Intravascular ultrasound (IVUS) has become a standard imaging technique in high volume, experienced interventional laboratories, but most of its use is relegated to the coronary arteries. While conventional angiography provides a single-plane “shadow” of the vascular lumen, it has limited ability to accurately and reproducibly measure vessel stenosis and characterize plaque morphology. The high-definition, cross-sectional images of the arterial lumen and the arterial wall provided by IVUS allow a far more detailed analysis of the target vessel and peripheral interventional success. Nonetheless, IVUS has not enjoyed wide deployment in the peripheral interventional arena. This chapter will provide the information needed to utilize IVUS during peripheral interventions by reviewing the fundamentals of IVUS technology, image interpretation and standardized measurements, its general application during peripheral vascular interventions, and some specific considerations for each vascular territory.

One of the first IVUS catheters was designed in 1972s by Bom et al.¹ with the purpose of exploring the intracardiac chambers and cardiac structures. During the early and mid-1980s, new catheters were designed in order to evaluate and characterize the arterial structures. Because of their size and catheter stiffness, early clinical studies with IVUS in the late 1980s were done primarily in the periphery. In the early 1990s, rapid technical improvement in catheter design, transducer technology, and subsequently imaging quality produced clinically useful catheters. The standard IVUS catheter was reduced to 3.5 to 4.3 F catheter, which led to marked growth of their use in coronary interventional cardiology research and practice. In the last 10 years, IVUS has moved from being solely a research tool to an established modality in clinical practice. While the use in the coronaries remains more common than in the peripheral arteries, the primary techniques, diagnostic utility, and therapeutic benefits remain similar.

IVUS serves three important clinical purposes—diagnosis, guiding the interventional strategy, and optimizing the interventional result. Starting with diagnosis, IVUS is the gold standard method to assess intermediate stenosis, ambiguous lesions, bifurcations, unusual lesion morphology (aneurysms, calcium, thrombi), in-stent restenosis, and any other unusual angiographic finding.² From an interventional perspective, IVUS has been utilized extensively as a guide to the procedure, aiding in selection of optimal strategy, adjuvant device utilization (rotational atherectomy, predilation, etc.) as well as selection of stent diameter and length.³ Additionally, IVUS imaging enables optimization of stenting procedures, maximizing expansion and apposition, and identifying poststenting complications such as vessel rupture and dissection.

Two different IVUS transducer technologies are currently available—solid state (or phase array) and mechanical (Figure 21-1).⁴ Solid-state configuration has 64 transducer elements arranged as a collar around the catheter. The backscattered ultrasound information received from each transducer element is sent to a computer that performs real-time image reconstruction to formulate a cross-sectional image of the artery. The advantage of this catheter is that there are no mobile parts and therefore, no motion artifacts in the image. The principal disadvantage is a slightly lower spatial and temporal resolution than mechanical transducers and a “ring-down artifact,” which is characterized by the appearance of bright halos around the IVUS catheter. This creates a small zone of uncertainty in the immediate area around the catheter and requires an additional corrective step in the calibration process⁵ (Figure 21-2).

The second technology is a mechanical transducer, with a single rotating transducer mounted on a drive cable that allows a spinning transducer to build a tomography picture in real time. The resolution and dynamic ranges (gray scale) are superior with this method and clearer images can often be obtained. The disadvantage of a mobile drive shaft and transducers is the potential for an image artifact known as nonuniform rotational distortion (NURD). NURD (Figure 21-2) results from mechanical binding of the spinning transducer,⁶ which can occur as a result of catheter kinking, excessive tightening of the hemostatic valve, or excessive vessel tortuosity.⁵

With coronary imaging, the transducers are typically between 30 and 40 MHz. This provides optimal resolution at a depth of penetration necessary for coronary arteries and for small peripheral vessels such as the renal or infrapopliteal lower extremity arteries. The frequency of the catheters used in peripheral arteries range from 10 to 40 MHz transducers, depending on the size of the vessel being imaged and the necessary depth of penetration. There are currently two principal IVUS manufactures: Boston Scientific Corporation, Inc. (Maple Grove, MN) and Volcano Therapeutics (VT) (Rancho Cordova, CA). Both systems provide either a portable unit that can be wheeled from room to room of the catheterization laboratory or a permanent unit that is fully integrated into the catheterization laboratory (Figures 21-3A and 21-3B). Boston Scientific Corporation (BSC) has recently released Atlantis® PV Peripheral Imaging Catheter, mainly designed for aortic imaging (of aneurysms) with a wide field of view. It is an over-the-wire catheter requiring 8 F catheter introducer sheath and has a 15-MHz transducer capable of 25-cm pullback with 1 cm graduated markers for measurements. For other peripheral territories, BSC offers Sonicath® Ultra^™ catheters ranging from 9 to 20 MHz as well as its coronary catheters, which are useful in renal and distal femoral arterial beds. VT has two catheters designed for peripheral vascular imaging. The detailed technical specifications of these catheters are summarized in Tables 21-1 and 21-2.

The important principles and steps necessary to integrate IVUS into the clinical setting are summarized in Table 12-3. Essentially, designation and training of appropriate IVUS technologists and interpreting physicians will allow for a seamless bridge between image acquisition, interpretation, and application. Once established, it can be expected that IVUS should add no more than a few minutes to the procedure time. A basic IVUS image acquisition protocol that can be used in a busy clinical lab is summarized in Table 12-4. The important point is that early identification for IVUS use allows the machine and catheter to be prepared so that imaging becomes a simple catheter exchange at the appropriate time. Motorized pullbacks are very useful in coronary arteries but can become time consuming in long peripheral lesions. If a long lesion is being imaged, a slow manual pullback often is sufficient as long as care is taken not to move “back and forth” and the rate of withdrawal is kept relatively slow and constant. If lesion length measurements (or volume measurements) are needed, then a motorized pullback is required.

Successful IVUS image interpretation is dependent on understanding the fundamentals of the image formation and the anatomy of the arterial structure. IVUS image formation is based on the same principles as standard ultrasound imaging. An abrupt change in density (acoustic impedance) between adjacent tissue layers produces a strong reflection, resulting in an apparent boundary on the ultrasound image displayed on the screen. Applying this to vascular anatomy, the IVUS catheter is in the lumen where there is normal flowing blood, which reflects little to no sound and will appear black with a slight “speckle” from the blood cells. If blood cells are stagnant and have rouleaux formation, there will there be a substantial interface for the ultrasound to “bounce off.” In this situation, blood appears bright on the screen. When ultrasound waves hit a very reflective object (such as a calcified plaque) most of the sound waves are reflected back toward the catheter, and the image obtained will be very bright (white) on the screen. Objects between these two extremes are displayed on a gray scale, with darker gray representing tissues with higher water or lipid content and lighter gray representing tissue that is more fibrous.

Figure 21-4 depicts the normal three layers of the arterial wall depicted by IVUS. From the innermost to the outermost are the intima, media, and adventitia. The intima is composed of the endothelial cell layer and the underlying basal membrane with the internal elastic membrane representing external boundary of this layer. The media is composed of smooth muscle cells, elastin, and collagen and encircled by the external elastic membrane (EEM) representing the outer layer of the vessel. The adventitia is mainly composed of fibrous tissue. As can be seen in Figure 21-4, each of the arterial layers has different acoustic properties and each appears different (distinct) on the ultrasound screen. The intima reflects ultrasound and appears brighter than the blood in the lumen. The media, made mostly of homogenous smooth muscle cells, does not reflect a lot of ultrasound and is dark. The adventitia has “sheets” of collagen serving as several layers of interface for the ultrasound to be reflected off and is therefore very bright. While these layers are always present in muscular arteries, young people free of disease will have an intimal thickness below the resolution of IVUS and thus the three layers may be difficult to visualize (and it will appear as a “monolayer” with the blood apparently against the adventitia).

Standard IVUS measurements are shown in Figure 21-5. There are two primary interfaces: (1) the lumen-intima interface and (2) the media-adventitia interface. From these two “circles,” virtually all required measurements can be obtained. The area enclosed by the lumen–intima interface is the luminal area. The area between the two interfaces is the plaque (or sometimes referred as the plaque and media or atheroma) area. The area enclosed by the media–adventitia interface is the vessel area (or media or EEM) area. Diameters are obtained by going through the center of the image. These measurements are performed at the lesion and at the proximal and references. Furthermore, length measurements can be performed based on the amount of time it takes to get from one location to the next with a known pullback speed. Thus, the reported measures include reference lumen area and diameter, lesion area and minimal lumen diameter, and lesion length. These standard measurements can also be applied to stents (by adding another “circle” that follows the contour of the stent), enabling one to assess the stent deployment. In 2001, Mintz et al.⁵ published the ACC consensus document on IVUS. Although this document refers primarily to coronary imaging, the principles outlined and the direct and derived measurements reviewed also pertain to peripheral imaging.

Plaque morphology can also be assessed by IVUS. Based on the degree of echogenicity, plaque can be characterized as soft (fatty), hard (fibrous), calcified, or mixed (Figure 21-6).^5,7,8,9,10 For instance, fatty plaque has a low echogenicity, where calcific plaque has high echogenicity and appears bright. Fibrous plaque has an intermediate echogenicity and mixed plaque contains more than one subtype.⁵

Intraluminal thrombus and dissection represent two additional tissue characteristics that can be seen by IVUS. Thrombus (Figure 21-7) is visualized as a mass in the lumen that often has a layered appearance. It is relatively echolucent and can occasionally be confused with atheroma or with stasis.^5,11 Dissection (Figure 21-8) is visualized as a discontinuity in the luminal wall with blood speckle behind it. Five categories of dissection are intimal, medial or adventitial (depending on the depth to which a dissection penetrates), intramural hematoma, and intrastent.⁵