Intravascular Ultrasound

General description of procedure, equipment, technique

Intravascular ultrasound provides cross-sectional (tomographic) views of the artery to assess the anatomy. The catheters are approximately 1 mm in diameter and consist of two types:

• A mechanical catheter in which the ultrasound crystal rotates around the catheter within a protective sheath
• A multiple array catheter with multiple crystals around the catheter tip, which are fired sequentially

The catheters are inserted into an artery over a guidewire using standard percutaneous coronary intervention techniques, including contrast injection to guide the catheter and anticoagulation. IVUS catheters are connected to a free-standing imaging console or by a connector to systems integrated with the imaging system of the catheterization laboratory.

Indications and patient selection

The indications include patients having coronary angiography or PCI for:

• Assessment of intermediate stenoses (e.g., 40% to 70%) by angiography
• Clarifying the anatomy where angiography is uncleara. Assessing lesions that are hazy on angiography for dissection, thrombus, intramural hematomab. Assessing extent of calcification
• Guiding chronic total occlusion PCI (e.g., to determine an intimal vs. medial or adventitial location)
• Optimizing stent deployment (e.g., stent apposition, expansion, and edge dissections)
• Assessing changes in plaque, arterial remodeling, and plaque composition

Contraindications

Patients who cannot have anticoagulation during the procedure

Details of how the procedure is performed

Arterial access, aspirin use, and anticoagulation are per standard PCI procedure. The target artery for IVUS is usually selected with a guide or sheath and a guidewire inserted into the artery using standard PCI techniques.

Prior to insertion in the body, mechanical IVUS catheters require fastidious flushing of the sheath to remove air from the sheath to prevent air embolization and poor image quality. Multiple array catheters usually only require a flush of the wire port prior to insertion in the body.

The catheter is connected to the imaging console or patient interface module. After anticoagulation and intracoronary nitroglycerin, the catheter is advanced over a guidewire through the guide and into the artery.

Imaging is started and the image optimized by flushing mechanical catheters, or a procedure to remove “ring-down” artifact on multiple array catheters. The catheter is advanced into the artery using fluoroscopic guidance. The catheter should not be forced and if it does not proceed easily should be withdrawn to avoid arterial injury.

Once the catheter is at the designated location in the artery (usually a little distal from the area of interest), recording is started. Imaging is usually achieved by a slow pullback using a manual technique or by mechanical pullback sleds with rates of pullback of 0.5 or 1.0 mm/sec. The catheter is withdrawn into the guide and the image reviewed on playback.

Once IVUS is completed, a final angiogram checks that there is no injury or other changes to the artery. Measurements of the lumen area (bounded by the lumen wall interface) and plaque area (bounded by the internal elastic membrane) are usually stored and incorporated in the patient’s medical record.

Interpretation of results

Assessing intermediate lesions

IVUS provides a high resolution image of the anatomy of the artery and does not directly assess the physiologic importance of an intermediate lesion. In comparison to stress imaging and fractional flow reserve (FFR), cutpoints for minimum lumen area (MLA) can act as a rough guide to the significance of a lesion.

In the major coronary arteries (left anterior descending [LAD], left circumflex [LCx], and right coronary arteries [RCA]) a MLA of >4 mm² usually indicates a lesion that is not hemodynamically significant. An MLA of ≤4 mm²may be physiologically significant, although this is best determined by fractional flow reserve.

The criteria for left main disease are less well established, but an MLA of >6 to 7 mm² is generally considered not hemodynamically significant. These IVUS criteria do not take into account other lesion characteristics important in determining lesion severity, such as lesion length, tortuosity, or distal myocardial viability.

Clarifying anatomy

IVUS can determine whether lesions that are hazy on angiography are due to eccentric stenosis, dissection, plaque rupture, thrombus, calcification, or intramural hematoma. The significance of these findings may determine whether revascularization is warranted and what adjunctive approaches to PCI are required.

Chronic total occlusions

IVUS can help direct crossing total occlusions by showing the location of the wire in plaque or medial/adventitial or extravascular locations. These may lead to choosing another path through the lesion and whether to attempt balloon inflations.

Stent deployment

IVUS can show inadequate stent apposition (struts not touching the lumen interface and free in the lumen), stent expansion (minimal stent areas of 80% to 90% of the reference areas are suggested), and whether there are edge dissections. These are thought to increase the risk of restenosis (especially in bare metal stents) or stent thrombosis and may justify further, more aggressive balloon dilations postdeployment.

Artifacts

Artifacts can occur due to variable rotational speeds of mechanical catheters (e.g. with very tortuous arteries) leading to kidney and egg-shaped images of the arteries, which will affect the accuracy of measurements. Intraarterial calcium can obscure ultrasound from reaching deep structures and characteristically have ultrasound void areas behind bright echogenic signals from the calcium.

Performance characteristics of the procedure (applies only to diagnostic procedures)

Performance characteristics are mainly reported for assessing significant lesions compared to physiologic techniques such as fractional flow reserve. Sensitivity and specificity differ in studies, but using the criteria above, MLA exceeding the cutpoints are generally considered specific, but not necessarily sensitive. Thus, exceeding the MLA thresholds usually indicates a lesion that is not physiologically significant, but an MLA less than the criteria may not always identify physiologically significant lesions.

Outcomes (applies only to therapeutic procedures)

The stent criteria above are related to restenosis, particularly in bare metal stents with greater lumen expansion corresponding to less restenosis. In drug eluting stents, the criteria may determine the risk of future stent thrombosis.

Alternative and/or additional procedures to consider

Fractional flow reserve is considered a more specific indicator of a physiologically significant lesion, but does not provide information on anatomy. Noninvasive stress testing, with or without imaging may determine the physiologic significance of a lesion and downstream myocardial viability. These tests may be helpful if IVUS fails. In most cases, cardiac CTA does not have sufficient resolution to determine lesion assessment in cases of angiographic ambiguity.

Complications and their management

Arterial injury including dissection is rare, but may occur particularly if the catheter is used too aggressively. Arterial thrombosis may occur in a patient who is not adequately anticoagulated. These complications may require stenting, bypass surgery, or aspiration thrombectomy. Spasm may occur which is typically relieved by intracoronary nitroglycerin.

What’s the evidence?

McDaniel, MC. ” Contemporary clinical applications of coronary intravascular ultrasound”. J Am Coll Cardiol Intv. vol. 4. 2011. pp. 1155-67. (Describes different applications for IVUS in the coronary arteries.)

Garcia-Garcia, HM. ” Imaging of coronary atherosclerosis: Intravascular ultrasound”. Eur Heart J. vol. 3. 2010. pp. 2456-69. (Describes different applications for IVUS from a European group.)

Kang, S-J. ” Comprehensive intravascular ultrasound assessment of stent area and its impact on restenosis and adverse cardiac events in 403 patients with unprotected left main disease”. Circ Cardiovasc Interv. vol. 4. 2011. pp. 562-9. (Describes criteria which could be used to for IVUS criteria to assess left main stenosis and outcomes after stenting.)

Anderson, WD. ” American College of Cardiology clinical expert consensus document on standards for acquisition, measurement and reporting of intravascular ultrasound studies (IVUS)”. J Am Coll Cardiol. vol. 37. 2001. pp. 1478-92. (Older document which describes the methods and characteristics for use in reports of IVUS procedures.)

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