Current clinical applications of coronary optical coherence tomography
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Optical coherence tomography (OCT) is an intra-coronary diagnostic technique that provides detailed imagings of blood vessels in the current cardiac catheterization laboratory. The higher resolution of OCT often provides superior delineation of each structure compared with intravascular ultrasound (IVUS), and it can reliably visualize the microstructure of normal and diseased arteries. The capabilities of OCT are well suited for the identification of calcified plaque and neointima formation after stent implantation. It has been reported that OCT-guided percutaneous coronary intervention (PCI) resulted in equivalent clinical and angiographic outcomes in comparison with IVUS-guided PCI. Recently, the three-dimensional reconstruction of OCT and a real-time point-to-point correspondence between coronary angiographic and OCT/OFDI images have been developed and provide useful information to PCI operators. The unique capabilities of OCT as an investigational tool for high-risk lesions will serve the cardiology community well, as it moves us toward a better understanding of atherosclerotic plaque. In addition, because of the development of new OCT technology, OCT has become a notable catheter-based imaging technology that can provide practical guidance for PCI in clinical settings.
KeywordsOptical coherence tomography Imaging Vulnerable plaque Thrombus Coronary intervention
Optical coherence tomography (OCT) is an intra-coronary diagnostic technique that provides detailed imagings of blood vessels in the current cardiac catheterization laboratory. The first OCT system was developed by a group of James G. Fujimoto in 1991 . By the early 2000s, the first images of human coronary atherosclerosis were recorded by Yabushita and colleagues . In 2008, the first commercially available time-domain OCT system (M2 OCT imaging system, LightLab Imaging, Inc., Westford, MA, USA) was introduced and included under insurance coverage in Japan. However, the time-domain OCT system needed an over-the-wire-type catheter with an occlusion balloon to obtain continuous long-sectional images due to the limitation of pullback speed. Therefore, the application of time-domain OCT was mostly limited to research purposes. More recently, new generation OCT systems, such as frequency-domain OCT and optical frequency-domain imaging (OFDI) systems, have been developed to overcome this limitation . Since then, OCT has become a noticeable catheter-based imaging technology that can provide scientific insights into vascular biology and practical guidance for percutaneous intervention (PCI) in clinical settings. In the current review article, updates on OCT image interpretation and clinical applications of coronary OCT are discussed.
OCT image interpretation
Coronary artery morphology
The higher resolution of OCT often provides superior delineation of each structure compared with intravascular ultrasound (IVUS), and it can reliably visualize the microstructure (i.e., 15–50 vs. 150–200 µm for IVUS) of normal and diseased arteries. Typically, the media of the vessel appear as a lower signal intensity band than the intima and adventitia, providing a three-layered appearance. There is good agreement in the intimal thickness between OCT and histological examination (r = 0.98, p < 0.001, mean difference = 0.01 ± 0.04 mm) . Most adults undergoing cardiac catheterization have intimal thickening and a signal-rich thick intimal band, even in angiographically normal segments. OCT signal penetration through the diseased arterial wall is generally more limited (no more than 2 mm with current OCT devices), making it difficult to investigate deeper portions of the artery or to track the entire circumference of the media–adventitia interface.
Detection of vulnerable plaque
One of the most valuable challenges for OCT is its role in the detection of vulnerable plaque. Myocardial infarction, sudden cardiac death, and unstable angina arise from coronary thrombosis, which mainly develops as a result of ruptured vulnerable plaque. Autopsy studies have identified several histological characteristics of plaques that correlate with the risk of rupture and subsequent acute coronary events. These characteristics include: (1) a large necrotic core with an overlying thin fibrous cap (<65 µm), so-called thin-cap fibroatheroma (TCFA); (2) activated macrophages near the fibrous cap; and (3) neoangiogenesis . A TCFA is characterized as a necrotic core with an overlying fibrous cap. A fibrous cap consists of vascular smooth muscle cell and extracellular matrix, which is often a signal-rich band, overlying a signal-poor region indicating the necrotic core. In the OCT image, the necrotic core is characterized as signal-poor regions with poorly defined borders, similar to that of a lipid pool. Histologically, a lipid pool is converted to a necrotic core by macrophage infiltration and apoptosis forming the early necrotic core. In previous studies by a combination of pathohistology and OCT, researchers did not attempt to differentiate a necrotic core from a lipid pool, although these structures have different clinical relevance [2, 7]. Recently, Fujii and colleagues reported that the histological findings underlying the false-positive diagnoses of OCT for TCFA included foam cell accumulation on the luminal surface, microcalcifications at the surface, hemosiderin accumulation, or organized thrombus . It seems likely that the diagnostic accuracy for OCT-derived TCFA was not higher than expected, so the ability of OCT to characterize a lipid pool containing necrotic core needs to be clarified in future histologic studies.
The unique capabilities of OCT as an investigational tool for high-risk lesions will serve the cardiology community well, as it moves us toward a better understanding and identification of vulnerable plaque, thereby improving our ability to more precisely treat our patients, both acutely and for the long term.
Assessment of neointima
The capabilities of OCT are well suited for the identification of neointima formation, where the relevant morphologic features are primarily localized within 500 μm of the luminal surface. Previous ex vivo studies have reported that OCT can characterize neointimal tissue morphologies following coronary stent implantation [19, 20, 21]. Homogeneous neointima including predominantly smooth muscle cells with collagen fibers is typically observed after bare-metal stent implantation, and it has been reported that homogeneous neointima is a predictor of late neointimal regression during 18 month follow-up . On the other hand, heterogeneous neointima contains various tissue components, such as proteoglycan-rich myxomatous matrix, calcium deposition, foam cell accumulation, and fibroatheroma. Heterogeneous neointima with high attenuation that cause invisible stent struts indicates a large amount of foam cell accumulation and large fibroatheroma/necrotic core within neointima. Such heterogeneous neointima, so-called neo-atherosclerosis, is a frequent finding in lesions with drug-eluting stent implantation and observed earlier than in bare-metal stent implantation . Although the mechanism of very late stent thrombosis is multifactorial , rupture of an atheromatous lesion within the stented segment (neointimal rupture) may be associated with the occurrence of very late stent thrombosis. OCT is useful for monitoring the morphological changes that occur in neointima formation after stent implantation. In addition, when various types of PCI, including plain old balloon angioplasty, paclitaxel-coated balloon dilatation, and drug-eluting stent implantation, for in-stent restenosis (ISR) lesions were compared, target lesion revascularization (TLR) rates of lesions with a homogeneous neointima were significantly higher in the plain old balloon angioplasty group than in the paclitaxel-coated balloon dilatation group (ISR: 54.8 vs. 19.1%, p < 0.001; TLR: 38.7 vs. 10.6%, p < 0.001) and drug-eluting stent group (ISR: 54.8 vs. 19.6%, p = 0.002; TLR: 38.7 vs. 10.7%, p = 0.005), whereas there were no differences among the three groups in lesions with a heterogeneous structure . These results suggested that treatment using a paclitaxel-coated balloon or drug-eluting stent group might be preferable for ISR lesions with homogeneous neointima compared with plain old balloon angioplasty. Morphological assessment of neointima in lesions with ISR using OCT provides useful information about suitable PCI strategies for ISR lesions.
Guidance for coronary interventions
In terms of the comparison between angiography alone vs. angiography plus OCT-guided PCI, it has been reported that OCT-guided PCI had a significantly lower 1-year risk of cardiac death (1.2 vs. 4.5%, p = 0.010), cardiac death or myocardial infarction (6.6 vs. 13.0%, p = 0.006), and the composite of cardiac death, myocardial infarction, or repeat revascularization (9.6 vs. 14.8%, p = 0.044) . This study was a case–control study and further large-scale prospective, randomized studies are needed to reveal the clinical superiority of OCT-guided PCI against angiography alone PCI. Regarding the comparison between OCT-guided and IVUS-guided PCI, Habara and colleagues first reported that OCT guidance for stent implantation was associated with smaller stent expansion than the conventional IVUS guidance in a single-center study. This result might be due to the poor visibility of the vessel border using OCT, which contributes to smaller stent expansion than with IVUS guidance. However, in a recent post hoc analysis, OCT and IVUS guidance for PCI resulted in a comparable degree of stent expansion . More recently, in a randomized, controlled trial (ILUMIEN III: Optimize PCI trial), OCT-guided PCI using a specific reference segment external elastic lamina-based stent optimization strategy was safe and resulted in a similar degree of stent expansion to that of IVUS-guided PCI . It has been reported that OFDI-guided PCI resulted in equivalent clinical and angiographic outcomes at 12 months in a multicenter, randomized, controlled trial (OPINION trial) . Further large-scale prospective studies are needed to compare the outcomes between OCT-guided PCI and IVUS-guided PCI.
Co-registration of OCT with coronary angiography
The unique capabilities of OCT as an investigational tool for high-risk lesions will serve the cardiology community well, as it moves us toward a better understanding of atherosclerotic plaque. In addition, because of the development of new OCT technology, OCT has become a notable catheter-based imaging technology that can provide practical guidance for PCI in clinical settings.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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