Intravascular Imaging: IVUS and OCT for PCI Optimisation

Definition and Pathophysiology

Intravascular imaging comprises catheter-based techniques that directly visualize the coronary lumen, vessel wall, plaque, thrombus, and implanted stents. The principal modalities used in the catheterization laboratory are intravascular ultrasound (IVUS) and optical coherence tomography (OCT). Unlike coronary angiography, which produces a two-dimensional luminogram and has limited specificity for plaque and vessel-wall pathology, intravascular imaging provides cross-sectional and longitudinal information about the artery.

The clinical value of these techniques rests on their ability to characterize the anatomy responsible for a stenosis, define plaque morphology, identify occult lesions, and optimize percutaneous coronary intervention (PCI). Imaging can therefore influence several stages of intervention:

• lesion preparation;

• selection of stent diameter and length;

• identification of the proximal and distal reference segments;

• assessment of stent expansion and apposition;

• recognition of edge dissection, tissue prolapse, thrombus, or other acute complications;

• investigation of restenosis and stent thrombosis.

Intravascular imaging may also clarify the mechanism of an acute coronary syndrome (ACS), particularly when angiography shows no significant obstructive disease or when the culprit lesion is uncertain. In these settings, OCT or IVUS may demonstrate plaque rupture, thrombus, spontaneous coronary artery dissection (SCAD), or another occult structural abnormality.

IVUS

IVUS generates images by transmitting ultrasound from an intracoronary catheter and recording echoes reflected from the vessel wall. Contemporary systems use frequencies ranging from approximately 20 to 60 MHz. Their principal advantage is tissue penetration, generally allowing assessment several millimetres beyond the lumen and visualization of the external elastic membrane (EEM). IVUS is consequently useful for determining vessel size, plaque burden, and the dimensions of lesions that may not be fully characterized by angiography.

Standard gray-scale IVUS demonstrates the principal vessel-wall layers and allows quantitative measurements of:

• minimum and maximum lumen diameter;

• lumen area;

• EEM or vessel area;

• atheroma area;

• plaque burden;

• minimum stent area.

The limitations of gray-scale IVUS include lower spatial resolution than OCT and imperfect discrimination between plaque components. Radiofrequency or virtual-histology IVUS can provide additional information regarding calcific, fibrotic, fibro-fatty, and necrotic components. Virtual-histology IVUS has also been used to identify thin-cap fibroatheroma, a plaque phenotype associated with subsequent major adverse cardiovascular events in observational studies.

OCT

OCT uses near-infrared light rather than ultrasound. Its resolution is substantially higher than that of IVUS, approximately 5–20 µm depending on the system, compared with roughly 70–200 µm for gray-scale IVUS. This allows detailed visualization of the lumen surface, fibrous cap thickness, thrombus, stent struts, tissue prolapse, and strut coverage.

OCT has limited tissue penetration, generally approximately 1–3 mm, and may not show the EEM through substantial plaque or calcium. Blood clearance is required because blood causes strong backscatter of the light signal. Imaging is therefore typically performed during contrast injection or another method of temporary blood displacement.

OCT is particularly useful for:

• characterizing the surface of atheroma;

• measuring fibrous cap thickness;

• distinguishing thrombus from plaque;

• evaluating occult plaque rupture;

• identifying spontaneous coronary dissection or intramural hematoma when angiography is inconclusive;

• assessing stent expansion, apposition, edge dissections, and endothelial coverage.

Complementary Characteristics

IVUS and OCT are complementary rather than interchangeable. IVUS generally provides better visualization of overall vessel dimensions, plaque burden, large vessels, ostial lesions, chronic total occlusions, and the left main coronary artery. OCT provides superior resolution for superficial plaque morphology, thrombus, fibrous caps, and stent-strut assessment, but its lower penetration and need for blood clearance restrict its use in some circumstances.

Clinical Presentation and Symptoms

Intravascular imaging is not a disease with a characteristic symptom complex. It is an adjunctive diagnostic and procedural technique used in patients undergoing coronary evaluation or PCI.

The clinical settings in which it is most relevant include:

• stable angina or chronic coronary syndrome with an intermediate or anatomically complex lesion;

• ACS with a clear culprit lesion requiring PCI;

• ACS with no significant obstructive coronary disease on angiography;

• ACS in which the culprit lesion is ambiguous or multiple potential culprit lesions are present;

• left main coronary artery disease;

• long lesions, true bifurcations, ostial lesions, heavily calcified lesions, and chronic total occlusions;

• suspected stent failure, including restenosis or stent thrombosis;

• suspected SCAD when angiography is diagnostically uncertain.

In patients with ACS, culprit-lesion ambiguity is clinically important because more than one lesion may be responsible for the presentation, while angiography may fail to distinguish thrombus, plaque disruption, calcification, or other causes of luminal haziness. OCT or IVUS can provide direct structural information in these circumstances.

Evaluation and Physical Examination

Physical examination does not determine whether IVUS or OCT is required and cannot replace angiographic, physiological, or intravascular assessment of a coronary lesion. The decision to use intravascular imaging depends principally on the clinical presentation, angiographic findings, lesion complexity, and whether PCI or another revascularization strategy is being considered.

A key preliminary consideration is whether the lesion is anatomically or clinically suitable for imaging. In SCAD, for example, intracoronary imaging must be approached cautiously. If angiography has already established SCAD and medical therapy is planned, additional coronary instrumentation is not recommended on safety grounds. When diagnostic uncertainty justifies imaging, the guidewire must first be confirmed to lie in the true lumen. Vessel tortuosity, small vessel diameter, and distal lesion location may make imaging hazardous or technically impractical.

Diagnostics

Coronary Angiography and Intravascular Imaging

Angiography remains the principal method for identifying the coronary arterial tree and guiding catheter-based intervention, but it provides only a two-dimensional representation of the lumen. It may underestimate plaque burden, fail to define the true vessel size, and provide limited information about the mechanism of an angiographically ambiguous lesion.

IVUS and OCT add cross-sectional information and allow the operator to correlate the angiographic appearance with:

• lumen dimensions;

• plaque distribution and burden;

• vessel-wall architecture;

• calcium;

• thrombus;

• lesion length;

• proximal and distal reference anatomy;

• stent geometry.

Coregistration of intravascular imaging with x-ray angiography permits point-to-point localization of the imaged segment. This can improve lesion characterization and stent positioning. In a prospective single-arm study of elective PCI, the angiographically identified target segment would have been left uncovered by the stent in approximately 70% of cases if coregistered OCT information had not been available.

IVUS Acquisition and Interpretation

IVUS is performed through a coronary guide catheter over a 0.0014-inch guidewire using standard catheterization techniques. The catheter is advanced distal to the lesion, followed by automated or manual pullback, commonly at approximately 0.5 mm/s. A bolus of intracoronary nitroglycerin is used to reduce arterial spasm and improve image assessment, and adequate anticoagulation is used for thrombus prevention.

The IVUS image normally displays the lumen, intima, media, and adventitia. The EEM represents the boundary between media and adventitia and is important for estimating the true vessel size. Measurements should be obtained at the lesion and at suitable proximal and distal reference segments.

Important quantitative parameters include:

• minimum lumen area (MLA);

• minimum lumen diameter (MLD);

• reference lumen area;

• EEM area and diameter;

• plaque or atheroma area;

• plaque burden;

• minimum stent area (MSA);

• stent expansion;

• stent apposition.

Stent expansion can be expressed as the MSA divided by the average reference lumen area. In the example provided, an MSA of 7.0 mm2 corresponded to stent expansion of 88.2% relative to the average proximal and distal reference lumen areas.

OCT Acquisition and Interpretation

OCT requires temporary clearance of blood from the imaged segment, usually by simultaneous contrast injection. The resulting high-resolution images provide detailed information about the lumen surface and superficial plaque.

Characteristic OCT appearances include:

OCT can detect stent malapposition, in which stent struts are separated from the vessel wall. It can also demonstrate incomplete expansion, edge dissection, tissue prolapse, thrombus, and delayed or incomplete strut coverage.

Physiological Correlation

Intravascular imaging and pressure-based physiology answer related but distinct questions. FFR and other physiological assessments determine whether a lesion is functionally significant, whereas IVUS and OCT define structural anatomy.

For IVUS, the following values are presented as correlates of ischemia:

An IVUS MLA below 6.0 mm2 in the left main coronary artery correlates with an FFR below approximately 0.75–0.80. However, current practice has largely replaced IVUS assessment of borderline stenoses with FFR, although IVUS remains important for left main disease and for PCI optimization.

FFR is generally recommended for intermediate lesions, whereas it is not recommended for lesions with very severe angiographic stenosis or in certain culprit-lesion settings. In lesions with FFR ≤0.80, PCI is selected in the algorithm provided; with FFR >0.80, optimal medical therapy is selected.

Assessment of Stent Failure

IVUS and OCT can determine the mechanism of restenosis and stent thrombosis. Potential findings include:

• inadequate stent expansion;

• malapposition;

• edge dissection;

• residual disease at the stent margin;

• tissue prolapse;

• thrombus;

• neoatherosclerotic or other intrastent pathology.

OCT offers particularly detailed evaluation of strut apposition and tissue coverage, while IVUS may better define larger vessel dimensions and deeper structural abnormalities.

Biomarkers and Laboratory Findings

The source material does not describe specific biomarker or laboratory findings for intravascular imaging itself. IVUS and OCT are imaging procedures and do not have characteristic circulating biomarker profiles. Their use in ACS is determined by the clinical syndrome and coronary findings rather than by a biomarker threshold specified here.

Treatment and Management

Role in PCI

The principal therapeutic role of IVUS and OCT is to guide and optimize PCI rather than to serve as stand-alone treatment. Imaging may alter the procedure by identifying the need for lesion preparation, selecting the appropriate stent size and length, and directing post-deployment optimization.

A practical imaging-guided PCI sequence includes:

Pre-intervention assessment

Stent selection

Post-stent assessment

IVUS and OCT are particularly valuable in complex PCI, including:

• unprotected left main interventions;

• long lesions;

• true bifurcations;

• ostial lesions;

• chronic total occlusions;

• heavily calcified lesions;

• lesions with uncertain vessel size;

• procedures in which stent failure would have major clinical consequences.

Intravascular imaging is considered essential for optimization of bioresorbable vascular scaffolds in the source material.

Acute Coronary Syndromes

For ACS with a clear culprit lesion suitable for PCI, intravascular imaging can guide the intervention and optimize stent deployment. For ACS without a clear culprit lesion, imaging may help establish the diagnosis and select a treatment strategy, with OCT generally favored when the principal question concerns superficial plaque disruption, thrombus, or an ambiguous culprit.

In patients with ACS and no significant obstructive coronary artery disease on angiography, imaging can help exclude or identify an atherothrombotic cause in the major coronary arteries. This distinction may influence both the immediate invasive strategy and decisions regarding prolonged antithrombotic treatment.

Spontaneous Coronary Artery Dissection

In SCAD, imaging should not be used routinely when angiography is diagnostic and medical therapy is planned. If the diagnosis remains uncertain after angiography and the potential benefit of clarification outweighs the risks of coronary instrumentation, IVUS or OCT may be considered. The guidewire must be confirmed within the true lumen before the imaging catheter is advanced.

PCI in SCAD is reserved for patients with symptoms and signs of ongoing myocardial ischemia, a large jeopardized myocardial territory, and reduced antegrade flow. When PCI is undertaken, intravascular imaging should be considered for guidance.

Procedural Considerations and Risks

IVUS catheters commonly have a crossing profile of approximately 2.9–3.2 Fr and are compatible with 5–6 Fr guide catheters. Complications are uncommon and generally self-limited, although coronary dissection or perforation has been estimated at approximately 1.6% in the material provided. Risk is influenced by vessel size and the force required to advance the catheter.

OCT requires contrast for blood clearance and should therefore be used cautiously in patients with chronic kidney disease. IVUS may be preferable when blood clearance is problematic or when deep vessel-wall assessment is required.

Guideline Recommendations

Chronic Coronary Syndrome and Complex PCI

Intracoronary imaging with IVUS or OCT is recommended when PCI is performed in anatomically complex lesions, particularly:

• left main stem disease;

• true bifurcations;

• long lesions.

This recommendation is classified as class I, level A in the cited European guideline.

The 2021 ACC/AHA revascularization guidance considers IVUS or OCT reasonable in selected patients to optimize stent implantation, with particular emphasis on left main coronary artery stenting. The 2021 guideline also supports IVUS for angiographically indeterminate left main disease and considers it reasonable for selected indeterminate non-left-main lesions. Routine IVUS assessment is not recommended when revascularization with PCI or coronary artery bypass grafting is not being contemplated.

European guidance recommends IVUS for assessing the severity of unprotected left main lesions and for optimizing their treatment. IVUS should also be considered for selected patients undergoing stent implantation and for detecting stent-related mechanical problems causing restenosis.

Acute Coronary Syndrome

For ACS with a clear culprit lesion suitable for PCI, the European guidance assigns intravascular imaging a class IIa, level A recommendation for PCI guidance.

When the culprit lesion is ambiguous, imaging—preferably OCT—may be considered; this is classified as class IIb, level C in the cited recommendation. The purpose is diagnostic clarification and treatment selection rather than routine imaging of every ACS patient.

Stent Restenosis and Thrombosis

IVUS is considered reasonable for determining the mechanism of stent restenosis. It may also be reasonable for investigating the mechanism of stent thrombosis. OCT can provide additional high-resolution information about malapposition, strut coverage, thrombus, and edge pathology.

European and American Recommendations for IVUS

Evidence from Clinical Trials

The available randomized evidence supports imaging-guided PCI over angiography-guided PCI, particularly in complex lesions, although the strength and consistency of evidence differ between IVUS and OCT.

IVUS-guided PCI has been associated with reductions in major adverse cardiovascular events, cardiovascular death, myocardial infarction, and target-lesion revascularization compared with angiography-guided PCI. In the ULTIMATE trial, which included 1,448 all-comer patients, IVUS-guided drug-eluting stent implantation reduced target-vessel failure at 12 months.

In the RENOVATE-COMPLEX PCI trial, predominantly IVUS-guided PCI, with a smaller OCT component, reduced the two-year composite of cardiac death, target-vessel myocardial infarction, or clinically driven target-vessel revascularization compared with angiography-guided PCI: 7.7% versus 12.3%.

OCT-guided trials have produced more heterogeneous findings:

• In ILUMIEN IV, OCT produced a larger MSA than angiography guidance and reduced definite or probable stent thrombosis, but did not significantly reduce two-year target-vessel failure.

• In OCTOBER, which evaluated technically challenging bifurcation lesions, two-year major adverse cardiovascular events occurred in 10.1% of the OCT-guided group and 14.1% of the angiography-guided group.

• In OCTIVUS, OCT-guided PCI was noninferior to IVUS-guided PCI for the composite of cardiac death, target-vessel myocardial infarction, or ischemia-driven target-vessel revascularization at one year.

A network meta-analysis incorporating IVUS- and OCT-guided studies found that intravascular imaging-guided PCI reduced target-lesion failure by 31% compared with angiography-guided PCI. The analysis also reported reductions in cardiac death, target-vessel myocardial infarction, target-lesion revascularization, and stent thrombosis. The evidence was strongest for IVUS, while uncertainty remained greater for OCT.

Prognosis and Follow-up

Intravascular imaging improves characterization of coronary disease and can identify procedural abnormalities that are not evident on angiography. The strongest prognostic implications relate to PCI quality: larger final stent area, improved expansion and apposition, and recognition of mechanical causes of restenosis or thrombosis.

Observational data indicate that patients undergoing IVUS-guided PCI may receive longer and larger stents and higher post-dilation pressures. Randomized and pooled data support reductions in adverse outcomes with imaging-guided PCI, particularly in complex lesions.

The presence of vulnerable plaque on imaging may identify lesions associated with later major adverse cardiovascular events. In the PROSPECT study, nonculprit-lesion events were associated with plaque burden of at least 70%, an MLA of 4 mm2 or less, and thin-cap fibroatheroma with a cap thickness below 65 µm. At three years, however, major adverse events were related to both culprit and nonculprit lesions.

After PCI, imaging is useful when clinical or angiographic findings suggest:

• restenosis;

• stent thrombosis;

• persistent ischemia;

• uncertain stent expansion;

• suspected malapposition;

• edge injury or dissection.

The source material does not specify a universal routine follow-up imaging schedule after PCI. Rather, repeat IVUS or OCT is used selectively to investigate complications, assess stent failure, or resolve diagnostic uncertainty. In cardiac transplantation, IVUS and coronary angiography are considered reasonable at 4–6 weeks and one year to exclude donor coronary disease, detect rapidly progressive cardiac allograft vasculopathy, and provide prognostic information.