Читать книгу Interventional Cardiology - Группа авторов - Страница 235

Interpretation of OCT data and their application in clinical situations is limited by image artifacts (Figure 9.1). Blood contamination usually results from inadequate flushing and can be prevented by using power injection through a pump at a speed greater than the maximal coronary flow. Residual red blood cells disturb the OCT light beam, reducing the visibility and brightness of the vessel wall (Figure 9.1b); blood swirling along the vessel wall can be mistaken for thrombus. Physiologic phenomena such as cardiac motion, vessel pulsatility, or, to a lesser extent, catheter movement and respiratory movements are associated with typical artifacts (Figure 9.1c). Dense objects such as guidewires, metallic stent struts (Figure 9.1a) completely obstruct the OCT signal, leading to a loss of signal and no visualization behind them. Compensation algorithms are been tested to reduce shadowing and improve signal from the deepest tissues [7]. “Sew‐up” artifacts appear as a result of rapid vessel movement during imaging, but they are less prominent than in the much slower IVUS pullbacks and have become clinically irrelevant at the high pullback speeds of the newest OCT systems. Imaging modalities that use a mechanically rotated endoscopic probe to scan an artery often suffer from image degradation caused by a variation in the rotational speed of rotating optical components during image acquisition [8]. This occurs in the presence of acute angulations, tight hemostatic valve, kinking of the imaging sheath, a defective catheter, or while the catheter crosses a tight stenosis. Saturation artifact occurs when the signal from a highly reflective surface exceeds the dynamic range of the detector (Figure 9.1d). Tangential signal dropout happens when the beam strikes the tissue with a near parallel angle, a signal‐poor area with diffuse borders, covered by a thin signal‐rich layer arises (Figure 9.1e), and mimics a lipid‐rich plaque with a fibrous cap [9].

Figure 9.1 Frequent artifacts in optical coherence tomography imaging. (a) Shadowing of guidewire (asterisk) and stent struts. (b) Residual blood. (c) Motion artifact (“sew‐up”). (d) Saturation artifact. (e) Tangential signal drop‐out artifact. Please note that this artifact causes a signal‐rich area overlying a signal‐poor region in an area of adaptive intimal thickening. (f) Bubble in the catheter causes a shadow on the vessel wall (arrow). (g) Multiple reflections. (h) Fold‐over artifact.

Blooming artifact is the effect of intense signal generated by the reflection of light [10]. This is most commonly caused by stent struts, which appear thicker. Bubble artifact is the result of air bubbles in the catheter sheath. Bubbles also form in the silicon lubricant used to reduce friction between the sheath and the revolving optic fiber in TD‐ OCT systems [11]. Bubbles can attenuate the signal along a region of the vessel wall, and images with this artifact are unsuitable for tissue characterization (Figure 9.1f). Multiple reflections are caused by the reflected surface of catheters creating one or more circular line within the image (Figure 9.1g). Strut orientation artifacts appears when the OCT catheter resides close to a stented artery wall, imaging metal coronary stents deployed appear as a bending of stent struts toward the imaging catheter. This so‐called sunflower effect occurs when the catheter occupies an eccentric position within the vessel lumen and the struts appear as a straight line [12]. Fold‐over artifact is more specific to FD‐OCT systems. It occurs when the vessel is larger than the ranging depth, thus it is typically observed in large vessels or side branches. Consequently, the vessel might appear to be folded over in the image (Figure 9.1h).