Interventional Cardiology. Группа авторов

arteries, because of the wide atrioventricular groove movement between systole and diastole.

      Ring‐down

      Ring‐down artifacts usually appear as a series of parallel bands or halos of variables thickness surrounding the catheter obscuring near field imaging. Phased‐array systems tend to have more ringdown artifacts.

      Non‐uniform rotational distortion

      Non‐uniform rotational distortion (NURD) arises from frictional forces to the rotating elements in mechanical catheters. NURD creates stretched or compacted portions of the images. Because accurate reconstruction of IVUS two‐dimensional images is dependent on uniform rotation of the catheter, non‐uniform rotation can create errors during IVUS measurements.

      Reverberations

      Strong spatial tissue heterogeneity creates acoustic noise and pulse reverberations—multiple echoes reaching the transducer before the next pulse transmission to give rise to multiple copies of the anatomy. Reverberation artifacts are more common from strong echoreflectors such as stents, guidewires, guiding catheters, and calcium (especially after rotational atherectomy).

      Other artifacts

      A few other artifacts can also interfere in IVUS interpretation; side lobes and ghost artifacts also generated from strong echoreflectors such as calcium and stent metal [5]. In longitudinal or L‐mode display, catheter motion artifacts during the pullback results in a “saw tooth” appearance.

      Catheter position also has an important role in image quality. Off axis position of the catheter can alter vessel geometry in an elliptical fashion to mislead the operator to overestimate the lumen and vessel area. Axial (antegrade–retrograde) movement of the IVUS probe during the cardiac cycle scrambles consecutive image slices that can have implications for three‐dimensional reconstruction and attempts to assess coronary artery compliance.

Image acquisition and presentation

Two important consensus documents have been published: Clinical use of intracoronary imaging. Part 1: guidance and optimization of coronary interventions. An expert consensus document of the European Association of Percutaneous Cardiovascular Interventions [4] and the Clinical use of intracoronary imaging. Part 2: acute coronary syndromes, ambiguous coronary angiography findings, and guiding interventional decision‐making: an expert consensus document of the European Association of Percutaneous Cardiovascular Interventions [5].

      IVUS is displayed as a tomographic cross‐sectional view. A longitudinal view (L‐mode or long‐view) can be also displayed, but this should be done only when using motorized transducer pullback. Longitudinal representation of IVUS images is useful for lengths measurements, for interpolation of shadowed deep arterial structures (i.e. external elastic membrane behind calcium or stent metal). There are advantages and disadvantages to using manual or motorized pullback; however, motorized pullback is usually preferable. Using motorized transducer pullback allows assessment of lesion length, volumetric measurements, consistent and systematic IVUS image acquisition among different operators, and uniform and reproducible image acquisition for multicenter and serial studies.

      In standard image acquisition after anticoagulation and intracoronary nitroglycerin administration, the IVUS catheter should be placed distal to the segment of interest (aiming for 20 mm of distal reference), and a continuous pullback to the aorta should be recorded. The preferred pullback speed is 0.5 mm/s but 1 mm/s is often used.

Normal artery morphology

The ultrasound appearance of normal human arteries in vitro and in vivo has been reported [6–8]. In muscular arteries such as the coronary tree there are three layers: intima, media, and adventitia. Normal intima thickness increases with age, from a single endothelial cell at birth to a mean of 60 μm at five years to 220–250 μm at 30–40 years of age [19]. The definition of abnormal intimal thickness by IVUS is still controversial; in general, the threshold of “normal intimal thickness” is <300 μm (0.3 mm). The innermost layer of the intima is relatively echogenic compared with the lumen and media and displayed on the screen as a single bright concentric echo. The lower ultrasound reflectance of the media is due to its homogeneous smooth muscle cells distribution and smaller amounts of collagen, elastic tissue, and proteoglycans. The thickness of media histologically averages 200 μm, but medial thinning occurs in the presence of atherosclerosis [9]. In advanced atherosclerotic disease, the media may not appear as a distinct layer around the full circumference 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