(sec)
Catheter preparation, calibration, image acquisition and fluoroscopic co‐localization
Purging with contrast is an essential step of catheter preparation. Because of the difficulty to inject the highly viscous contrast through the long thin hole around the probe, a high‐pressure syringe is provided. After connection and testing, calibration is the next essential step and must be performed when the catheter is already inserted at the start‐point of the pullback to correct for changes in length of the optic fiber caused by bending of the imaging wire and proximity to the vessel wall [6]. The rapid OCT pullback should be started only after optimal blood clearance that can be achieved via a manual injection through the guiding catheter or, better, using an automatic injector (ACIST Bracco, Italy; Medrad, Bayer, Germany). While the injection rate is similar or slightly higher than the customary 3 mL/s for the RCA and 4 mL/s for the LCA, the contrast volume should be increased to 15–20 mL to ensure full clearance throughout the pullback. High volume users mainly prefer manual selection of the pullback to avoid a too early start often observed, relying on the automatic system triggering. The use of contrast to replace blood is very efficient because it avoids swirling of blood when less viscous fluids are used but is contraindicated in patients with severe renal insufficiency. In most patients it is possible to obtain all the required information performing 2–3 imaging runs (before stenting and after stent optimization) sparing contrast in avoiding multiple views not required when the full three‐dimensional anatomy of the vessel can be clarified with OCT, especially in combination with coregistration that also eliminates the need of injections for balloon and stent positioning. The substitution of contrast with dextran or saline may offer a future solution for all these problems.
If an angiographic acquisition is performed throughout the pullback, efficient built‐in software is available to follow the imaging tip during pullback and allow accurate matching of OCT with angiographic images. This facilitates accurate ballon/stent positioning during fluoroscopy at the desired sites. The AptiVue software upgrade has new features that will assist in achieving optimization targets as it was designed to follow the ILUMIEN IV protocol (NCT03507777). The software permits automated and rapid determination of stent expansion and apposition. The stent expansion analysis offers two modes: dual and tapered. In both, the stent is automatically detected, rendered, and divided in half to account for natural vessel tapering. It is important to note that all areas should be checked and manually corrected by the operator, when needed, prior to any treatment.
Artifacts
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).
Normal coronary vessel anatomy
With the exception of the left main stem, coronary arteries are muscular arteries and are histologically organized into three layers. The intima consists of a lining layer of endothelial cells supported by a subendothelial layer [13], which is exceedingly thin at birth and grows progressively with age, eventually reaching OCT resolution limits [14]. In OCT, the intima can be visualized as a signal‐rich luminal layer. The intimal thickens with age, and nearly all adult coronaries display an extent of intimal thickening [15]. There is no established cut‐off for the identification of pathologic intimal thickening; however, some authors use, rather arbitrarily, a cut‐off of 300 μm to identify intimal thickening, and above 600 μm for pathologic intimal thickening in the absence of a lipid pool or calcified region >1 quadrant [16]. The medial layer is a signal‐poor region isolated from the intimal layer and the adventitia by the brighter lines of the internal and the external elastic membrane, respectively [17]. The adventitia is recognized as a heterogeneous high signal outer layer.
