Image orientation within the artery is another important aspect of image interpretation (FIGURE 15.4). In general, the IVUS beam penetrates beyond the coronary artery, often providing images of perivascular structures, such as the myocardium, pericardium, and cardiac veins.

These structures have characteristic appearances when viewed from various positions within the arterial tree. Therefore, in combination with the branching patterns of arteries, they can offer useful landmarks. The pericardium appears as a bright and relatively thick layer with varying degrees of “spokelike” reverberations created by the interwoven fibrous strands. The myocardium is often viewed on the side opposite to the pericardium as a variable pattern of homogenous, low-echoic gray-scale signals. The cardiac veins are visualized by IVUS as echolucent luminal structures with no connection to the coronary lumen, often showing compression during the cardiac cycle.

Current IVUS systems display the cross-sectional image as if viewing from a proximal position looking in a distal direction. Therefore, in the left anterior descending artery (LAD), the left circumflex (LCX) artery and the diagonal branches should emerge approximately 90° counter-clockwise from the pericardium, whereas septal branches typically emerge on the side opposite to the pericardium (ie, on the myocardium side). The distal LAD is accompanied by 1 or 2 anterior interventricular vein (AIV) branches, which run parallel to the LAD for a variable distance. It is important to recognize that there is no default rotational orientation of the image relative to the anatomy as the image initially appears on the screen—that is, what appears as “up” on the screen depends on where the beam begins its cycle and will be different from case to case. However, it is possible for the operator to rotate the image on the system to a standard view, using the anatomic clues described earlier, to provide the same image orientation for each IVUS examination.

In the LCX artery, the great cardiac vein (GCV) runs superior to the LCX in most cases and immediately inferior to the left auricle (FIGURE 15.5). Therefore, viewed from the LCX, the recurrent arterial branches generally emerge in an orientation directed toward the GCV. In contrast, the obtuse marginal (OM) and the posterolateral (PL) branches emerge opposite to the GCV and course inferiorly to cover the lateral myocardial wall.

A clear, echo-free space, called the transverse sinus (TVS), is often observed outside the left main coronary artery (LMCA) (FIGURE 15.6). This space is formed by the tenting of pericardium descending from the aorta onto the surface of the heart. Opposite to the plane of imaging, the TVS is appreciated adjacent to the left main lumen, immediately outside the left superior aspect of the aortic root. When imaging near the aortoostial junction, the left main lumen enlarges markedly into the aortic root, and often the ultrasound plane captures the inferior aspect of the left coronary cusp.

Unlike other epicardial coronary arteries typically accompanied by parallel venous structures, the proximal and mid-right coronary artery (RCA) shows a unique vein appearance—the vein arc crossing around the RCA in a “horseshoe” pattern, often at a position adjacent to the right ventricular (RV) marginal branches (FIGURE 15.7). The RV branches commonly have a geographic relationship with the pericardium similar to the diagonal branches of LAD—roughly 90° counterclockwise from the pericardium. The recurrent atrial branches typically emerge opposite to the RV marginal branches.

Mechanical catheters require flushing with saline to remove air bubbles from inside the catheter (FIGURE 15.8). This preparation should be performed before inserting the IVUS catheter to avoid air embolism in the coronary artery. Incomplete flushing can leave microbubbles adjacent to the transducer, resulting in poor image quality. Air bubbles can cause various patterns of noise including concentric rings around the imaging catheter with poor image penetration (FIGURE 15.8A) or a deep sector of interference, producing a blind area (FIGURE 15.8B).

When blood flow is stagnant, red cells aggregate and reflect ultrasound more strongly (FIGURE 15.9). This produces an appearance in the lumen that looks white or “foggy” and the lumen-intima boundary (yellow arrows) may be blurred. Injection of contrast or saline may disperse the stagnant flow from the lumen.

Ring-down artifact is the most common image artifact with IVUS, which is manifested as multiple bright rings surrounding the catheter (FIGURE 15.10). This artifact is more commonly seen with the solid-state systems (FIGURE 15.10A) than with mechanical rotational systems (FIGURE 15.10B). Solid-state catheters require digital subtraction to mask this artifact before being inserted into the coronary artery. If this is incorrectly performed, digital subtraction can be a potential cause for removal of real information (white arrows in FIGURE 15.10C). In mechanical rotating systems, near-field resolution is commonly excellent so that digital subtraction is not required. Ring-down artifact is minimized by the fact that the transducer is offset from the surface of the catheter by design. If a significant ring-down artifact is observed with a mechanical catheter (white arrows in FIGURE 15.10D), the likely cause is microbubbles within the protective sheath, requiring repeated saline-flush procedures until the artifact is removed.

The solid-state catheter includes no moving parts and thus is free of NURD (FIGURE 15.11). This artifact can occur with mechanical systems when bending or friction of the drive cable interferes with uniform transducer rotation, causing a wedge-shaped, smeared image to appear in 1 or more segments of the image (between 10 and 6 o’clock in this example).

“White cap” artifacts caused by side lobe echoes (arrows) originate at the edges of a strong reflecting surface, such as metal stent struts or calcification (FIGURE 15.12). Smearing of the strut image can lead to the mistaken impression that the struts are protruding into the lumen.

Radiofrequency (RF) noise appears as alternating radial spokes or random white dots in the far field (FIGURE 15.13). The interference is usually caused by other electrical equipment in the cardiac catheterization laboratory.

“Reverberation” refers to an artifact where multiple, radial ghost images are created at regular intervals (yellow arrows) beyond a bright interface such as the leading edge of calcium or a stent strut (FIGURE 15.14). These artifacts are caused by repeated “round trips” of a portion of the ultrasound energy that is reflected back from the bright interface in the vessel and then, in turn, bounces off the transducer and makes another trip (or 2 or 3), which the IVUS scanner presents as ghost images.

IVUS has an intrinsic distance calibration based on the known speed of ultrasound in tissue. This is implemented in the form of a measurement grid on the image, as calipers and area tracing capability in the system software and as tag information in exported DICOM files. In principle, all ultrasound measurements should be performed at the “leading edge” of boundaries (that is, the edge closest to the transducer) because of the higher accuracy and reproducibility compared with those at the trailing edge.

For cross-sectional analysis, electronic caliper (diameter) and tracing (area) measurements can be performed (FIGURE 15.15). For vessel measurement, the interface between the media and the leading edge of adventitia that corresponds to the external elastic membrane (EEM) is used, because the outer border of the adventitial cannot be defined by IVUS. In cross sections with large plaque burden or significant calcification, the circumference of EEM may not be fully identifiable because of ultrasound signal attenuation or acoustic shadowing. Extrapolation from the closest identifiable EEM border is acceptable only if the attenuation or acoustic shadowing involves a relatively small arc (<90°). For lumen measurement, the interface between the lumen and the leading edge of the intima is used. When the blood-tissue interface is obscure on a still image, a review of moving images can help identify the TL border. During the procedure, saline or contrast medium can be injected through the guide catheter to reduce blood speckle. Plaque area (or more accurately, the plaque plus media area) is calculated as the difference between EEM and lumen areas. The ratio of plaque to EEM area is termed the percent plaque area, plaque burden, or cross-sectional narrowing. For EEM and lumen diameter measurements, the maximum (solid arrows) and minimum (dotted arrows) diameters are determined. For plaque, the maximum (solid arrows) and minimum (dotted arrows) thicknesses are measured.

Metal struts of stents are seen as bright, focal points in a circular-arrayed pattern on the IVUS scan, and stent area is measured by tracing the leading edge of the stent struts (FIGURE 15.16). For tissue within the stent (ie, plaque prolapse, in-stent thrombus, or neointimal hyperplasia at follow-up imaging), the area is calculated as the difference between stent area and lumen area. As in the case of nonstented segment analysis, a review of moving images or saline/contrast flush during the procedure can help identify the TL-tissue interface. For diameter measurements, the maximum (solid arrows) and minimum (dotted arrows) stent diameters are determined, and the ratio of maximum to minimum diameter defines a measure of stent symmetry. For neointima, the maximum (solid arrows) and minimum (dotted arrows) thickness are measured.

Arterial remodeling refers to a change in vessel caliber (either an increase or decrease) that occurs during the development of atherosclerosis (FIGURE 15.17). In serial IVUS studies, evidence of remodeling can be directly derived from serial changes in the EEM area by 2 or more IVUS measurements obtained at different times at the same transducer location. A remodeling index (the ratio of EEM area at the lesion site versus the reference site) as a continuous variable may also be used, in combination with the categorical classifications (positive remodeling = remodeling index >1.0 or 1.05; negative remodeling = remodeling index <1.0 or 0.95).

In single time point studies, the EEM area measurements at reference sites proximal and/or distal to the segment of interest are used as an indirect surrogate for the original vessel size before the lesion site became diseased (FIGURE 15.18). The reference segment is selected as the most normal-looking (largest lumen with smallest plaque burden) cross section within 10 mm from the lesion with no intervening major side branches.

In gray-scale IVUS, atheromatous plaques are classified based on the echogenicity and association with specific acoustic findings, such as signal attenuation, shadowing, and reverberation (FIGURE 15.19). The brightness of the adventitia can be used as a gauge to discriminate between predominantly fatty from fibrous plaque (plaque that appears darker than the adventitia is considered fatty). Regions of calcification are strongly echoreflective and create a dense shadow peripherally from the catheter, known as acoustic shadowing (between 7 and 11 o’clock in the example).

Calcium deposits are described qualitatively as superficial (FIGURE 15.20A), deep (FIGURE 15.20B), or mixed (FIGURE 15.20C) according to the leading-edge location of the acoustic shadowing of the deposit. If the edge is within the inner half of the plaque plus media, the deposit is considered superficial. Extensive superficial calcium, particularly circumferential “napkin-ring” calcification (FIGURE 15.20D), may require plaque modification with rotational atherectomy or “scoring” before stent implantation. Conversely, even for lesions with significant calcification on fluoroscopy, IVUS may show that the calcification is distributed in a deep portion of the vessel wall or has a limited arc (less than 180°). In these cases, stand-alone stenting without modifying the calcium deposit usually achieves adequate lumen expansion. The shadowing of the image by calcium precludes determination of the thickness of a calcific deposit as well as visualization of vessel structures behind the calcium. Ultrasound reverberation due to calcium (see FIGURE 15.14) should not be misinterpreted as true vessel structures.

By gray-scale IVUS, morphologic features associated with clinical instability or high risk for cardiovascular events after percutaneous coronary intervention (PCI) include noncalcified plaque with ultrasound attenuation and an echolucent zone within plaque (FIGURE 15.21). In particular, echoattenuated plaque (or attenuated signal plaque, defined as the absence of the ultrasound signal behind plaque that is either hypoechoic or isoechoic to the reference adventitia but contains no bright calcium) likely represents either fibroatheroma with a necrotic core or pathologic intimal thickening (PIT) with a lipid pool.

Plaque rupture is diagnosed when a hypoechoic cavity within the plaque is connected with the lumen and a remnant of the ruptured fibrous cap is observed at the connecting site (FIGURE 15.22). Ruptured plaques are often eccentric, less calcified, large in plaque burden, positively remodeled, and associated with thrombus.

Dissection appears as a fissure or separation within the intima or plaque (FIGURE 15.26). The severity of a dissection can be characterized according to the depth and extent.

The ratio of plaque area to total vessel area is termed “plaque burden” (FIGURE 15.28). Residual plaque burden after stenting is known as a major and independent influence on the late outcome.²

Incomplete stent apposition (ISA) or stent malapposition occurs when part of the stent structure is not fully in contact with the vessel wall (FIGURE 15.29). This is defined by IVUS as 1 or more struts clearly separated from the vessel wall with evidence of blood speckle behind the strut in a segment not associated with any side branches. Isolated ISA observed after deployment usually is not directly linked to adverse clinical events, provided that the lumen is large enough to preserve blood flow. On the other hand, significant ISA possibly increases local flow disturbances, delays healing, and increases the potential risk for stent deformation at subsequent procedures. An early clinical study demonstrated an unexpectedly high percentage of these IVUS-detected stent deployment issues, even after angiographically successful results. These observations led to the concept of the high-pressure stent deployment technique used today.