Introduction

The use of intracoronary optical coherence tomography (OCT) into percutaneous coronary intervention (PCI) workflows has significantly refined lesion assessment and therapeutic decision-making process, advancing the field from angiography-guided to anatomy-guided intervention [1, 2]. The interpretation of OCT imaging for procedural guidance requires a systematic approach that includes both pre-intervention assessment and post-intervention optimization [3]. In the pre-procedural setting, OCT provides essential insights into plaque morphology and phenotype, facilitating the identification of high-risk features and informing lesion preparation strategy [4]. In addition, OCT allows for precise geometric measurements, including reference vessel diameter and lesion length, that guide stent sizing.

Following stent implantation, post-PCI OCT evaluation enables a detailed assessment of procedural results. Through the identification of key mechanical aspects such as stent expansion, apposition, edge dissections, and intraluminal abnormalities, OCT supports immediate procedural improvement and contributes to the long-term success of the intervention [3, 5].

Pre-PCI Guidance

Assessment of Atherosclerotic Plaque Morphology and Phenotype

The characterization of coronary plaque morphology plays a pivotal role in PCI procedural planning [1, 3]. It represents a critical step that directly influences both technical procedural success and long-term clinical outcomes. Thanks to its high spatial resolution, OCT enables in vivo identification of the key structural components of atherosclerotic lesions and provides accurate insight into plaque composition (Fig. 1). Pathologic segments exhibit distinct light-attenuation profiles depending on their histologic characteristics. Fibrous plaques typically appear as low-attenuating, signal-rich regions that preserve the trilaminar structure of the vessel wall and are often distinguished from normal segments by eccentric intimal thickening. Lipid-rich plaques demonstrate high signal attenuation with poorly defined deep borders and signal-poor cores, usually covered by a fibrous cap of variable thickness. Fibrous and lipid-rich plaque lesions are frequently treated with a direct stenting strategy or a conventional balloon. Calcific plaques are visualized as sharply demarcated, signal-poor regions with posterior shadowing, reflecting the high refractive index of calcium. Notably, calcified lesions are associated with stent underexpansion and worse clinical outcomes and may require more aggressive lesion preparation [6]. In this context, OCT allows assessment of different patterns of calcium distribution (deep, superficial, nodular) and provides information on arc, thickness, and calcium length [7]. These three characteristics constitute the basis for an OCT-derived calcium scoring system proposed to predict the risk of stent underexpansion [8]. The score assigns two points for a calcium arc >180°, and one point each for calcium thickness >0.5 mm and calcium length >5 mm. A cumulative score ≥ 4 identifies lesions at high risk of suboptimal expansion. Based on these findings, the expert consensus document on the clinical use of intravascular imaging by the European Association of Percutaneous Cardiovascular Interventions (EAPCI) recommends ad-hoc lesion preparation when calcium arc exceeds 180° or calcium thickness is >0.5 mm [9]. In cases of mild-to-moderate calcification (score ≤ 2), non-compliant balloon predilation is often sufficient. However, moderate-to-severe calcification (score ≥ 3) may necessitate debulking techniques—such as rotational atherectomy, laser, cutting balloons, or intravascular lithotripsy—prior to stent deployment.(Fig. 2)

Fig. 1
Fig. 1
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Pre-PCI OCT assessment

Fig. 2
Fig. 2
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Post-PCI OCT assessment

Other than the identification of signal attenuation originating from the vessel wall, OCT also detects signal attenuation originating from the lumen, strongly suggesting the presence of intraluminal thrombus. This is particularly relevant in acute coronary syndromes (ACS), where thrombus identification influences both procedural strategy and adjunctive pharmacotherapy. Red thrombus, rich in erythrocytes, appears as a high-attenuating, shadowing structure, whereas white thrombus presents as a low-attenuating, protruding mass with minimal shadowing.

Beyond plaque morphology, the phenotypic characterization of atherosclerotic plaques by OCT has significantly advanced the understanding of coronary pathophysiology, particularly in ACS, and plays a key role in guiding PCI strategy [1]. The most frequent pathophysiologic mechanism underlying ACS is plaque rupture, which is identified on OCT as a discontinuity in the fibrous cap with a visible cavity within a lipid-rich plaque, often accompanied by overlying thrombus [10]. By contrast, plaque erosion presents with fibrous cap integrity and is characterized by intraluminal thrombus, often white, overlying an intact surface [11]. Erosions are often associated with lower plaque burden, preserved distal flow, and non-occlusive thrombus. These characteristics have led to increasing interest in conservative management of OCT-confirmed plaque erosion in selected patients with stable hemodynamics [12]. Another distinct phenotype recognizable by OCT is calcified nodule [11]. Among its variants, the eruptive calcified nodule—corresponding pathologically to fragments of calcium protruding through disrupted fibrous caps—has been clearly implicated in plaque destabilization and thrombus formation [13]. Importantly, all forms of calcified nodules have been associated with increased periprocedural complications and suboptimal PCI outcomes, reinforcing the need for phenotype-specific strategies during lesion preparation and stenting [14].

Identification of Proximal and Distal Reference Size and Lesion Length

Accurate identification of both proximal and distal reference segments is essential for optimal stent sizing and to provide adequate lesion coverage. If not adequately planned, geographic miss, edge dissections, suboptimal expansion, or incomplete lesion coverage may occur, each of which contributes to early stent thrombosis, restenosis, and target lesion failure [3]. By integrating both geometric and plaque morphology information, OCT provides a highly refined approach to stent selection and deployment (Fig. 1). Identification of proximal and distal reference segments, achieved by scrolling through the OCT pullback, results in the determination of lesion length. Optimal stent landing zones are selected within approximately 5 mm of healthy or minimally diseased vessel [4]. Importantly, landing zones should avoid segments containing lipid pools, thin-cap fibroatheromas, or eccentric calcification, all of which have been associated with an increased risk of edge complications and periprocedural myocardial infarction [15]. Once proximal and distal boundaries are set, modern OCT systems automatically calculate lesion length, assisting in the selection of appropriate stent length. Stent diameter selection is based on the measurement of the reference vessel size, ideally using the external elastic lamina (EEL) as a marker of true vessel dimension. When EEL is clearly visible on both sides of the vessel at the reference sites, the mean EEL diameter should be used, particularly in the distal segment, which often defines the minimum diameter and helps avoid oversizing. In diffusely diseased or tapering vessels, where the distal reference may appear undersized or the EEL is poorly visualized, a lumen-based strategy is recommended. This typically involves measuring the mean distal lumen diameter and then rounding up the stent diameter by 0.25–0.5 mm (e.g., 3.76 mm → 4.00 mm). Alternatively, when EEL-based sizing is possible, operators may round down to the nearest nominal size to avoid oversizing (e.g., EEL of 3.76 mm → 3.50 mm stent). These principles also guide the selection of post-dilation balloon sizes, ensuring adequate expansion without excessive trauma. In vessels with diffuse disease, it may be necessary to land in a segment that is only near-normal, rather than plaque-free.

Post- PCI Guidance

Stent Expansion

Stent expansion refers to the degree of luminal enlargement achieved by the stent after deployment and represents a direct consequence of lesion preparation quality and appropriate device sizing during procedural planning [9]. Adequate stent expansion is identified as the strongest predictor of future adverse events after DES implantation [5, 9]. Multiple indices have been proposed to quantify stent expansion, the most commonly used being the ratio of the minimum stent area (MSA) to the average reference lumen area, calculated as the mean of the proximal and distal reference areas. Current EAPCI consensus document on intracoronary imaging guidance recommends targeting a relative expansion of at least 80%, calculated as MSA divided by the average reference lumen area [9]. In non–left main lesions, an absolute MSA >4.5 mm² is considered acceptable, with some studies proposing 5.0 mm² as an optimal safety margin in complex lesions or large-caliber vessels [16, 17]. In a pre-specified analysis of ILUMIEN IV, an increase in the risk of TLF has been observed when the MSA falls below 4.0 mm², with every 1 mm² increase in MSA associated with a 24% reduction in TLF at two years, underscoring the importance of maximizing stent expansion in clinical practice [18]. The physiological implications of suboptimal expansion are also substantial. A small MSA has been strongly associated with lower post-PCI fractional flow reserve (FFR) [19]. When stent under-expansion is identified on post-procedural OCT pullback, operators are encouraged to reassess the proximal and distal reference segments—typically defined as the 5 mm of vessel adjacent to either stent edge—to confirm whether vessel dimensions have changed compared to pre-PCI imaging. If vessel size is unchanged and high-pressure post-dilatation has already been performed (e.g., ≥ 18 atm with a non-compliant balloon), further optimization may offer limited additional benefit [3]. Conversely, if vessel dimensions appear underestimated or if no post-dilatation was previously performed, balloon upsizing and further high-pressure inflation should be considered, ideally guided by EEL- or lumen-based sizing algorithms. The decision to pursue additional post-dilatation must balance the potential benefits of increased stent area and improved expansion ratios against the incremental procedural risks, including vessel injury or perforation.

Stent Apposition

Stent apposition refers to the complete contact between the outer surface of the stent struts and the luminal contour of the vessel wall [9]. A strut is considered malapposed when the axial distance between its surface and the intima exceeds the nominal strut thickness, including any polymer coating. The clinical significance of acute malapposition remains controversial. The potential implications of malapposition are likely influenced by anatomical and procedural factors. In large-caliber vessels, minor separation may be negligible, whereas in smaller arteries or bifurcation segments, it may hinder flow, or make difficult further guidewire recrossing, increasing the risk of abluminal passage and suboptimal procedural results [3, 4]. According to the EAPCI consensus document, routine correction of acute malapposition is not recommended when the axial separation is less than 400 micrometers and the longitudinal extension is under 1 mm [9]. In such cases, spontaneous neointimal coverage is anticipated, and procedural manipulation may be avoided. Thus, while acute malapposition is not in itself an indication for further intervention, its presence should be evaluated based on vessel size, lesion complexity, and procedural goals. If needed, in the case of isolated malapposition without stent underexpansion, low-pressure inflation with a semi-compliant balloon is generally sufficient. High-pressure inflation with non-compliant balloons is usually unnecessary and may pose additional risks.

Edge Dissection

Edge dissections is defined as a tear of the luminal surface occurring at the proximal or distal stent edges [9]. These injuries may range from minor intimal flaps to deeper lesions involving medial or adventitial layers. Approximately the 80% of edge dissections are angiographically silent, and the majority heal spontaneously over time without clinical consequence [20]. Nevertheless, detailed characterization of dissection morphology, including longitudinal extension, circumferential arc, dissection depth (intimal, medial, or adventitial), flap thickness, and cavity depth (the distance between flap and the underlying plaque) is essential during results assessment, since a progression to intramural hematoma may result in acute vessel closure, or promote adverse remodeling in the long term [3]. OCT enables detailed characterization of dissection morphology, including longitudinal extension, circumferential arc, dissection depth (intimal vs. medial or adventitial), flap thickness, and cavity depth—the latter representing the distance between the flap and the underlying plaque. Although no universal criteria currently exist for defining major dissections, there is general agreement that certain morphological features confer a higher risk of adverse outcomes. While minor intimal flaps may be safely left for a conservative approach, dissections with extensive morphological features—especially when associated with medial involvement or intramural hematoma—should be treated by additional stent implantation [4, 9]. In the CLI-OPCI registry, a linear rim of tissue exceeding 200 μm in thickness was used to define a major dissection [21]. When present at the distal edge, this morphology conferred a 2.5-fold increased risk of major adverse cardiac events, including death, myocardial infarction, and target lesion revascularization. Interestingly, the same morphology did not appear to carry significant prognostic weight when observed at the proximal edge. A pre-specified analysis of the ILUMIEN IV trial demonstrated that edge dissections involving intramural hematoma with an arc ≥ 60° and a length >3 mm were independently associated with a 1.7-fold increased risk of target lesion failure, cardiac death, and target vessel myocardial infarction [18]. Notably, this elevated risk was observed even for proximal edge dissections, challenging the traditional view that distal dissections are more clinically significant. According to the EAPCI consensus document, high-risk features include a circumferential arc exceeding 60°, longitudinal extension greater than 2 mm, involvement of deep vessel layers, the presence of residual plaque burden at the dissection site, and localization at the distal stent edge [9].

Tissue Prolapse

Tissue prolapse refers to the intrusion of atheromatous or thrombotic material through the stent struts into the lumen [2]. This finding is particularly common in the setting of acute coronary syndromes, where lipid-rich plaques and thrombus are prevalent [16, 21]. OCT enables clear delineation of prolapsed material, differentiating between organized thrombus, necrotic core, and plaque fragments. Tissue prolapse is observed more frequently when stents are deployed over thin-cap fibroatheromas or large necrotic cores. The clinical relevance of tissue prolapse remains uncertain. In most cases, it is not associated with immediate or delayed adverse events and it does not need any additional intervention. However, extensive prolapse in combination with suboptimal stent expansion or inadequate stent apposition may be a signal of incomplete lesion coverage. The decision to perform further optimization should be guided by the size, and composition of the prolapsed tissue, as well as the overall procedural context. In select cases, further balloon inflation may be justified to restore optimal luminal geometry and minimize residual thrombotic risk.

Conclusions

OCT application for PCI guidance, from pre-procedural planning to post-stent optimization, has demonstrated significant potential in enhancing procedural precision and clinical outcomes. The increasing body of evidence supporting OCT-guided intervention, particularly in complex coronary lesions, has reshaped the role of intracoronary imaging in routine clinical practice, which has obtained class IA evidence in the latest European and American guidelines [22, 23]. In this evolving landscape, OCT use is essential to ensure optimal PCI results and optimize individual patient outcomes.

Key References

  • Ali ZA, Karimi Galougahi K, Mintz GS, Maehara A, Shlofmitz RA, Mattesini A. Intracoronary optical coherence tomography: state of the art and future directions. EuroIntervention 2021;17:e105-e123.

    • [Comprehensive state-of-the-art review summarizing OCT principles, clinical applications, and emerging frontiers. Chosen as a cornerstone reference to provide an updated overview and future perspectives].

  • Romagnoli E, Burzotta F, Vergallo R et al. Clinical impact of OCT-derived suboptimal stent implantation parameters and definitions. Eur Heart J Cardiovasc Imaging 2023;25:48–57.

    • [Focuses on how OCT-defined parameters of stent underexpansion, malapposition, and edge complications correlate with outcomes. Chosen for its direct relevance to post-PCI optimization].

  • Raber L, Mintz GS, Koskinas KC et al. Clinical use of intracoronary imaging. Part 1: guidance and optimization of coronary interventions. An expert consensus document of the European Association of Percutaneous Cardiovascular Interventions. EuroIntervention 2018;14:656–677.

    • [EAPCI consensus providing practical recommendations on OCT/IVUS for PCI planning and optimization, establishing standardized criteria for clinical use].

  • Landmesser U, Ali ZA, Maehara A et al. Optical coherence tomography predictors of clinical outcomes after stent implantation: the ILUMIEN IV trial. Eur Heart J 2024;45:4630–4643.

    • [The largest global RCT of OCT-guided PCI]