The images are displayed in cross-sectional, longitudinal or 3D mode according to the operator's choice ( A good-quality blood clearance enables differentiation of lumen contour and vessel wall allowing for reliable automatic measurement of lumen diameters, lumen area, and marking the maximal stenosis and distal as well as proximal reference segment. Due to necessity of blood clearing, imaging of the proximal left main artery can be challenging, and in this case, IVUS is generally preferred. The recommended settings vary according to vessel diameter with flow of 3 to 4 mL/s and total volume of 10 to 14 mL. This could be performed manually or with automated injector. A difference and mild disadvantage compared with IVUS is the necessity of blood clearing with contrast medium in the site of interest, since the near-infrared light is attenuated by blood. Another advantage of OCT system is an option to use 3D reconstruction of the vessel or stent, which offers the operator better understanding of anatomical relations, for example, in bifurcation stenting. This allows to register the lens movement during the pullback and consequentially coregistration with CA. Both the lens and proximal markers are affixed to the imaging core. There are three radio-opaque markers the distal marker 4 mm from tip of the catheter, the lens marker 2 mm proximal to the lens, and the most proximal marker located 50 mm proximal to the lens, marking the imaging region. The crossing profile of catheter is 2.7 F with insertable length of 135 cm and the catheter is compatible with 0.014 in guidewires. The examiner has a choice of survey mode-75 mm pullback range with a pullback rate of 36 mm/s or high-resolution mode-pullback range of 54 mm with a rate of 18 mm/s, that is, pullback time 2.1 or 3 seconds, which is significantly lower than one of IVUS. The most widely used OCT catheter in Northern America and Europe is the Dragonfly OPTIS imaging catheter. The limited penetration depth of OCT in general and especially in lipid tissues limits its ability to visualize deeper vessel structures and plaque burden cannot reliably be measured compared with IVUS (penetration depth up to 4 mm). The latest third generation of swept-source OCT is able to produce images with frame rate of 180 fps with penetration depth of signal from 1 to 1.5 mm according to the type of tissue. In a phantom model, the mean lumen area was equally measured by OCT, whereas IVUS overestimated mean lumen area by 8%. In the OPUS-CLASS study investigating the measurement accuracy of quantitative coronary angiography (QCA), IVUS, and OCT, IVUS measurements of minimal lumen area (MLA) were 10% higher compared with OCT and the interobserver variability of IVUS was twice as high as of OCT. This fact reduces the interobserver and intraobserver variabilities of OCT measurement, otherwise common in CA. Optical coherence tomography is a modality that uses a fiber-optic catheter that emits waves of near-infrared light (1–1.3 µm) and registers its scattered signal producing high-resolution images at a resolution of 10 to 15 µm, far exceeding the resolution of an angiography and 10-fold resolution of IVUS. Technical Overview, Advantages and Disadvantages of OCT Technology

Optical coherence angiography