Cardiac Optical Coherence Tomography

For more than 2 decades since the first imaging procedure was performed in a living patient, intravascular optical coherence tomography (OCT), with its unprecedented image resolution, has made significant contributions to cardiovascular medicine in the realms of vascular biology research and percutaneous coronary intervention. OCT has contributed to a better understanding of vascular biology by providing insights into the pathobiology of atherosclerosis, including plaque phenotypes and the underlying mechanisms of acute coronary syndromes such as plaque erosion, neoatherosclerosis, stent thrombosis, and myocardial infarction with nonobstructive coronary arteries. Moreover, OCT has been used as an adjunctive imaging tool to angiography for the guidance of percutaneous coronary intervention procedures to optimize outcomes. However, broader application of OCT has faced challenges, including subjective interpretation of the images and insufficient clinical outcome data. Future developments including artificial intelligence–assisted interpretation, multimodality catheters, and micro-OCT, as well as large prospective outcome studies could broaden the impact of OCT on cardiovascular medicine.

expectedly found some parts were damaged.
The second team brought parts to replace those that were damaged.Like any other new procedures, unexpected complications could occur, but complications were of particular importance because OCT is an intracoronary procedure.A critical complication not only would have harmed the patient, but it could have also jeopardized the future of cardiac OCT.Fortunately, intracoronary OCT imaging in living patients was safely performed in 2000 without any complications.In 2002, the first report on plaque characteristics of culprit lesion in patients with acute myocardial infarction (AMI), acute coronary syndromes (ACS), and stable angina was published, confirming the previous pathology studies. 3Since the prototype OCT system was not equipped with automated pull-back, the catheter was manually pulled back to the region of interest and saline was injected to remove blood to acquire several images for each plaque.After the research use of the prototype OCT system, a commercial system was developed by LightLab Imaging Inc. in Westford, Massachusetts, as an MIT startup in a joint venture with Carl Zeiss Meditec.
The first commercially available intravascular OCT system used time-domain (TD) detection (TD-OCT).The TD-OCT catheter had an outer diameter of 0.019-inch, containing a 0.006-inch optic fiber, that appeared like a guide wire (ImageWire LightLab Inc).TD-OCT provided a slow image acquisition at 15 to 20 frames/second (200-240 axial scans per frame), resulting in a maximum pull-back speed of 2 mm/second, which required proximal occlusion of the blood flow with a dedicated balloon and saline infusion from the tip of the occlusion balloon (Figure 2). 4 With the advent of commercially available equipment, multiple groups conducted validation studies to show the accuracy and feasibility of a commercial intravascular OCT imaging system.Kume et al 5 examined 35 lipid-rich plaques from 38 human cadavers and evaluated the correlation between the OCT-based measurement of fibrous cap on lipid-laden plaque and histology examination and reported an excellent correlation between OCT and histology measurements of fibrous cap (R ¼ 0.90; P < 0.001).
The same group reported the categorization of intraluminal thrombus by OCT.In the examination of 108 coronary arterial segments from 40 cadavers, red thrombus was characterized by high-backscattering protrusion with signal-free shadowing, whereas white thrombus was characterized by lowbackscattering projection. 6

OCT's CONTRIBUTIONS TO CARDIOVASCULAR MEDICINE.
There are 2 primary applications of intravascular OCT imaging: in vivo vascular biology research and PCI optimization.Intravascular OCT provides highresolution images of coronary arteries with a 10-to 20-mm spatial resolution, nearly 10 times higher than that of IVUS examination (Table 1).Moreover, the greater signal-to-noise ratio and the ability to differentiate plaque components enables detailed visualization of vascular structures, often referred to as an optical biopsy.These benefits have facilitated the use of OCT in research to study vascular biology, which previously could not be done by existing modalities, and during PCI procedures where OCT can accurately assess vessel size and stent apposition.Given that the strength of OCT lies in visualizing microstructures related to plaque vulnerability, such as lipid-rich plaque, fibrous cap thickness, macrophage accumulation, and intraluminal thrombus, OCT drew attention as an emerging imaging modality to evaluate plaque vulnerability in vivo.Before the commercial OCT system became available, Jang et al reported higher prevalence of lipid-rich plaque, thinner fibrous cap, and more frequent thin-cap fibroatheroma (TCFA) defined as having $2 quadrants of lipid and fibrous cap <65 mm in patients with AMI compared reported that OCT was used in a mere 0.3% of PCI procedures performed in 2014. 16Similarly, in Europe, OCT use was reported at only 1.3% of PCI cases between 2005 and 2015, although the rate slightly increased during that period. 17In contrast, OCT penetration rates were higher in Asian countries, particularly in Japan, where the national guidelines granted Class I recommendations for the use of intravascular imaging for stent optimization, and imaging modalities were reimbursed by national health insurance. 18According to the annual report from the Japanese Circulation Society, OCT was used in 29,700 of 255,416 (11.6%)PCI procedures, and IVUS examination was used in approximately 80% of cases in 2014.These data indicate that >90% of PCI procedures were guided by intravascular imaging.In general, intravascular imaging techniques are used more commonly in Asian countries, including Japan, Korea, and China.However, recent data on the exact rate of OCT use in PCI have not been reported in the literature.Thus, although OCT-guided PCI rates varied among countries, the overall penetration rate remained limited.Over the last decade, a greater number of randomized controlled trials were conducted to evaluate the advantages of imaging-guided PCI, primarily based on IVUS.[28] [30][31][32][33][34] The minimum stent area (MSA) has been one of the most significant predictors of stent failure since the bare metal stent era. 35After the advent of drug-eluting stents, multiple IVUS studies advocated various cut-off thresholds of MSA to avoid stent failure: 5.0 mm 2 for sirolimuseluting stents 36 and 5.7 mm 2 for paclitaxel-eluting stents. 37These thresholds have been used for procedural endpoints in IVUS-guided PCI.However, OCT underestimates the lumen area in comparison with IVUS by approximately 10%. 38This discrepancy must be taken into consideration when adopting IVUS-derived thresholds for OCT-guided PCI.From the data of MGH OCT registry, including 900 lesions Yonetsu and Jang History and Perspective of OCT in 786 patients, Soeda et al 30 investigated the OCT-derived predictors of device-oriented cardiac events and found that the best cut-off threshold for MSA was 5.0 mm 2 .This paper also reported that irregular protrusion inside the implanted stent was an independent predictor of device-oriented cardiac events.In addition to MSA and irregular protrusion, edge dissection 32,33 and stent malapposition 39,40 have been reported as potential predictors of subsequent stent failure (Figure 3).A significant advantage of OCT in PCI is the 3-dimensional reconstruction of the implanted stent structure over bifurcated lesions. 41cent OCT consoles are equipped with 3-dimensional reconstruction software, facilitating the identification of stent struts, link connections, and guidewire recrossing positions (Figure 4).Previous studies indicated that the likelihood of incomplete stent apposition after kissing balloon dilatation to the bifurcation could be predicted by the type of link connection of stent strut and the position of the recrossed wire. 42This information could contribute to an optimization of stent implantation in bifurcation lesions, potentially decreasing the risk of late complications, including stent thrombosis.Calcified lesions present a considerable challenge in the optimization of PCI, often preventing sufficient stent expansion and increasing the risk of stent failure. 43like IVUS examination, which exhibits acoustic shadows behind the calcification, OCT depicts calcification as a clearly delineated, signal-poor region, enabling the measurements of the thickness of calcific plates. 44In cases of a thick and circumferential calcification, balloon angioplasty is not effective to dilate the vessel. 13Instead, debulking devices such as rotational atherectomy, orbital atherectomy, or lithotripsy should be considered to achieve sufficient stent expansion.Although OFDI is based on the same FD-OCT technology as FD-OCT produced by LightLab/St.Jude/ Abbott, image acquisition rates and pull-back speeds are different.Kubo et al 26,46   Yonetsu and Jang History and Perspective of OCT than angio-guided PCI, whereas no significant difference was observed in the rate of malapposition. 47 of this writing, definitive superiority of OCTguided PCI over IVUS-guided PCI has not been demonstrated in prospective studies in terms of PCI optimization.
OCT AS A RESEARCH TOOL PLAQUE EROSION.One of the most significant advantages of OCT is its ability to characterize plaque phenotype in the culprit lesions of ACS.A diagnostic algorithm for underlying plaque phenotypes, such as plaque rupture, plaque erosion, or calcified nodules was developed in 2013 using 128 patients with ACS from the MGH OCT registry. 48This study demonstrated that OCT could differentiate plaque phenotypes of the culprit lesions, showing a similar proportion of each plaque type compared with previous pathology studies. 49,50The diagnostic algorithm introduced in this study has been widely used for identification of OCT-defined plaque erosion, characterized by an intact fibrous cap without plaque rupture and intraluminal thrombus formation (Figure 7).Considering the nonocclusive thrombosis and smaller plaque burden with plaque erosion, several studies attempted conservative management without stent implantation for lesions with plaque erosion. 51,52The EROSION (Effective Anti-Thrombotic Therapy Without Stenting: Intravascular Optical Coherence Tomography-Based Management in Plaque Erosion) study, published in 2017, was a prospective observational study that investigated the feasibility of a conservative strategy with antithrombotic therapy for patients with ACS with OCT-defined plaque erosion in the culprit lesions.
The study revealed a significant decrease in thrombus volume with antiplatelet therapy without stenting at the 1-month OCT follow-up, showing no adverse cardiovascular events in the 60 enrolled patients with erosion. 52The subsequent EROSION III study, which randomized 226 patients with STEMI into OCT-guided  with ACS, with NSTE-ACS showing a greater prevalence of erosion compared with STEMI (47.9% vs 29.8%; P < 0.001).This study found that age <68 years, anterior ischemia, no diabetes mellitus, a hemoglobin of >15.0 g/dL, and normal renal function were independent predictors of OCT-derived erosion. 54The predictors of OCT-derived plaque erosion at the culprit lesions in patients with STEMI showed that younger age, current smoking, absence of coronary risk factors, larger vessel size, and the spatial location with close proximity to a bifurcation were independent predictors of erosion. 55  In the OPINION study, the stent diameter was determined as 0 to 0.25 mm greater than mean lumen diameter at distal reference site.In the present case, lumen diameter at distal reference was 2.45 mm (white arrows), and the operator implanted a 2.75-mm stent according to the OPINION protocol.   OCT is able to identify the multiple layers of healed coronary plaque (Figure 8), which was validated with pathology examination by Shimokado et al. 57 As suggested by previous pathology studies, 56 plaque disruption followed by organization of residual thrombus may contribute to rapid progression of the lesion.This progression results in the formation of layered pattern of coronary plaque on OCT. 58,59Thus, the OCT-defined healed coronary plaque, characterized by the layered plaque, represents indicators of previous plaque destabilization.

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It has been reported that the healed coronary plaque at the culprit lesions was associated with plaque vulnerability at both culprit and nonculprit sites.Usui et al 60 reported that untreated healed coronary plaque was associated with future adverse events, predominantly ischemia-driven revascularization.As mentioned elsewhere in this article, because healed coronary plaque can be detected by OCT, it has gained research attention as a potential marker of plaque vulnerability.Calcified plaque can be a cause of ACS, after plaque rupture and plaque erosion, of which 3 subtypes can be identified by OCT: eruptive calcified nodules, superficial calcific sheet, and calcified protrusion.ACEs ¼ acute coronary syndrome; other abbreviation as in Figure 1.including virtual histology-derived TCFA, large plaque burden, and small lumen area in the nonculprit lesions of patients with ACS using virtual histology IVUS examinatoin. 63After this landmark study, several OCT studies aimed to identify vulnerable features using OCT (Table 3).The MGH group conducted a retrospective analysis of 1,474 patients and reported that lipid-rich plaque identified in the nonculprit regions of the target vessel was associated with subsequent adverse events at 4 years. 64The OCT examination of all 3 main epicardial coronary arteries after primary PCI.They reported that TCFA and a minimum lumen area of <3.5 mm 2 were predictive of adverse cardiac events in both lesion-level and patient-level analyses, and, when these 2 factors were present, the HR for lesion-specific analysis was 15.5 (95% CI: 6.89-34.89). 66These OCT-based studies consistently showed that TCFA/lipid-rich plaque with a small minimum lumen area was the hallmark of adverse events.Vulnerable features have  2.

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Yonetsu and Jang History and Perspective of OCT been identified by various imaging modalities, which typically include lipid-laden plaque characteristics, a small lumen area, and a large plaque burden.However, in all studies, the primary endpoint was driven by unplanned revascularization and not by a hard endpoint.The CLIMA study was an outlier.Furthermore, we need to consider the diverse mechanisms of coronary thrombosis, including plaque erosion and calcified nodules.Currently, no diagnostic method is available to detect plaques prone to endothelial denudation and occlusive thrombus formation leading to plaque erosion.Considering all these issues, the ability to predict all future cardiovascular events with intravascular imaging modalities including OCT may be in the distant future. 67SCULAR BIOLOGY RESEARCH.One of the most significant contributions of OCT has been its role in enhancing our understanding of atherosclerosis.
Because intravascular OCT provides high-resolution images of vessel microstructures close to the lumen, it has been extensively used to examine various aspects of vascular biology in coronary arteries, particularly in ACS.In 2012, Kato et al 68 investigated nonculprit plaque characteristics in 104 patients with ACS and non-ACS and reported that nonculprit lesions in patients with ACS had larger lipid indices (defined as the average lipid arc multiplied by lipid length), and more prevalent TCFAs compared with those in stable patients.This finding supports the important concept of panvascular inflammation in patients with ACS. 69,70In line with this finding, when patients with ACS were divided into those with plaque rupture at the culprit lesion and those without, nonculprit plaques showed rupture in 26% of patients with plaque rupture at the culprit lesion, whereas no plaque rupture was observed in the counterpart (26% and 0%; P ¼ 0.02). 70These results underscore the idea that ACS is a panvascular process with local manifestations and that prevention and treatment should be patient focused rather than plaque focused.
OCT has been used to evaluate the morphological features of the culprit lesions of ACS as described, 48 and numerous studies have shed light on the panvascular nature of atherosclerosis.Araki et al 71  NEOATHEROSCLEROSIS.OCT has been used to evaluate the underlying mechanisms of stent failure in previously implanted stents, as IVUS was recommended by earlier versions of American and European guidelines. 15Gonzalo et al 72 reported in 2009 that OCT can depict various patterns of neointima within stents with restenosis, which was not provided by IVUS or other imaging modalities (Figure 10).In addition to homogeneous, heterogeneous, or layered pattern neointima, Takano et al 73 reported the development of lipid-laden neointima observed by OCT within stents.Notably, lipid-laden neointima was more frequently observed in bare-metal stents implanted >5 years previously as compared with younger bare-metal stents implanted within 6 months (67% and 0%; P < 0.01).0][81] Park et al 79   resolution at the expense of signal penetration depth.
Additionally, because higher resolution images contain more data and place a greater load on the system, the image acquisition rate of mOCT is not sufficient for clinical use.Owing to these unresolved limitations, mOCT has not been tested in humans.

CONCLUSIONS
TD-OCT acquired the CE mark in Europe in 2004; however, because of the need for proximal occlusion, the U.S. Food and Drug Administration did not approve it.Subsequently, in 2006, frequency-domain (FD)-OCT was introduced, which revolutionized intracoronary imaging.Unlike TD-OCT, FD-OCT was user friendly and provided much faster image acquisition enabling faster pullback speeds of 20 mm/second.With FD-OCT, >50 mm could be imaged within 3 seconds.This nonocclusive image acquisition facilitated the use of OCT and, finally in 2010, the U.S. Food and Drug Administration approved intravascular OCT for clinical use.In the same year, the MGH OCT Registry was launched.Data have been collected at 21 sites from 6 countries around the world.This registry included a broad spectrum of patients including those with ACS and chronic coronary syndrome, those who underwent 3 vessel OCT imaging, and those with previously implanted stents.Since the OCT registry was launched, >40 fellows from around the globe have been trained at the Cardiology Laboratory for Integrated Physiology and Imaging at MGH in Boston, Massachusetts.

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= thin-cap fibroatheroma TD-OCT = time-domain optical coherence tomography Yonetsu and Jang J A C C : A S I A , V O L . 4 , N O . 2 , 2 0 2 History and Perspective of OCT F E B R U A R Y 2 0 2 4 : 8 9 -1 0 7with non-ST-segment elevation MI (STEMI) or patients with stable angina patients.7This report marked the beginning of in vivo vascular biology research using OCT.In terms of visualization of the culprit lesions of AMI, Kubo et al8 reported that OCT is superior to IVUS examination and coronary angioscopy for detection of plaque rupture with thrombus.OCT has been tested for its use to optimize PCI in clinical practice.OCT-guided PCI comprises 3 phases: vessel morphological assessment, vessel size determination, and stent optimization.Before interventional procedures, OCT can provide detailed information about plaque characteristics, including the presence and extent of lipid-rich plaques and calcification.Plaques containing a substantial lipid component are susceptible to distal embolization, potentially resulting in slow flow phenomenon or periprocedural MI, complications that can be predicted with baseline OCT images. 9,10Lee et al 10 observed 131 patients undergoing PCI and reported that TCFA was independently associated with an increased risk of periprocedural MI.Although the usefulness of distal protection devices remains contentious, predicting periprocedural MI may aid in devising PCI strategies to circumvent distal embolization.Moreover, one advantage of OCT over IVUS examination is its superior delineation of calcified plaques, enabling the quantification of calcium arc and thickness. 11,12A study revealed that calcium thickness predicts the vessel's crackability by balloon angioplasty, 13 potentially informing the decision to use debulking devices, such as rotational or orbital atherectomy, or intravascular lithotripsy to achieve sufficient stent expansion.Apart from plaque CENTRAL ILLUSTRATION History of Intravascular OCT 1992: Invention of OCT 1998: MGH cardiac OCT group 2000: FIM study in Seoul 2004: TD-OCT CE mark in Europe 2006: FD-OCT CE mark in Europe 2010: FD-OCT FDA approval in the U.S. 2010: MGH OCT registry 2016: ILUMIEN III trial 2017: EROSION study 2021: Recommendation in U.S. guideline Yonetsu T, et al.JACC: Asia.2024;4(2):89-107.Timeline of intravascular OCT development and landmarks is summarized.For 20 years since its invention at the Massachusetts Institute of Technology, intravascular OCT has emerged as an imaging tool for coronary arteries, enhancing our understanding of atherosclerosis and aiding percutaneous coronary interventions procedures.CE ¼ Communauté Européenne; FDA ¼ U.S. Food and Drug Administration; FD-OCT ¼ frequency-domain optical coherence tomography; FIM ¼ first-in-man; MGH ¼ Massachusetts General Hospital; OCT ¼ optical coherence tomography; TD-OCT ¼ time-domain optical coherence tomography; U.S. ¼ United States.J A C C : A S I A , V O L . 4 , N O . 2 , 2 0 2 Yonetsu and Jang F E B R U A R Y 2 0 2 4 : 8 9 -1 0 7 History and Perspective of OCT characterization, OCT enables precise measurements of vessel size and lesion length, facilitating the selection of appropriate coronary stents.After stent implantation, OCT is adept at detecting minor complications like intimal dissection or stent malposition, 4,14 in addition to offering quantitative measurements of stent expansion.Because insufficient stent expansion is a critical predictor of stent failure, further postdilatation may be performed to optimize stent expansion.Before the integration of OCT into clinical practice, angiograms and IVUS examinations were used to guide optimal stent implantation.However, the superior image quality of OCT may potentially supersede these modalities.Consequently, OCT is expected to provide indispensable information on vascular biology and to assist in PCI guidance for physicians.The following sections offer a synopsis of the history and current state of OCT research in these 2 directions.CURRENT STATUS In the previous version of the American College of Cardiology/American Heart Association/Society for Cardiovascular Angiography and Interventions PCI guidelines published in 2011, 15 OCT technology was mentioned without specific recommendations, whereas IVUS examination received Class IIa recommendations for ambiguous left main disease and stent restenosis.Owing to the lack of guideline-based recommendations, intracoronary OCT use remained minimal in the United States for a decade.In fact, a large database from the Healthcare Cost and Utilization Project's National Inpatient Sample, encompassing 3 million cases of coronary angiography,

FIGURE 2
FIGURE 2 Time-Domain and Frequency-Domain OCT PCI has been tested in several prospective studies.The ILUMIEN I (Observational Study of Optical Coherence Tomography in Patients Undergoing Fractional Flow Reserve and Percutaneous Coronary Intervention) study was a single-arm prospective observational study which investigated the impact of OCT assessment on physician decisionmaking. 45This study found that 57% of strategies were changed by examining baseline plaque morphology with OCT before PCI, and additional procedures were performed after OCT assessment of implanted stents in 27% of cases.This finding highlights that OCT assessment may change PCI strategy when added to angiography-guided PCI.Since OCT has been introduced, it has been compared with IVUS examination in various aspects.The benefits of OCT encompass high-resolution imaging, the capacity to distinguish lipid-containing plaque, and the visualization of coronary artery microstructures.This allows a meticulous evaluation of atherosclerosis.IVUS examination, however, facilitates the visualization of

FIGURE 3
FIGURE 3 Complications After Stent Implantation

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A C C : A S I A , V O L . 4 , N O . 2 , 2 0 2 History and Perspective of OCT F E B R U A R Y 2 0 2 4 : 8 9 -1 0 7 reperfusion and angio-guided reperfusion groups revealed that the OCT-guided reperfusion strategy resulted in fewer stent implantations than the angioguided group; no significant difference was observed in the rate of adverse events. 53The EROSION study facilitated the application of OCT in patients with ACS, and investigators are currently exploring a potential paradigm shift in ACS management with the identification of plaque erosion and conservative management without coronary stenting in certain subsets of patients.These preliminary findings should be validated in large-scale randomized trials in the future.Since the feasibility of the OCT-derived diagnostic algorithm of plaque erosion was reported, research interest in the clinical and morphological characteristics of plaque erosion has grown.Yamamoto et al 54 investigated 1,241 patients with ACS undergoing OCT examination of culprit lesions, identifying plaque erosion in 38.4% of all patients However, differentiating plaque erosion from plaque rupture using clinical characteristics alone remains challenging.Future studies should aim to identify additional diagnostic methods for the noninvasive diagnosis of erosion.HEALED CORONARY PLAQUE.Coronary thrombosis after plaque rupture or erosion does not always

FIGURE 5
FIGURE 5 Measurement of Lumen Profile and Selection of Stent Size

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A C C : A S I A , V O L . 4 , N O . 2 , 2 0 2 Yonetsu and Jang F E B R U A R Y 2 0 2 4 : 8 9 -1 0 7History and Perspective of OCT

FIGURE 6
FIGURE 6 Angio-Coregistration in a Contemporary Console

FIGURE 7
FIGURE 7 Ruptured Plaque and Plaque Erosion

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Yonetsu and JangJ A C C : A S I A , V O L . 4 , N O . 2 , 2 0 2 History and Perspective of OCT F E B R U A R Y 2 0 2 4 : 8 9 -1 0 7 cause clinical symptoms.Even after plaque rupture, an occlusive thrombus may not develop in the coronary artery, which may heal with endogenous antithrombotic mechanisms.Previous pathology studies have indeed found multiple healed sites from previous ruptures underneath the acute rupture site, evident as layered tissues overlying a necrotic core.

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A C C : A S I A , V O L . 4 , N O . 2 , 2 0 2 Yonetsu and Jang F E B R U A R Y 2 0 2 4 : 8 9 -1 0 7 History and Perspective of OCT CALCIFIED PLAQUE IN ACS: A NEW CLASSIFICATION.OCT has enriched our understanding of coronary calcification.When compared with other intravascular imaging modalities, OCT is superior in the assessment of the morphological characteristics of vascular calcification.Previous OCT studies investigated the morphological characteristics of ACS culprit lesions and identified calcified nodules as one of the causes of coronary thrombosis, corroborating findings from pathology studies.Additionally, as more clinical data accumulated, the morphological variety of calcification at the culprit lesions of ACS have been noted.Sugiyama et al 61 reported that calcified plaques represented 141 (12.7%) of the culprit lesions in 1,241 patients with ACS.More important, they found 3 distinct subtypes: eruptive calcified nodules (which corresponded with pathologically defined calcified nodules), superficial calcific sheets (which displayed a sheet-like appearance of a superficial calcific plate without erupted nodules or protruding masses into the lumen), and calcified protrusions (which corresponded with pathologically defined nodular calcification overlaid by smooth fibrous tissue) (Figure 9).Interestingly, only one-fourth of these were eruptive calcified nodules; the majority were superficial calcific sheets.Thus, close observation of ACS culprit lesions using OCT led to discovery of 3 subtypes of calcified plaques in vivo and the development of a novel classification that extends beyond existing knowledge based on autopsy studies.VULNERABLE PLAQUE RESEARCH.The term vulnerable plaque was introduced in 1989 by Muller et al 62 to describe 2 conditions of coronary plaque: vulnerable plaque, which refers to a plaque susceptible to rupture triggered by various stimuli such as

FIGURE 9
FIGURE 9 New Classification for Calcified Culprit Lesions in ACS

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A C C : A S I A , V O L . 4 , N O . 2 , 2 0 2 History and Perspective of OCT F E B R U A R Y 2 0 2 4 : 8 9 -1 0 7 circadian rhythm, and nonvulnerable plaque.The initial definition of vulnerable plaque focused on plaques that were prone to rupture, potentially leading to coronary thrombosis and the onset of ACS.However, owing to the low incidence of hard endpoints such as AMI and SCD, clinical trials included soft endpoints such as revascularization.Hence, we should be cautious in interpreting the results of clinical studies.In 2011, the PROSPECT (Providing Regional Observations to Study Predictors of Events in the Coronary Tree) study identified virtual histology IVUS-derived vulnerable plaque features, conducted a comprehensive analysis of the distribution of coronary plaques with different phenotypes using 3-vessel OCT data, discovering that TCFA was particularly clustered in the proximal segment of the left anterior descending artery (LAD) in patients with ACS.In contrast, fibrous plaques were evenly distributed throughout the coronary trees.This study provided new insights into the in vivo distribution of various types of coronary plaques and their associations with clinical presentations.71

FIGURE 10
FIGURE 10 Neointimal Patterns Within Stents Observed by OCT History and Perspective of OCT potential role of OCT in the diagnosis and management of patients with MINOCA.PERSPECTIVEAs described, intravascular OCT, with its superior spatial resolution and capability for plaque characterization, has made significant contributions to cardiovascular medicine, mainly in optimizing PCI procedures and improving our understanding of vascular biology.PCI optimization has been largely dependent on the software and hardware for OCT imaging and has been driven by industry.Although vascular biology research emerged in the early 2010s, its popularity has decreased and is now limited to a small number of research groups.In daily practice, the penetration rate of OCT has not surpassed other imaging modalities such as IVUS examination.To increase the widespread use of OCT imaging and establish it as an essential modality in clinical practice, several issues must be addressed.One issue lies in the interpretation of OCT images, which requires additional training and experience.Artificial intelligence (AI) seems to be a promising solution and has been the focus of intensive research in recent years.
Since the invention of OCT at MIT and the first-inman study of intracoronary OCT >2 decades ago, this technology has made significant strides in cardiovascular medicine.With its high spatial resolution and detailed plaque characterization, OCT has primarily impacted PCI optimization and vascular biology research.Although not currently an essential mode of coronary imaging in clinical practice, history moves forward.The integration of AI and new technologies, along with the development of nextgeneration OCT software and hardware, promises to bring us closer to the ideal optimization of PCI and a deeper understanding of vascular biology.These advancements have the potential to improve both the prevention and outcomes of coronary artery disease in the future.

TABLE 1
Comparison Between Intravascular Ultrasound Examination and OCT in the Identification of Plaque Morphology