What Is IVUS and How Does It Work?

Introduction

Coronary angiography shows the silhouette of blood flow through your arteries. Intravascular ultrasound shows the arteries themselves. This distinction matters more than most patients realize. An angiogram that reports “40% stenosis” reveals nothing about the composition of the plaque, the true vessel size, or whether the artery has remodeled outward to accommodate disease that remains hidden from the lumen view.

IVUS addresses these limitations by imaging the vessel wall directly. A miniaturized ultrasound transducer at the tip of a catheter creates cross-sectional images as it travels through the coronary artery, revealing plaque burden, composition, and anatomical details invisible to angiography. This article explains how the technology works, what it measures, and why those measurements matter for clinical decisions.

Understanding IVUS basics provides the foundation for evaluating when it might be useful for your own care. The subsequent articles in this series address how IVUS compares to other imaging technologies, what the clinical evidence shows, and how to advocate for IVUS if you believe it could benefit you.

What is intravascular ultrasound and what does the procedure involve?

Intravascular ultrasound is an invasive imaging technique performed during cardiac catheterization. A thin catheter containing a miniaturized ultrasound transducer is threaded through the coronary arteries, typically over a guidewire already positioned for diagnostic angiography or intervention. The transducer emits high-frequency sound waves that reflect off tissue interfaces, creating real-time cross-sectional images of the vessel wall.

The procedure adds roughly 10-15 minutes to a standard catheterization. Patients typically experience no additional discomfort beyond what accompanies the catheterization itself. The IVUS catheter is withdrawn slowly through the artery of interest, with automated pullback systems ensuring consistent image acquisition. Some centers perform IVUS routinely during complex interventions, while others reserve it for specific clinical scenarios.

IVUS can be performed through radial (wrist) or femoral (groin) arterial access, following the same approach used for the diagnostic portion of the procedure. The imaging catheter requires adequate guide catheter support, which may influence access site selection in some cases. Recovery follows the same timeline as diagnostic catheterization.

How does IVUS create images from inside the coronary arteries?

The IVUS catheter contains either a mechanically rotating transducer or a solid-state phased array of piezoelectric elements arranged circumferentially around the catheter tip. Mechanical systems rotate a single transducer at high speed to sweep ultrasound beams through 360 degrees. Phased array systems activate sequential elements electronically to achieve the same circumferential coverage without moving parts.

Sound waves at frequencies between 20-45 MHz penetrate the vessel wall and reflect back from interfaces between tissues of different acoustic properties. The blood-intima interface, the intima-media boundary, and the media-adventitia transition all produce distinct echoes. Computer processing converts these reflected signals into grayscale images that display vascular anatomy in real time.

The resulting images show the artery in cross-section, like slicing through a tube. Automated pullback at a constant speed allows reconstruction of the entire vessel segment, creating what amounts to a virtual longitudinal slice through the artery. This pullback data enables precise measurements of plaque volume and distribution along the length of the vessel.

What can IVUS visualize that cannot be seen on a standard angiogram?

Angiography depicts the lumen alone. IVUS reveals the vessel wall. This fundamental difference means IVUS can detect atherosclerosis that angiography completely misses due to positive remodeling, where arteries expand outward to accommodate growing plaque while preserving lumen diameter. Studies consistently show that angiographically normal segments often harbor substantial disease on IVUS examination.

Beyond detection, IVUS provides measurements of true vessel size that inform stent selection. Angiography tends to underestimate vessel diameter and lesion length. The MUSIC study established criteria for IVUS-guided stent optimization that required minimum stent area greater than 90% of the distal reference lumen area (de Jaegere et al., 1998). Meeting these criteria correlated with reduced restenosis rates.

IVUS also reveals complications invisible to angiography. Edge dissections at stent margins, incomplete stent apposition where struts fail to contact the vessel wall, and tissue prolapse through stent struts all have prognostic significance but may go undetected without intravascular imaging. The ADAPT-DES registry demonstrated that IVUS-identified findings like stent underexpansion predicted subsequent adverse events (Witzenbichler et al., 2013).

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How does IVUS characterize plaque composition?

Grayscale IVUS differentiates plaque types based on echogenicity and acoustic shadowing. Soft or lipid-rich plaque appears echolucent, meaning darker than surrounding tissue. Fibrous plaque generates intermediate echogenicity. Calcified plaque produces bright echoes with acoustic shadowing behind it, blocking visualization of deeper structures. Mixed plaque contains elements of multiple compositions.

These distinctions matter clinically. Heavily calcified lesions may require rotational atherectomy before stenting. Soft plaque at stent edges raises concerns about distal embolization. The presence and distribution of calcium influences decisions about lesion preparation and stent expansion strategies.

However, grayscale IVUS has limitations in plaque characterization. The differentiation between soft plaque and lipid-rich necrotic core is imprecise. Acoustic shadowing from superficial calcium can obscure deeper plaque elements. These limitations prompted development of advanced tissue characterization techniques discussed in the plaque characterization article.

What is the difference between grayscale IVUS and virtual histology IVUS?

Grayscale IVUS relies on visual interpretation of echogenicity patterns. Virtual histology IVUS (VH-IVUS) applies radiofrequency signal analysis to classify tissue into four categories. fibrous, fibro-fatty, dense calcium, and necrotic core. The system displays these classifications as color-coded overlays on the grayscale image, providing more detailed plaque composition assessment.

The PROSPECT trial used VH-IVUS to identify thin-cap fibroatheromas, a plaque morphology associated with vulnerability to rupture. Lesions with large plaque burden, small lumen area, and VH-IVUS classification as thin-cap fibroatheroma predicted future events at non-culprit sites (Stone et al., 2011). The VIVA study similarly found that VH-IVUS-identified fibroatheromas were associated with major adverse cardiac events at both plaque and patient levels (Calvert et al., 2011).

Despite these research applications, VH-IVUS has not achieved widespread clinical adoption. Validation studies showed imperfect correlation with histopathology. The technology requires specific hardware that not all IVUS systems support. Most clinical applications of IVUS rely on grayscale imaging, reserving tissue characterization for research protocols or specific clinical questions about plaque composition.

How long does the IVUS procedure take and what does the patient experience?

The IVUS imaging portion of a procedure typically requires 10-15 minutes per vessel examined. Automated pullback at 0.5-1.0 mm per second through a 30-40 mm vessel segment takes about one minute of actual imaging time. Additional time goes to catheter exchanges, positioning, and interpretation. For complex multivessel assessment or when IVUS guides iterative optimization, total imaging time may be longer.

Patients generally perceive no difference between procedures with and without IVUS. The imaging catheter does not cause pain. Contrast injection for angiography typically produces a transient warm flushing sensation that patients notice, but IVUS imaging itself generates no perceptible sensation. Sedation protocols follow institutional standards for cardiac catheterization and do not change based on IVUS use.

The overall procedure length depends more on the complexity of the intervention than on IVUS use. A diagnostic catheterization with IVUS assessment might add 15-20 minutes to total lab time. During percutaneous coronary intervention, IVUS imaging occurs alongside other procedural steps, so the incremental time impact is modest. Recovery and discharge criteria remain unchanged.

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What are the key measurements and metrics reported on an IVUS study?

IVUS reports focus on vessel dimensions, plaque characteristics, and stent deployment parameters when applicable. Minimum lumen area (MLA) quantifies the tightest point in the vessel segment. External elastic membrane (EEM) area measures the total vessel size at the media-adventitia boundary. Plaque burden, calculated as (EEM area minus lumen area) divided by EEM area, expresses atherosclerotic volume as a percentage of total vessel area.

For stent assessment, minimum stent area (MSA) represents the smallest cross-section within the stented segment. Stent expansion ratio compares MSA to reference vessel dimensions. Incomplete stent apposition is present when a gap exists between stent struts and the vessel wall. Edge dissection refers to intimal tears at stent margins. Each of these findings has prognostic implications for subsequent events.

Reference segment measurements inform procedure planning and stent selection. The ULTIMATE trial required minimum stent area greater than 5.0 mm² or greater than 90% of the distal reference lumen area for optimal IVUS-guided outcomes (Zhang et al., 2018). These quantitative criteria distinguish IVUS guidance from angiography-alone approaches where such precision is not achievable.

How do interventional cardiologists learn to interpret IVUS images?

IVUS interpretation skills develop through structured training during interventional cardiology fellowship. Most fellowship programs include didactic instruction on image acquisition and interpretation, followed by supervised clinical experience. Case volume requirements for board certification include intravascular imaging cases, though specific IVUS numbers vary by training program.

Competency requires recognizing normal arterial anatomy, identifying artifacts that can mimic pathology, and understanding the clinical significance of various findings. Distinction between true lumen and false lumen in dissections, recognition of intramural hematoma, and identification of thrombus all require interpretive skill that improves with experience. High-volume operators develop pattern recognition that enables rapid assessment during procedures.

Published consensus documents from professional societies provide standardized measurement protocols and reporting frameworks. The American College of Cardiology and European Society of Cardiology have issued guidance on intravascular imaging use. These resources establish quality benchmarks but do not substitute for hands-on training and clinical experience in IVUS interpretation.

Conclusion

IVUS provides information about coronary arteries that angiography cannot deliver. By imaging the vessel wall directly, it reveals plaque burden, composition, and remodeling patterns invisible to lumen-only assessment. During intervention, IVUS guides stent sizing and confirms adequate deployment through quantitative criteria that improve outcomes.

The technology is mature, well-validated, and widely available. The principal barriers to broader use relate to economics and workflow rather than clinical uncertainty. Understanding what IVUS is and what it measures positions patients to engage meaningfully in discussions about whether intravascular imaging might benefit their care.

The next article examines how IVUS compares to other coronary imaging technologies, including OCT, CT angiography, and functional assessment with FFR. Subsequent articles address clinical indications, the evidence base, and practical guidance on accessing IVUS as a patient.