Expert review document part 2: methodology, terminology and clinical applications of optical coherence tomography for the assessment of interventional procedures

Abstract

This document is complementary to an Expert Review Document on Optical Coherence Tomography (OCT) for the study of coronary arteries and atherosclerosis.1 The goal of this companion manuscript is to provide a practical guide framework for the appropriate use and reporting of the novel frequency domain (FD) OCT imaging to guide interventional procedures, with a particular interest on the comparison with intravascular ultrasound (IVUS).1–4 In the OCT Expert Review Document on Atherosclerosis, a comprehensive description of the physical principles for OCT imaging and time domain (TD) catheters (St Jude Medical, Westford, MA, USA) was provided.1 The main advantage of FD-OCT is that the technology enables rapid imaging of the coronary artery, using a non-occlusive acquisition modality. The FD-OCT catheter (DragonflyTM; St Jude Medical) employs a single-mode optical fibre, enclosed in a hollow metal torque wire that rotates at a speed of 100 r.p.s. It is compatible with a conventional 0.014″ angioplasty guide wire, inserted into a short monorail lumen at the tip. The frequency domain optical coherence tomography lateral resolution is improved in comparison with TD-OCT, while the axial resolution did not change. These features, together with reduced motion artefacts and an increased maximum field of view up to 11 mm, have significantly improved both the quality and ease of use of OCT in the catheterization laboratory.3,4 However, the imaging depth of the FD-OCT is still limited to 0.5–2.0 mm.5 The main obstacle to the adoption of TD-OCT imaging in clinical practice is that OCT cannot image through a blood field, and therefore requires clearing or flushing of blood from the lumen.1 The 6 Fr compatible DragonflyTM FD-OCT catheter is so far the only one in the market, as two other systems from Volcano and Terumo, which have function similar to the DragonflyTM, are not yet available. The DragonflyTM catheter is first advanced over a regular guide wire, distal to the region of interest. A dedicated marker, located 10 mm distal to the OCT lens, enables the pull-back starting point selection. The acquisition of an OCT image sequence requires a bolus of crystalloid solution (usually contrast) injected through the guiding catheter. The acquisition speed can be set up in a range between 5 and 40 mm/s, based on the used OCT system. Most expert users advocate the use of automated contrast injection to optimize the image quality. The previous experience with TD and FD-OCT technology shows OCT acquisition to be safe,1,4,6 effective,1,4,6–8 and highly reproducible for the assessment of the luminal areas and length.9–11 A fair correlation between OCT and IVUS quantitative measurements of the lumen areas was reported,9–11 despite comparative studies showing that IVUS tends to slightly overestimate lumen areas, while stent and neointimal areas are slightly higher on OCT.9–11 Frequency domain optical coherence tomography image quality depends on an accurate acquisition technique and proper guiding catheter engagement is needed to optimize directional contrast flushing. Metallic stent struts are highly reflective structures that lead to specular reflection with the excessive signal usually resulting in ‘blooming’ of the strut surface. Optical coherence tomography shows only the reflection of the luminal surface of the stent strut and is unable to provide a direct measurement of the strut thickness.8,12 Two features indicate a strut: a highly reflective spot and/or an associated shadow behind the strut. The presence of only one of these two defining features is sufficient for strut identification. For bioresorbable stents, the assessment of the whole thickness is possible, since light can usually penetrate non-metallic materials. They can be followed by serial images; the degradation process, characterized by the presence of empty spaces replacing stent struts, was documented for the first generation design,13 but was not visible with the second-generation model, which maintains its mechanical properties.14 Based on preliminary unpublished data, the bioabsorbable magnesium stent has different properties; it is visible after implantation and tends to disappear at the follow-up due to magnesium degradation. Stent overlap occurs in the cross sections in which two superimposed layers of stent struts are visible (Figure 1). Incomplete stent apposition or malapposition occurs if there is a separation of a stent strut from the vessel wall. Malapposition can be addressed at a cross-section level and expressed as an area, or can be evaluated at the strut level analysis.15 In the latter case, it is defined as a measured distance greater than the strut thickness for bare metal stents or greater than the sum of the thickness of the strut plus polymer for drug eluting stents (DES)8,15 (Figure 2). Acute malapposition is diagnosed immediately after stent deployment (Figure 1),14 late malapposition at the follow-up, while late acquired malapposition