Basic interpretation of intracoronary ultrasound and optical coherence tomography images: examples
Latest advancements in intracoronary imaging have enabled the insertion of miniaturized catheters with imaging probes in coronary arteries in order to provide detailed imaging information in a cross-sectional image format after an automated pullback. Cross-sectional images can then be also used for longitudinal reconstructions for the vessel segment imaged. There are two main broadly available intracoronary imaging modalities, namely intravascular ultrasound (IVUS) and optical coherence tomography (OCT), which provide information on (1) the lumen and wall morphology as well as the anatomic and tissue characteristics of atherosclerotic plaques, and (2) endovascular implants 1, 2. The main imaging features that are visualized during IVUS or OCT imaging in everyday clinical practice in the catheterization laboratory will be presented using intravascular images from routine cases. Intravascular ultrasound imaging enables the visualization of both the lumen and the arterial wall. The lumen is not void of echo signal due to blood speckling. In coronary arteries, three layers of the arterial wall are frequently visible moving outward: the intima, media (less echogenic), and adventitia (Figure 1A). Due to echogenic properties of these layers, the lumen and media–adventitia (corresponding to the external elastic membrane [EEM]) borders can be readily identified and can be used for quantitative measurements of the lumen area, atheroma/plaque area (i.e., area between the lumen border and the media–adventitia border), and consequently, the plaque burden (plaque area divided by EEM area). It should be noted that because the internal elastic membrane cannot be well delineated in IVUS images, IVUS measurements cannot determine true histological atheroma area (i.e., the area bounded by the internal elastic membrane), and thus the area bounded on the outer side by the EEM and the inner side by the lumen border (i.e., intima plus media) is used as a surrogate of the atheroma area. Online measurements (Figure 1B) on cross-sectional images can be easily performed during IVUS interrogation at different locations along the length of the artery which can be easily identified using the longitudinal view of the IVUS pullback (Figure 1B, bottom). During IVUS imaging, most branches (Br) can be easily detected (Figure 2). The guidewire causing a characteristic artifact is denoted with an asterisk. IVUS provides deep penetration into the arterial wall, and thus branches may also appear within the arterial wall as they approach the main vessel (Figure 2C, 8 o’ clock). High lipid content results in low echogenicity, whereas calcific plaques are highly echogenic. Fibrous plaques have an intermediate echogenicity between echolucent (i.e., “soft”) atheromas and calcified regions. Atherosclerotic regions usually contain more than one plaque subtype; these mixed plaques are described as fibrofatty/fibrolipidic and fibrocalcific plaques. Figure 3A portrays a primarily fibrous plaque spanning from 7 to 1 o'clock. A fibrolipidic plaque is shown in Figure 3B with an echolucent area (arrow) probably corresponding to a lipid pool and a fibrous cap inward. Figure 3C demonstrates a fibrocalcific plaque with calcified regions at 3 and 8–9 o'clock which obstruct the penetration of ultrasound and cause “acoustic shadowing.” Extensive calcification is easily identified in IVUS images and can be quantified by measuring the arc of calcium (in degrees); a cross-sectional image with a ~230° arc of calcium is shown in Figure 3D. Metallic stent struts (Figure 4A, asterisks) appear as small echogenic arcs along the circumference of the vessel (due to strong reflection of ultrasound). Intimal hyperplasia is usually visualized as tissue of intermediate echogenicity within the area bounded by the stent struts (Figure 4A, arrow). Figure 4B portrays an area with stent malapposition as there is a distance between some stent struts (asterisks) and the wall, and thus, there is blood flow between those struts and the underlying wall. Optical coherence tomography provides images of high resolution (10–15 microns) enabling the detailed morphology of the lumen and lesions (Figure 5); a highly irregular lumen cross-section due to heavy atherosclerotic disease in the arterial wall is visible in this cross-sectional image. During OCT imaging, branches (Br) of any size and variable take-off angle are easily detected (Figure 6). The guidewire causing a characteristic shadow is denoted with an asterisk. Fibrous plaques have high backscattering and a relatively homogeneous signal (Figure 7, white arrows). Calcified regions have poor signal and are rather heterogeneous with sharply delineated border (Figure 7, red arrows). Lesions with lipid pools (i.e., necrotic core) appear as signal-poor regions with poorly defined or diffuse borders (Figure 8, asterisks). The high resolution of OCT enables the measurement of fibrous cap thickness (arrows) over the necrotic core, and thus is suitable for identifying thin-cap fibroatheromas, which are considered as the primary precursor for acute coronary events. OCT images show thrombotic material inside a stent (Figure 9, arrows denote thrombus). An intimal flap (red arrow) is portrayed within 5 mm proximal to a stent as shown in the longitudinal view of the OCT images (Figure 10, top). The OCT consecutive cross-sectional images (distal to proximal) show frame-by-frame the intimal flap spanning a length of more than 6 mm (Figure 10, bottom, white arrows denote the distal edge of the dissection). In-stent restenosis demonstrating a homogeneous, high-backscattering neointima is shown in Figure 11A; stent struts with their shadow are visible both in the cross-sectional and longitudinal OCT images. In Figure 11B, in-stent restenosis with a focal calcified region (arrow) beneath the neointima is shown. Stent malapposition becomes evident when stent struts (asterisks) are distant from the luminal surface of the arterial wall (Figure 12). Dr. Papafaklis has nothing to disclose.
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