Optical Coherence Tomography of the Coronary Arteries

Abstract

Intravascular imaging, particularly optical coherence tomography, has brought significant improvement in diagnostic and therapeutical approaches to coronary artery disease and has offered superior high-resolution visualization of coronary arteries. The ability to obtain images of intramural and transmural coronary structures allows the study of the process of atherosclerosis, effect of therapies, mechanism of acute coronary syndrome and stent failure, and performance of new devices and enables the interventional cardiologist to optimize the effect of percutaneous coronary intervention. In this review, we provide the summary of the latest published data on clinical use of optical coherence tomography as well as practical algorithm for optical coherence tomography-guided percutaneous coronary intervention for daily interventional practice.

Coronary angiography (CA), after decades, remains the mainstay imaging method of coronary arteries and for guidance of percutaneous coronary intervention (PCI). However, the angiography provides only visual evaluation of the contrast-filled two-dimensional luminogram of three-dimensional (3D) coronary vessel with spatial resolution of less than 0.2 mm and thus cannot visualize the vessel wall, characterize the plaque composition, or assess the result of stenting. ¹ As a result to overcome these limitations, intravascular ultrasound (IVUS) and optical coherence tomography (OCT) were developed and gradually implemented in diagnostic and therapeutic processes as well as research during the last three decades. This article aims to summarize the latest knowledge and practical applications of OCT.

Technical Overview, Advantages and Disadvantages of OCT Technology

Optical coherence tomography is a modality that uses a fiber-optic catheter that emits waves of near-infrared light (1–1.3 µm) and registers its scattered signal producing high-resolution images at a resolution of 10 to 15 µm, far exceeding the resolution of an angiography and 10-fold resolution of IVUS. This fact reduces the interobserver and intraobserver variabilities of OCT measurement, otherwise common in CA. ² In the OPUS-CLASS study investigating the measurement accuracy of quantitative coronary angiography (QCA), IVUS, and OCT, IVUS measurements of minimal lumen area (MLA) were 10% higher compared with OCT and the interobserver variability of IVUS was twice as high as of OCT. In a phantom model, the mean lumen area was equally measured by OCT, whereas IVUS overestimated mean lumen area by 8%. ³ The latest third generation of swept-source OCT is able to produce images with frame rate of 180 fps with penetration depth of signal from 1 to 1.5 mm according to the type of tissue. The limited penetration depth of OCT in general and especially in lipid tissues limits its ability to visualize deeper vessel structures and plaque burden cannot reliably be measured compared with IVUS (penetration depth up to 4 mm).

The most widely used OCT catheter in Northern America and Europe is the Dragonfly OPTIS imaging catheter. The examiner has a choice of survey mode—75 mm pullback range with a pullback rate of 36 mm/s or high-resolution mode—pullback range of 54 mm with a rate of 18 mm/s, that is, pullback time 2.1 or 3 seconds, which is significantly lower than one of IVUS. The crossing profile of catheter is 2.7 F with insertable length of 135 cm and the catheter is compatible with 0.014 in guidewires. There are three radio-opaque markers; the distal marker 4 mm from tip of the catheter, the lens marker 2 mm proximal to the lens, and the most proximal marker located 50 mm proximal to the lens, marking the imaging region. Both the lens and proximal markers are affixed to the imaging core. This allows to register the lens movement during the pullback and consequentially coregistration with CA. Another advantage of OCT system is an option to use 3D reconstruction of the vessel or stent, which offers the operator better understanding of anatomical relations, for example, in bifurcation stenting. A difference and mild disadvantage compared with IVUS is the necessity of blood clearing with contrast medium in the site of interest, since the near-infrared light is attenuated by blood. This could be performed manually or with automated injector. The recommended settings vary according to vessel diameter with flow of 3 to 4 mL/s and total volume of 10 to 14 mL. Due to necessity of blood clearing, imaging of the proximal left main artery can be challenging, and in this case, IVUS is generally preferred. A good-quality blood clearance enables differentiation of lumen contour and vessel wall allowing for reliable automatic measurement of lumen diameters, lumen area, and marking the maximal stenosis and distal as well as proximal reference segment. The images are displayed in cross-sectional, longitudinal or 3D mode according to the operator's choice ( Fig. 1 ). The advantages and disadvantages of each imaging method are summarized in Table 1 .

Fig. 1.

OCT image display. ( A ) Angiographic coregistration with highlighted stented segment (white lines) and white marker in the middle of the lumen marking the corresponding cross-sectional image. ( B ) Cross-sectional image with automatically measured lumen area and marked stent struts (white dots) on the luminal surface. ( C ) Lumen profile with automated measurements of minimal lumen area and proximal and distal reference area, mean diameter, area stenosis (AS), and length. ( D ) Longitudinal view with rendered stent, color-coded apposition markers, and side branches of the artery. OCT, optical coherence tomography.

Table 1. Comparison of IVUS and OCT methods.

IVUS	OCT
Advantages	Advantages
Better signal penetration and thus better adventitia visualization	×10 higher resolution, ability to detect fine details
Generally longer clinical experience with IVUS	Images are easier to interpret
Availability of RCT data on clinical outcomes of IVUS-guided PCI	Various automatic measurements and more user-friendly interface
Better imaging of proximal left main coronary artery	Faster pullback
	Better thrombus detection
	Ability to characterize the calcium depth
Disadvantages	Disadvantages
Images and tissue interpretation are more difficult	Additional use of contrast
Thrombus and tissue prolapse detection is limited	Necessity to flush the vessel lumen
Strut coverage for low resolution is not possible	No data from RCT of OCT-guided PCI to prove improvement of clinical outcomes
Strut apposition assessment is limited	Limited signal penetration depth of OCT

Abbreviations: IVUS, intravascular ultrasound; OCT, optical coherence tomography; PCI, percutaneous coronary intervention; RCT, randomized controlled trial.

Clinical and Imaging Outcomes of OCT-Guided PCI

There is currently limited data to support routine OCT-guided PCI in clinical practice to improve clinical outcomes and no results of randomized controlled trial (RCT) of clinical outcomes, which compares angiography-guided and OCT-guided PCIs, were published up to this date. The observational CLI-OPCI study compared OCT-guided PCI and matched angiography-guided PCI. Patients in OCT-guided PCI group had a significantly lower 1-year risk of cardiac death (1.2 vs. 4.5%, p  = 0.010) and major adverse cardiovascular events (MACE) (9.6 vs. 14.8%, p  = 0.044). ⁴ In CLI-OPCI II study, a suboptimal OCT stent deployment was associated with increased MACE (hazard ratio [HR]: 3.53; 95% confidence interval [CI]: 2.2–5.8; p  < 0.001). ⁵ Some registry studies including patients with ST-segment elevation myocardial infarction (STEMI) showed the reduced number of stents used ⁶ and larger in-stent minimal lumen diameter in OCT-guided PCI. ⁷ One of the first prospective studies assessing OCT's impact on PCI strategy in prestent phase and the association with post-PCI fractional flow reserve (FFR) values was the ILUMIEN-I study. ⁸ Prestenting OCT imaging affected the operator's decision-making in 57% compared with 27% cases in post-PCI imaging group. The study failed to prove difference in post-PCI FFR values between optimization groups. ILUMIEN II study was a post hoc analysis which showed that OCT and IVUS guidances resulted in a similar degree of stent expansion (72.8 vs. 70.6%, respectively, p  = 0.29). ⁹ ILUMIEN-III study ¹⁰ was a multicenter RCT comparing the effects of OCT-guided, IVUS-guided, and angiography-guided PCIs on post-PCI minimum stent area (MSA) using a predefined external elastic lamina (EEL)-based stent optimization protocol. OCT showed to be noninferior to IVUS (MSA of 5.79 vs. 5.89 mm ² ), but not superior to angiography guidance (MSA of 5.79 vs. 5.49 mm ² ). The study was underpowered ( n  = 450) to demonstrate any differences in clinical end points, but the OCT-guided PCI resulted in higher minimum and mean stent expansion than angiography-guided PCI group (87.6 vs. 82.9%, p  = 0.02 and 105.8 vs. 101.4%, p  = 0.001, respectively) and fewer untreated major dissections and persisting major malappositions than IVUS-guided PCI group (14 vs. 26%, p  = 0.009 and 11 vs. 21%, p  = 0.02, respectively). The noninferiority of OCT-guided versus IVUS-guided PCI (HR: 1.07, p  = 0.042) confirmed OPINION trial, ¹¹ in which target vessel failure (5.2 vs. 4.9%), in-stent (1.6 vs. 1.6%, p  = 1.00), and in-segment restenosis (6.2 vs. 6.0%, p  = 1.00) was comparable after 1 year. In the large national observation Pan-London registry ¹² with 87,166 patients after PCI including 1.149 OCT-guided and 10.971 IVUS-guided PCI, the OCT guidance was associated with lower all-cause mortality at a median of 4.8 years compared with IVUS-guided and angiography-guided PCIs (7.7, 12.2, and 15.7%, respectively; p  < 0.0001). The difference persisted in elective as well as acute coronary syndrome (ACS) subgroup, after multivariate Cox analysis and in case of propensity matching also in OCT versus angiography cohort. In the randomized multicenter DOCTORS trial ¹³ with non-ST-segmentation elevation ACS patients, OCT-guided group was associated with significantly higher FFR value than angiography-guided group (0.94 ± 0.04 vs. 0.92 ± 0.05, p  = 0.005). The result can be explained by OCT's superior ability of detecting stent malapposition, stent underexpansion, incomplete stent lesion coverage, and edge dissection, which led consequently to more frequent postdilatation and better stent expansion. Several studies evaluated strut coverage after several months interval. OCTACS study ¹⁴ with 100 patients with non-ST-segment elevation myocardial infarction, randomized 1:1 into OCT-guided or angiography-guided implantation of newer generation drug-eluting stent (DES) group, showed improved strut coverage in OCT guidance group (percentage of uncovered struts; 4.3 vs. 9.0%, p  < 0.01). Similarly, the DETECT-OCT study ¹⁵ of OCT-guided PCI in patients with stable coronary artery disease showed a better strut coverage at 3 months (7.5 vs. 9.9%, p  = 0.009). A recent study with patients undergoing OCT prior to DES or drug-eluting balloon treatment of in-stent restenosis (ISR) showed no significant differences in terms of MACE or target lesion revascularization (TLR). ¹⁶ Key findings of the above-mentioned studies are summarized in Table 2 .

Table 2. Key findings of OCT studies.

Study/first author, year	Number	Study design	Key findings
CLI-OPCI, 4 2012	670 patients	Observational study of OCT-guided vs. matched angiography-guided PCI	OCT guidance was associated with a significantly lower risk of cardiac death or MI at 1 y
OPUS-CLASS, 3 2013	100 patients	Prospective multicenter comparison of angiography, IVUS, OCT, and phantom models	OCT provides accurate and reproducible quantitative measurements of coronary dimensions
CLI-OPCI II, 5 2015	832 patients, 1,002 lesions	Retrospective multicenter analysis of OCT findings	Suboptimal stent deployment according to a specific OCT criteria was associated with an increased risk of MACE
ILUMIEN I, 8 2015	418 patients, 467 stenoses	Prospective observational study of pre- and post-PCI procedural practice using OCT and FFR	Physician decision-making was affected by OCT imaging prior to PCI in 57% and post-PCI in 27% of all cases. No difference in post-PCI FFR values between optimization groups
ILUMIEN II, 9 2015	940 patients	Retrospective matched-pair analysis of OCT-guided ILUMIEN I and IVUS-guided ADAPT-DES patients	OCT and IVUS guidance resulted in a comparable degree of stent expansion
OCTACS, 14 2015	100 patients	1:1 randomized OCT-guided or angiography-guided stent implantation	Improved strut coverage in OCT guidance group (4.3 vs. 9.0% uncovered struts) at 6 mo
Sheth et al, 7 2016	642 patients	2:1 propensity score matching of OCT guided and angiography guided in patients with STEMI	OCT-guided primary PCI for STEMI was associated with a larger final in-stent MLD
ILUMIEN III, 10 2016	450 patients	Multicenter RCT comparing the effects of OCT-guided, IVUS-guided, and angiography-guided PCIs	OCT-guided PCI using a specific reference segment EEL-based stent optimization strategy resulted in similar MSA to that of IVUS-guided PCI
FORMIDABLE-CARDIOGROUP IV and USZ registry, 6 2017	540 patients	Retrospective multicenter propensity score matching of OCT-guided and angiography-guided PCIs in patients with ACS	OCT-guided approach reduced the number of stents used. No statistically significant difference in clinical end points between groups
DOCTORS, 13 2016	240 patients	Multicenter RCT of OCT-guided and angiography-guided PCIs	OCT-guided PCI is associated with higher postprocedural FFR with similar procedural complications
OPINION, 11 2017	829 patients	Multicenter RCT noninferiority study of OCT-guided and IVUS-guided PCIs	OCT guidance was with number of target vessel failure, in-stent and in-segment restenoses noninferior to IVUS guidance at 1 y
DETECT-OCT, 15 2018	779 patients	Multicenter RCT of OCT-guided and angiography-guided PCIs	OCT-guided DES implantation improved early strut coverage compared with angiography-guided DES implantation
Pan-London, 12 2018	87,166 patients	Observational cohort study of OCT-guided, IVUS-guided, and angiography-guided PCIs	OCT guidance was associated with an improved clinical outcome

Abbreviations: DES, drug-eluting stent; EEL, external elastic lamina; FFR, fraction flow reserve; IVUS, intravascular ultrasound; MACE, mace major adverse cardiovascular events; MI, myocardial infarction; MLD, minimal lumen diameter; MSA, minimal stent area; OCT, optical coherence tomography; PCI, percutaneous coronary intervention; RCT, randomized controlled trial; STEMI, ST-segment-elevation myocardial infarction.

Currently, there are underway two major multicenter RCTs of 2-year clinical outcome after OCT-guided versus angiography-guided PCI. The ILUMIEN-IV trial (NCT03507777) is studying patients with high-risk clinical characteristics and/or angiographic lesions and the OCTOBER trial (NCT03171311) is investigating patients requiring complex bifurcation stent implantation. Finally, the CLIMA study (NCT02883088) is multicenter observational registry, which is investigating the correlation between OCT morphology of vulnerable atherosclerotic plaques in the left anterior descending artery with clinical outcome at 1 and 3 years intervals.

Position of Coronary OCT in Guidelines

Currently, there is no reference to the role of OCT in clinical practice in American College of Cardiology/American Heart Association/Society for Cardiovascular Angiography and Interventions guidelines for PCI guidelines. ^{17 18} The European Society of Cardiology (ESC) guidelines on myocardial revascularization recommend that OCT should be considered to detect stent-related mechanical problems leading to restenosis (class IIa, level of evidence C) and in selected patients to optimize stent implantation (class IIa, level of evidence B). ¹⁹ ESC guidelines for the management of STEMI from 2017 and for the management of ACS in patients presenting without persistent ST-segment elevation from 2020 include recommendation of OCT in their diagnostic flow chart, in case of myocardial infarction with nonobstructive coronary arteries, when thrombus, plaque erosion or rupture, or spontaneous coronary artery dissection (SCAD) are suspected. ^{20 21} According to a position paper of ESC on SCAD, an OCT imaging should be considered when uncertainty persists after diagnostic angiography. ²²

Preinterventional OCT Assessment

Plaque Morphology

OCT is an excellent method in morphologic assessment of arterial wall affected by atherosclerotic process. The normal vessel wall has three distinctive layers—intima, media, and adventitia. Therefore, the loss of triluminal structure immediately raises the suspicion of pathological process. Postmortem studies validated OCT in differentiation among fibrous, calcific, and lipid plaques with high sensitivity and specificity. ^{23 24} In comparison with IVUS or angioscopy, OCT has higher accuracy in identifying plaque rupture, erosion, dissection, intracoronary thrombus, and calcified nodule. ²⁵ The ability to differentiate between white (platelet-rich) and red (red blood cell and fibrin-rich) thrombi may have particular information value about the time of occurrence of acute event in patients with ACS. The basic morphology of individual types of tissue and structures on OCT image can be relatively easily identified by these five binary attributes: position (vessel wall/lumen), reflection (bright/dark), light attenuation (slight/distinct shadows), delineation (sharp/diffuse), and homogeneity of tissue (homogenous/heterogeneous). By this algorithm, the low-attenuating signal-rich bright region is fibrous plaque ( Fig. 2A ), high-attenuating, sharply delineated dark region covered with fibrous cap is lipid-rich plaque ( Fig. 2C ), and low-attenuating, sharply delineated homogenous region is calcific plaque ( Fig. 2B ). Inside the lumen, a low-attenuating bright structure is usually white thrombus and high-attenuating dark structure with bright surface is red thrombus.

Fig. 2.

Common pathological lesions and findings. ( A ) Fibrous plaque with calcified nodule (white arrow). ( B ) 360-degree calcified plaque. ( C ) Highly attenuated and sharply delineated signal representing a lipid plaque (white arrow). ( D ) White thrombi containing lesion (white arrows). ( E ) Stent underexpansion with severe malapposition (asterisk). ( F ) Multiple stent prolapses after stenting (white arrows). ( G ) Stent malapposition (asterisk) with tear of the vessel surface with an abluminal cavity. ( H ) 65% stent restenosis as a result of neoatherosclerosis.

One of the first steps in OCT-guided PCI is lesion evaluation, since the knowledge of plaque composition may guide the strategy of lesion preparation. The most common type of plaque, fibrous plaque, is defined as intimal thickness > 600 µm. ²⁶ Thin-cap fibroatheroma (TCFA) is defined as delineated necrotic core > 90 degrees on cross-sectional image with overlying fibrous cap with predetermined thickness < 65 µm. ²⁷ The cutoff minimal thickness is derived from histopathological samples, where sample shrinkage is not accounted for. Moreover, the minimal resolution of OCT is 10 µm; therefore, we recommend cutoff of 70 µm. A study comparing OCT-derived TCFA with histology as golden standard showed sensitivity, specificity, positive predictive value, negative predictive value, and overall diagnostic accuracy for TCFA 100, 97, 41, 100, and 98%, respectively. ²⁸ TCFA is associated with a higher risk of postprocedural myocardial infarction, lower thrombolysis in myocardial infarction grade, and no-reflow phenomenon, in case of later two, the risk increases with higher size of the lipid arc in the culprit plaque. ^{29 30} The use of distal protection filter failed to decrease the rates of periprocedural MI in lipid-rich regions. ³¹ In case of TCFA, a direct stenting without predilatation is recommended. It is also advised to avoid implanting the edges of stent into lipid-rich regions and instead, covering the entire plaque is recommended, since there is higher risk of edge-stent dissection and consequently in-stent thrombosis or later edge restenosis with higher rates TLR and MACE. ^{32 33 34}

OCT far exceeds the ability of CA ³⁵ in detecting and locating calcified plaque and in comparison with IVUS better quantify its thickness, since the signal is attenuated by calcium only moderately. ²⁷ The presence of calcified lesion is associated with stent underexpansion. ^{36 37} An OCT-based study of calcified lesions with a maximum angle of >180 degrees, maximum thickness >0.5 mm, and length of >5 mm showed increased risk of stent underexpansion. ³⁸ Therefore, according to the extent of calcified lesion, a cutting/scoring balloon, rotablation, or intravascular lithotripsy should be considered. The presence of calcium fractures is associated with greater stent expansion and lower TLR. ^{37 39} A pooled analysis of HORIZONS-AMI and ACUITY trials showed that the moderate or severe target lesion calcification was an independent predictor of 1-year definite stent thrombosis (HR: 1.62; 95% CI: 1.14–2.30; p  = 0.007) and ischemic TLR (HR: 1.44; 95% CI: 1.17–1.78; p  = 0.0007). ⁴⁰

Stent Sizing and Stent Length

Next important step after evaluation the plaque morphology is the determination of stent diameter. In practice, there are two options of stent sizing, either by using measurements of ELL or lumen diameter. Both protocols were used in already mentioned studies. ^{10 11} According to the ILUMIEN-III study, the maximum and minimum EEL diameters of distal and proximal reference segments are measured and mean EEL diameter of each reference segment is calculated. The smaller one is rounded down to the nearest 0.25 mm to derive stent diameter (e.g., 3.15–> 3.00 mm). ¹⁰ It is common agreement that IVUS is slightly better in imaging of EEL than OCT due to attenuation of light by lipid or calcified plaque in OCT. One study demonstrated the EEL was visible in 270 degrees only in 62.9% cases ⁴¹ ; however, in ILUMIEN-III study, EEL was visible in >180 degrees in 85% of cases. ¹⁰ The second option, lumen diameter-derived stent size, was recommended in OPINION trial, where calculated mean lumen diameter is up rounded by 0.25 mm (e.g., 3.15–> 3.25 mm). ¹¹

The stent length should be calculated as a distance between proximal and distance reference segments. Reference segments should have normal appearance and be close to target lesion as much as possible. As mentioned before, reference segments with TCFA should be avoided because of an association with edge dissection, stent thrombosis, and MACE. ^{32 33 34}

Angiographic coregistration enables to enhance stent placement accuracy, minimizes geographic miss, and eliminates angiographic ambiguity. The white marker directly project displayed OCT frame on the angiogram. 3D reconstruction may be used in bifurcation stenting.

Postprocedural OCT Assessment

The next step in optimization of OCT-guided PCI is assessment of stent expansion and apposition as well as searching for complications as tissue protrusion, dissection, or thrombus presence. Several studies proposed slightly different targets for stent optimization. ^{4 5 10 11 14} An algorithm for OCT-guided stent optimization and illustrative case is presented in Figs. 3 and 4 .

Fig. 3.

Algorithm for OCT-guided stent optimization. DRS, distal reference segment; EEL, external elastic lamina; MSA, minimal stent area; NC, noncompliant; OCT, optical coherence tomography; PCI, percutaneous coronary intervention; PRS, proximal reference segment.

Fig. 4.

Illustrative case of an OCT-guided PCI. (1) Visually 40% stenosis of LAD with positive hemodynamic fraction flow reserve measurement (FFR 0.80). Distal (1A), proximal (1C) reference segment, and minimal lumen area (1B) were identified and mean reference diameters were measured. The mean lumen diameter was calculated ([4.16 − 2.96]/2) as 3.55 mm. According to the algorithm, up rounded to the nearest stent size, the appropriate stent diameter would be 3.75 mm. Since the stent size of 3.75 mm was not available, the size of 3.50 mm was chosen instead. Due to calculated length between reference segments of 16.2 mm (1D), the length stent of 18 mm was selected. There were no lipid-rich plaques at the sites of reference segments. (2) Postinterventional OCT shows final result after implanting 3.5 × 18 mm DES at 16 atm and postdilatation with 3.75 × 8 mm balloon at 20 atm. The minimal stent area was detected with value of 8.65 mm ² ( A ) and residual area stenosis of 10.9% ( B ), thus meeting the criteria for target MSA > 4.5 mm ² . The achieved stent expansion was 95% ( C ).There was no tissue protrusion, no edge stent dissection, or malapposition ( D —coded as white stent struts). LAD, left anterior descending; OCT, optical coherence tomography; PCI, percutaneous coronary intervention.

Stent Expansion

IVUS studies confirmed that stent underexpansion ( Fig. 2E ) is an important predictor of stent failure. ^{42 43} Stent expansion can be evaluated by absolute measurement of MSA in cross-sectional view (absolute stent expansion) or by comparison of MSA to proximal, distal, or average reference segment (relative stent expansion). The OPTIS OCT system automatically detects minimal stent area and after highlighting the proximal and distal markers near stent edges automatically calculates the relative stent expansion. Studies showed that a greater absolute stent expansion is associated with a lower risk of stent failure and better clinical outcomes. ^{41 43 44} The MLA cutoff of ≥4.5 mm ² is commonly accepted as a MLA target value. The MLA value less or equal to 4.5 mm ² , both inflow and outflow MLA, is an independent predictor of MACE (HR: 4.65; p  < 0.001) after PCI. ⁵ The vessel caliber should be taken into consideration before effort to achieve target value, as using it in large vessels may result in undersizing and in small vessels may not be achievable. As for relative stent expansion value, different targets were studied. MSA greater than distal reference area, >90 or >80% of the average reference area was proposed as a target value. The consensus of experts considers MSA >80% of average reference area as the most reasonable. ⁴⁵ Moreover, the MSA value capable of prediction of FFR >90% was >79.4%. ¹³

Malapposition

The malapposition ( Fig. 2G ) is defined as axial distance >200 µm between the strut's surface and the luminal surface. OCT can automatically detect stent struts position in relation to lumen surface and automatically displays a rendered stent in the longitudinal or 3D view. The role of malapposition in clinical outcomes is still controversial and a subject of academic debate. Generally, the malapposition may appear in acute or late phase (positive arterial remodeling) and can lead to stent underexpansion and as a result, turbulent flows, whereas the proximal malapposition can prevent the re-entry in the vessel. There are studies that did not prove association between malapposition and adverse clinical outcome. ^{46 47 48} Contrary to these results, two large registries ^{49 50} showed a frequent malapposition in cases of acute, subacute (48%), as well as late and very late (31%) stent thromboses. ⁴⁹ Incomplete stent apposition (ISA) is associated with incomplete strut coverage by neointimal tissue. OCT studies ^{51 52} after PCI with second generation of DES showed that the best cutoff value for predicting late-persistent incomplete strut apposition is >350 µm, so achieving values below cutoff value should enable full neointimal integration. Analysis of Taniwaki et al ⁵³ reported a minimal longitudinal length of ISA segments >1 mm in patients with very late stent thrombosis.

Tissue Protrusion

Tissue protrusion or prolapse ( Fig. 2F ) is defined as the projection of intrastent protrusion of either nonthrombotic plaque or, in the context of ACS, atherothrombotic plaque. The definition of protrusion varies from 200 to 500 μm between studies. ^{4 5 10} It is recommended that the major protrusion is treated with either postdilatation or thromboaspiration in case of thrombus in ACS. Alternatively, more potent P2Y12 inhibitor in case of minor protrusions may be considered. Irregular protrusion and small MSA < 5.0 mm ² were independent predictors of adverse clinical outcomes, mainly driven by TLR. ⁵⁴ Tissue protrusion on OCT is an established predictor of acute and subacute stent thromboses. ^{55 56} In the CLI-OPCI and HORIZONS-AMI substudies with ACS patients, tissue protrusion was also associated with adverse clinical events. ^{55 57}

Edge Dissection

Dissections at the edge of stent were defined as a disruption of medial layer with extension of >60 degrees or >3 mm in length, both <5 mm within the stent edge. ^{10 33} Stent edge dissections may be overlooked in CA and are twice less to be detected by IVUS. ¹⁰ OCT revealed edge dissection at the distal stent edge, but not the proximal stent edge were associated with MACE (HR: 2.5) in the CLI-OPCI study. ⁴ In contradiction with this finding, two studies found that majority of minor OCT-detected edge dissections heal spontaneously, and are not associated with acute stent thrombosis or restenosis up to 8 and 12 months follow-up. ^{58 59} According to ESC expert consensus document, in line with IVUS studies, stent edge dissections are considered for OCT-defined predictors of early stent thrombosis. ⁴⁵

Fractional Flow Reserve and OCT Correlation

Several smaller studies investigated diagnostic efficiency of OCT-derived MLA in identifying hemodynamically significant coronary stenosis determined by FFR. The cutoff MLA value with ability to predict FFR value < 0.80 varies in studies between 1.59 and 2.43 mm ² with sensitivity 70 to 93% and specificity 63 to 90%. ^{60 61 62 63 64 65 66} The largest (203 intermediate coronary lesions) and most recent study ⁶⁷ compared OCT and IVUS in their performance of detecting significant functional ischemia in intermediate coronary stenosis. The best cutoff value for prediction of ischemia with FFR value < 0.75 was 1.39 mm ² (area under the curve [AUC]: 0.732, 95% CI: 0.660–0.804) with 46% false-positive results and 19% false–negative results. The study also showed that OCT-MLA value had significantly better diagnostic efficacy than IVUS-MLA value. This confirms another study, ⁶⁰ where in vessels having reference diameter <3 mm, the efficacy of OCT predictive value was higher than IVUS. In the study where reference diameter vessel was <3 mm, the best cutoff value of OCT-derived MLA for FFR ≤0.80 was 1.23 mm ² (AUC: 0.96, 95% CI: 0.83–1.0). ⁶² It can be concluded that OCT has only low to moderate diagnostic efficiency in identifying hemodynamically significant coronary stenosis, and although it is slightly superior to IVUS, it is not reliable enough to replace an important role of FFR in assessing of functional stenosis.

Mechanisms of Stent Failure

OCT should be considered to detect stent-related mechanical problems leading to stent thrombosis or restenosis (class IIa, level of evidence C). ¹⁹ OCT is considered a golden standard in thrombus detection, as it can distinguish thrombus from other tissue. According to large registries, ^{49 50 51} the most common finding in patients with early stent thrombosis in descending order is malapposition, underexpansion, and edge dissection, while in case of late and very late thromboses, it is malapposition, neoatherosclerosis ( Fig. 2H ), uncovered stent struts, and underexpansion. Although the data for tailored approach to stent failure are scarce, it is rational to choose PCI strategy with regard to the specific OCT findings, similarly as in IVUS. ⁶⁸ In case of underexpansion or malapposition, postdilatation may be sufficient, while in case of neoatherosclerosis, a DES implantation is an appropriated solution. The most frequent cause for stent restenosis is neointimal hyperplasia. Among other reasons is underexpansion (early ISR), stent fracture or neoatherosclerosis (late ISR), which can be only detected by OCT. ^{69 70}

Conclusion

The presented evidence confirms the justified present and future role of optical coherence tomography in interventional cardiology. In addition to IVUS, OCT offers better imaging resolution, faster pullback, angiography coregistration, various automatic measurements, and more user-friendly interface. Furthermore, there is growing evidence for improved imaging and clinical outcomes of OCT-guided PCI, although we still lack the data of randomized clinical trial of clinical outcomes. The key limitation that prevents further adoption of OCT guidance in PCI is the cost and variability in reimbursement that differs across the world with Japan being on top of the list, ⁷¹ followed by countries in Europe, North America, and rest of the world. To maximize the utility of OCT, physicians should recognize the clinical situations (summarized in Table 3 ), in which OCT brings added value and be trained to proceed effectively with prespecified protocol and in time-effective manner. The optimization of stenting and procedural/strategy guidance is currently the most common use of coronary OCT. The new hybrid OCT/IVUS catheters and further technological improvements will push the boundaries of intracoronary imaging even further.

Table 3. Situations in clinical practice where use of optical coherence tomography is recommended.

PCI guidance and optimization

Ambiguous findings on coronary angiography

Complex bifurcation stenting

PCI of chronic total occlusions

PCI of distal left main coronary artery

Search for culprit lesion of ACS

Identification of mechanisms of stent failure

Myocardial infarction with nonobstructive coronary arteries

Abbreviations: ACS, acute coronary syndrome; PCI, percutaneous coronary intervention.

Funding Statement

Funding None.