of the vessel; media thickness of coronary arteries is inversely related to lesion thickness [10]. The intima–media border is poorly defined because the intimal layer reflects ultrasound more strongly than the media. Conversely, the media–adventitia border, consistent with the location of the external elastic membrane (EEM), is accurately defined because a step‐up in echo reflectivity occurs without blooming. The outermost layer, the adventitia, is composed of collagen and elastic tissue; it is 300–500 μm thick. The outer border of the adventitia is also indistinct due to echo reflectivity similar to the surrounding peri‐adventitial tissues [6]. Therefore, the normal coronary artery is either (i) “mono‐layered” in cases of intimal thickness <100 μm because (if in case a 20 or 40 MHz is used) IVUS catheter resolution is less than 100 μm; or (ii) “three‐layered” to include a bright echo from the intima, a dark zone from the media, and bright surrounding echoes from the adventitia (Figure 8.2). The “three‐layered” has been better recognized with the 60 MHz IVUS catheter.
Figure 8.2 Normal coronary artery morphology in cross‐sectional view. In the magnified image on the right, the bright inner layer (intima), middle echolucent zone (media), and outer bright layer (adventitia) are representative of the “three‐layered” appearance of intravascular ultrasound (IVUS). In the magnified image on the left, only the outer bright adventitial layer is representative of the “mono‐layered” appearance.
Quantitative analysis
In non‐stented lesions there are two strong acoustic interfaces that are well visualized by ultrasound: the leading edge of the intima and the outer border of the media (or media–adventitia junction). Therefore, two cross‐sectional area (CSA) measurements can be defined by IVUS: the lumen CSA and the media–adventitia CSA (or EEM CSA). The atheroma or plaque&media (P&M) complex is calculated as EEM minus lumen; the media cannot be measured as a distinct structure. Thus, complete quantification of a non‐stented lesion is possible by tracing the EEM and lumen areas of the proximal reference, lesion, and distal reference; calculating derived measures (minimum and maximum EEM and lumen diameters, P&M area and thickness, and plaque burden; P&M divided by EEM); and measuring lesion length (distance between the proximal and distal reference) (Figure 8.3).
Figure 8.3 IVUS measurements pre‐intervention in a non‐stented artery. The proximal and distal reference and minimum lumen area (MLA) of the lesion are shown. The IVUS study is shown in duplicate: one unlabeled and one highlighted with lines to illustrate quantitative analysis. The dashed line highlights each external elastic membrane cross‐sectional area (EEM CSA), and the solid line indicates each lumen interface (lumen CSA). The minimal lumen cross‐sectional area (lumen CSA) at the lesion site is 2.1 mm2. Between the EEM CSA and lumen CSA, the atheroma or plaque&media (P&M) complex is calculated.
In stented vessels, the stent forms a third measurable structure (stent CSA). It appears as bright points along the circumference of the vessel. Complete quantification of a stented lesion is possible by tracing the EEM and lumen areas of the proximal and distal reference and the EEM, lumen, and stent areas of the stented lesion; calculating derived measures (minimum and maximum EEM, stent, and lumen diameters; peri‐stent P&M area and thickness; and intra‐stent intimal hyperplasia [IH, area and %IH); and measuring stent length. With the use of motorized pullback, area measurements can be added to calculate volumes using Simpson’s formula.
Qualitative analysis
Grayscale IVUS has some ability to differentiate plaque composition based on different echoreflectivity of the tissue. Atherosclerotic plaques are rarely homogeneous and contain a mixture of plaque components with different impedance (density). A standard approach is to compare the echointensity or “brightness” of the plaque to the surrounding adventitia that is used as a reference. Three basic types of lesions are distinguished according to plaque echogenicity: (i) “soft” or hypoechoic plaque does not reflect much ultrasound and appears dark with less echointensity compared to the adventitia (Figure 8.5), (ii) fibrous, and (iii) calcific plaques are characterized by equal or greater intensity than the adventitia. A plaque that is not so reflective as to cause shadowing is labeled “hard” or hyperechoic and is composed primarily of fibrous tissue. The presence of acoustic shadowing along with the brightest echoes and reverberations are characteristic of the presence of calcification (Figure 8.4). Extensive target lesion calcification may adversely impact the PCI procedure by affecting the ability for effective dilatation of a coronary stenosis and is associated with greater likelihood of stent underexpansion. In lesions with maximum circumferential extension of calcium >180 degree by IVUS, greater calcific burden was associated with a smaller stent area and greater stent eccentricity [4].
Figure 8.4 A pure soft or hypoechoic plaque is uncommon because atherosclerotic plaques are rarely homogeneous. (a) shows an example of a predominantly soft plaque – a thin fibrous cap (small arrows) and lipid core underlying it; the plaque is less bright than the adventitia a. In (b), fibrous plaque or hyperechoic plaque is shown. Hyperechoic plaque is as bright as or brighter than the adventitia a without shadowing. In this eccentric plaque, the thickness of the media behind the thickest part of the plaque b is an artifact caused by attenuation of the beam as it passes through the hyperechoic plaque. In reality, the media becomes thinner with increasing atherosclerosis. Note that the media behind the thinnest part of the plaque is also thinner – without artifacts. (c) shows superficial calcium – defined as calcium a that is closer to the intima than it is to the adventitia. Calcium shadows the deeper arterial structures; in this case, the arc of calcification is ~180°.
Intimal hyperplasia due to in‐stent restenosis often appears to have low echogenecity depending, in part, on age and adjunct therapies (i.e. brachytherapy).
The identification of thrombus is difficult by IVUS. It may appear as lobulated hypoechoic mass within the lumen, scintillating echoes, a distinct interface between the presumed thrombus imaging and underlying plaque, and blood flow through the thrombus (Figure 8.5j).
Figure 8.5 Diagnostic intravascular ultrasound was performed to assess the angiographic filling defect at the right coronary artery. The intravascular ultrasound images are shown from proximal (A) to distal (J). There is marked segmental positive remodeling (B to I) with severe malapposition and underexpansion throughout the entire length of the stent, which appear “sized” just to the smallest lumen (panel B). Notice the space between the stent strut and the intima and the blood speckle/thrombus
