) Here, P S φφ φ Δ= − is the phase difference between the P- and S-polarized heterodyne signals. Finally, 2D images of the above three parameters were obtained simultaneously by using repeated A-scan acquisition and mechanically scanning the specimens laterally through a focused 0.5 mW signal beam. In this experiment, the system sensitivity was obtained as 100 dB using a highly reflective plane mirror as the test object in this setup. The following section demonstrates our preliminary in vitro investigations of human aortic specimens using PS-OCT. In this study, we adapted a free-space PS-OCT system to precisely control the polarization state of the laser beam used in birefringent imaging. Several other groups have developed a high speed fiber-based PS-OCT system for application as a medical instrument in vivo (Guo et al., 2004; Park et al., 2001; 2004; Saxer et al., 2000). Moreover, an optically clear hemoglobin-based blood substitute has also been used to displace blood and enable OCT imaging with minimal patient discomfort (Villard et al., 2002). Further, several Fourier domain PS-OCT techniques (Park et al., 2005; Yamanari et al., 2006; Zhang et al., 2004) have been reported recently and have received considerable attention due to the high data acquisition rates (e.g., acquisition at 80 to 110 fps), which can eliminate motion artifacts and reduce ischemia during blood-free optical imaging. This allows for comprehensive scanning of long arterial segments during a short balloon occlusion or even 1 bolus liquid flush without occlusion. The first clinical study using this technology is being initiated in order to investigate vulnerable plaque hypothesis in a prospective multicenter manner. By combining the above features, PS-OCT can be used to measure reflected intensity, phase retardation, and fast-axis angle distributions, and thereby provide a greater contrast than is available with conventional OCT systems. 3. In vitro PS-OCT imaging of human atherosclerosis Specimens of the aorta with white or yellow plaque were obtained from heart transplant recipients at the National Taiwan University Hospital, Taiwan. The photographs of some specimens are shown in Fig. 2. The protocol was approved by the ethics committees of the National Taiwan University Hospital. The specimens were dipped in saline (4 ° C), cut into segments smaller than 1 × 1 cm, and examined. Each segment was mounted in a cuvette and moistened with a normal saline bath maintained at 37 ° C during the imaging. Only the intimal surface was exposed for PS-OCT imaging. The aortic specimen regions imaged with PS-OCT were marked for subsequent histopathological examination. After PS-OCT imaging, all the specimens were fixed in 10% neutral formalin for 24 h and then processed for standard paraffin embedding. Serial sections with 4 μm thickness were cut within the region of the PS-OCT examination, and stained with hematoxylin and eosin (H and E) for routine examination. The distribution of the collagen structure in the plaque lesion was also examined using Masson trichrome and picrosirius red staining procedures as well as a polarization microscope. Finally, the entire specimens were classified into normal vessel (N), lipid (L), fibrocalcific (C), and fibrous lesions (F) by a pathologist (J. J. Shyu). Polarization-Sensitive Optical Coherence Tomography in Cardiology 739 Fig. 2. Photographs of the aorta with white or yellow plaque. Fig. 3. Histological and PS-OCT images of a normal aortic wall (left column) and a plaque with lipid-loaded lesion (right column): (a) Histology (H and E; magnification ×100); (e) Histology (Masson’s trichrome; magnification ×40); (b), (f) Back-scattered intensity image; (c), (g) Phase retardation image (linear color scale degrees); (d), (h) Fast-axis angle image (linear color scale degrees). Advances in Lasers and Electro Optics 740 The PS-OCT images of representative specimens are shown in Figs. 3–6. The histological image of the normal vessel wall [Fig. 3(a)] showing a medial layer below the intima is compared with the PS-OCT image of the same specimen [Fig. 3(b)]. The signal-rich layer closest to the lumen is the intima. In the normal vessel wall, the phase retardation increases uniformly [Fig. 3(c)], and the pseudocolor distribution of the fast-axis angle signals is also uniform [Fig. 3(d)]. The pale area in Fig. 3(e) is a subintimal lipid-loaded region (L), which is morphologically composed mostly of the necrotic debris of foamy cells. Because of the paraffin embedding process, the solvent treatment removes the lipid from these lipid-loaded structures, which therefore appear as empty spaces in stained sections [Fig. 3(e)]. The corresponding PS-OCT image [Fig. 3(f)] reveals a decreased signal density under a thin homogeneous surface band. Moreover, the phase retardation and fast-axis angle signals are distributed in a slightly more random manner in the atherosclerotic lesion [Figs. 3(g) and 3(h), respectively] than in a normal vessel wall [Figs. 3(c) and 3(d)]. Moreover, the PS-OCT and histological images showed a plaque having small amounts of fibrous connective tissue (blue stain; black arrows) within a lipid-loaded area [Fig. 4(a)]. The signal density (arrows) was stronger, the backscattering signal was more heterogeneous [Fig. 4(b)], and the variation in the phase retardation [Fig. 4(c)] and fast-axis angle distribution [Fig. 4(d)] was more abrupt in the fibrous tissue than in the lipid-loaded region (L). Figure 4(e) shows a typically advanced plaque within the vascular intima; it is characterized by a necrotic lipid core covered by a thicker fibrous cap (CF ~250 μm; stained blue with Masson’s trichrome). Plaque development in the vascular wall involves a reorganization of intimal collagen fibers (Rekhter, 1999). Figure 4(f) shows a relatively deep Fig. 4. Histological and PS-OCT images of vessel wall with a small fibrous lesion in the lipid-loaded area (left column) and a lipid-loaded fibroatheroma with a thick fibrous cap (right column): (a), (e) Histology (Masson’s Trichrome; ×40); (b), (f) Back-scattered intensity image; (c), (g) Phase retardation image (linear color scale degrees); (d), (h) Fast-axis angle image (linear color scale degrees). [...]... to C shows significant difference in μs; Δn between C and Polarization-Sensitive Optical Coherence Tomography in Cardiology 745 Fig 8 Distributions of μs, geff, Δn, and β in normal vascular intima (N), lipid laden (L), fibrous (F), and fibrocalcific (C) plaques N, F and N, L and C, and L and F has significant differences; and β between C and N, F and N, L and N, and L and F has significant differences... potential to transform the powerful TPF technology for in vivo studies and clinical applications Recently, increasing interests have been focusing on the development of TPF endomicroscope with a small size which can go through the accessory port of a standard endoscope for in vivo and clinical studies while maintaining the TPF imaging ability similar to a standard TPF microscope Major challenges for TPF endomicroscopy... signals collection and fast beam scanning By replacing a bulky grating/lensbased pulse stretcher with a single photonic bandgap fiber for pulse prechirping, the system 764 Advances in Lasers and Electro Optics (a) (b) 20 µm 20 µm (c) (d) 20 µm 20 µm (e) (f) 20 µm 20 µm Fig 10 Typical depth-resolved TPF images obtained with the scanning all-fiber-optic endomicroscope bases on a GRIN lens: (a & b) Pig... fluorescence imaging have been under rapid development, aiming for non-invasive, high-resolution and high-speed imaging of tissue microstructures and assessment of tissue pathology in vivo The development of optical fibers (such as double-clad fibers and photonic bandgap fibers), miniature beam scanning mechanisms (such as MEMS scanners and fiber-optic resonant scanners) and miniature imaging optics (such... plaques are shown in Fig 6 The PS-OCT image showed a large sharply delineated, signal-rich area of heterogeneous backscattering [Fig 6(b) and 6(f)], as well as strong birefringence [Fig 6(c) and 6(g)] Different structural orientations were also indicated by the PS-OCT image [i.e., different orientations of a fast-axis angle signal in three 742 Advances in Lasers and Electro Optics parts of the tomogram;... improved to increase the signalto-noise ratio and resolve more detailed intracellular structures by introducing better compound lenses with higher NAs Two-photon Fluorescence Endomicroscopy 765 4 Summary In this book chapter, the general technological and engineering challenges in developing a scanning two-photon fluorescence fiber-optic endomicroscopy system has been discussed, including single-mode... a core/inner clad diameter of 16/ 165 μm and NA of 0.04/0.6 The large core of the PC-DCF reduces the nonlinear optical effects up to a certain excitation power (Bao & Gu, 2009) But the large core diameter and the related low NA make it challenging to focus the excitation beam to a small spot size with a given miniature objective lens The use of a PC-DCF would 754 Advances in Lasers and Electro Optics. .. respectively The GRIN lens with a 0.22 pitch and a 1.8-mm diameter (NSG America, Inc.) had a magnification of 0.5 from the DCF tip to the sample The compound 760 Advances in Lasers and Electro Optics lens was made of a pair of 3-mm miniature aspherical lenses (modified from Archer OpTx L150 and L110 lenses) The purpose of the use of the miniature compound lens is to achieve a tighter focus and increase the... fast two-dimensional beam scanning and high-quality miniature imaging optics Detailed design issues and imaging performance has been illustrated using our recently developed scanning all-fiber-optic TPF endomicroscope as an example The two-photon fluorescence endomicroscope has shown great flexibility and reliability for high-resolution imaging of internal luminal organs In summary, endomicroscopy technologies... tissues where birefringence is caused by form birefringence First, the user selected regions (such as the white rectangle shown in the left column of Fig 7) corresponding to those evaluated by histopathology The regions were then automatically divided into several regions of interest (ROIs) (e.g., green dashed inset in the left column of Fig 7) beginning from the intimal surface and including approximately . coherence tomography Advances in Lasers and Electro Optics 734 and fluorescence molecular imaging for simultaneous morphological and molecular imaging. Physics in Medicine and Biology, 55, 191-206 in saline (4 ° C), cut into segments smaller than 1 × 1 cm, and examined. Each segment was mounted in a cuvette and moistened with a normal saline bath maintained at 37 ° C during the imaging hematoxylin and eosin (H and E) for routine examination. The distribution of the collagen structure in the plaque lesion was also examined using Masson trichrome and picrosirius red staining procedures