Optical coherence tomography (OCT) is a method of obtaining tomographic images of a human organ based on the coherence of light. Around 1990, OCT was independently developed by two Japanese researchers, Naohiro Tanno of Yamagata University, Japan, and James G. Fujimoto of the Massachusetts Institute of Technology (MIT) in the United States, and patent applications were submitted at about the same time in Japan and the United States during 1991. In vitro observation of the retina and coronary artery was first performed in 1991 [1]. OCT was initially applied in the clinical setting in the ophthalmology field in 1996 and is now widely used to assess retinal diseases and other conditions.

In cardiology, OCT is an optical analog of intravascular ultrasound (IVUS), used to examine the coronary arteries. Ultrasound employed for IVUS examination is replaced by near-infrared light with a wavelength of about 1,300 nm, which is absorbed by red blood cells, water, lipids, and protein at relatively low levels. Through a rotating glass fiber-optic system, coherent infrared light can be directed and reflected within the tissue to create a detailed tissue image with extraordinarily high resolution, and IVUS-like cross-sectional tomographic images can be obtained. OCT and IVUS differ in several respects, as shown in Table 1. The resolution of OCT (to 10-20 µm) is about 10-fold higher than that of IVUS (to 100-150 µm), but the maximum depth of tissue penetration is lower with OCT (1-2 mm) than with IVUS (4-8 mm). Another important difference is related to the strong attenuation of light by blood, which originates from two sources: absorption by hemoglobin and scattering by red blood cells. To examine coronary arteries, blood must first be removed during an OCT examination to eliminate massive scattering of light by red blood cells.

Due to its high resolution (10-15 µm), OCT can differentiate the internal and external elastic laminae, as well as distinguish the intima/media/adventitia, which is impossible with the low resolution (100-150 µm) of IVUS [9]. OCT can also differentiate tissue characteristics based on polarization properties. High birefringence by polarization shift identifies fibrous tissue, collagen, and lipid composition. Low birefringence reflects calcium. By overlying low and high birefringent images on the OCT image map, tissue structures can be highlighted by tissue composition. In a comparison using autopsy specimens, Yabushita et al. [10] reported that OCT images of each type of plaque had the following features: 1) fibrous plaque: a homogenous high-signal region with low attenuation; 2) calcified plaque: a well-delineated, low-signal region with sharp borders; and 3) lipid-rich plaque: a low-signal region with diffuse borders. They reported > 90% sensitivity and specificity for detecting lipid-rich plaque, based on comparisons with pathological specimens. In another study, Kume et al. [11] also concluded that OCT had a high sensitivity for detecting lipid-rich plaque.

In addition to its ability to detect lipid-rich plaque, as mentioned above, the high resolution of OCT makes it possible to identify a very thin (< 100 µm) fibrous cap covering a lipid core, which is difficult to detect with other modalities [12]. Autopsy studies demonstrated that fibroatheroma with a thin fibrous cap of < 65 µm (thin-cap fibroatheroma [TCFA]) is often found in culprit lesions in acute coronary syndromes (ACSs) and is considered to be a vulnerable plaque [13]. Early diagnosis of TCFAs may lead to more aggressive medical/invasive treatment of symptomatic and asymptomatic coronary artery disease to improve prognosis and clinical outcomes.

OCT-derived TCFA was defined as a lipid-rich plaque (lipid arc within a plaque in ≥ 2 quadrants) with a thin fibrous cap (thickness at the thinnest segment < 65 µm) (Fig. 2) [14]. Our OCT examination of a patient with ACS revealed ruptured plaque with a thin fibrous cap at the site of a culprit lesion that could not be detected by IVUS (Fig. 3). Kubo et al. [15] assessed culprit lesions in patients with acute myocardial infarction by OCT, IVUS, and coronary angioscopy, and reported that OCT was the most sensitive imaging modality for detecting the rupture and erosion of culprit plaque, and most frequently detected TCFAs. Additionally, OCT can distinguish between red and white thrombi using differences in the signal attenuation curve, which are frequently observed in unstable plaque [16].

Moreover, OCT may be able to assess inflammatory cell infiltration, primarily active macrophages, another characteristic of unstable plaque. Based on OCT examination, MacNeill et al. [17] reported that plaque in patients with ACS had a high density of macrophages, and that this density was higher in ruptured plaque than in non-ruptured plaque.

While many authors have reported on the usefulness of OCT for tissue characterization, as indicated above, Manfrini et al. [18] performed a comparison with pathological findings and found that the detection rates by OCT differed for each type of plaque (45% for fibrous-cap atheroma, 68% for fibrocalcific plaque, and 83% for fibrous plaque). They suggested that the low detection rates for fibrous-cap atheroma and fibrocalcific plaque may be due to the inability to detect the lipid core and calcium deposits located behind a thick fibrous cap because of the low tissue penetration of OCT, and that the lipid core and calcium deposits were both seen as low-intensity areas that were difficult to distinguish. Low tissue penetration is a fundamental limitation of the current system. Moreover, distinguishing lipid from calcium is a subjective process and its precision could depend on the skill of the observer, as is the case for gray-scale IVUS. Thus, establishing objective diagnostic criteria is desirable.

Because OCT has a high resolution, it can provide detailed information about the vessel that cannot be obtained with other modalities. Example cases are shown in Figs. 4 and 5. The case depicted in Fig. 4 demonstrated spontaneous dissection that could not be found on angiography or IVUS. Fig. 5 shows an example of a cutting balloon angioplasty. Multiple tears in the intima were observed after dilatation with a cutting balloon that also could not be detected by angiography or IVUS. Thus, this technology offers detailed visualization of vessel injury after stent implantation, such as tissue prolapse, dissections, and thrombus, and enables systematic classification and quantification in vivo [19,20].

After implantation of a drug-eluting stent (DES), not only dissection, tissue prolapse, and thrombi, but also incomplete stent apposition (ISA), have attracted attention in relation to stent thrombosis and should be assessed in detail [19]. When assessing ISA, we should recall that the near-infrared light used in OCT does not penetrate metal stent struts, similar to IVUS. The stent strut is visualized as a linear structure with strong surface reflection and typical dorsal shadowing, and the posterior side of each strut near the vessel wall cannot be observed (Fig. 6) [21,22]. Thus, it is necessary to determine whether the strut has made an indentation on the intimal surface of the vessel (Fig. 6A) or to measure the distance between the surface reflection of the strut and the adjacent visible vessel surface while taking the thickness of the strut into account (Fig. 6B). To ensure the accurate assessment of ISA, we validated the accuracy of OCT measurement of the strut thickness of several commercially available stents in vitro. Differences in OCT stent strut measurements as compared with the manufacturers' nominal strut thickness data were small and reproducibility of the measurements was high, suggesting that the assessment of stent strut apposition using OCT is feasible [21].

As described above, the capability of OCT to examine the structure of the arterial wall before or after PCI is superior to those of other imaging modalities, such as angiography and IVUS, and the application of OCT during PCI seems logical and promising, yet the assessment of clinical significance to date has been limited to observations and has not necessarily included clinical outcomes. Furthermore, OCT has several inherent limitations. Given its limited penetration depth and/or scan range, the whole-vessel structure of a lesion with a large plaque burden or a large vessel cannot be visualized. Additionally, first-generation TD-OCT also requires displacement of the blood during imaging with balloon occlusion methods, making the PCI procedure more complex and precluding the use of this technique for ostial lesions, including the left main coronary artery (lesions). As the newly developed FD-OCT system does not require coronary artery occlusion and has a faster pullback speed [5,6], it may be more suitable for PCI. The clinical significance of OCT for PCI needs to be further established in larger-scale clinical trials.

As described above, OCT is a novel, promising imaging modality with characteristics that differ from those of traditional modalities, such as IVUS and angioscopy. However, OCT does not yet have the established clinical profile or ease-of-use of IVUS. As described, the advent of next-generation OCT, FD-OCT, is expected to make the procedure easier and faster. Since OCT has received more attention in Korea, Japan, China, and Europe, increasing numbers of OCT studies are being reported. Through these studies, we hope that the clinical role of OCT in PCI will be clarified in the future. We believe that the clinical usefulness of FD-OCT is likely to be greater than that of standard/previous-generation TD-OCT.

OCT is an imaging modality that can be expected to become increasingly popular in the future for the diagnosis and treatment of patients with symptomatic/asymptomatic coronary artery disease.