OCT systems mainly consist of a light source, a coupler- or a circulator-based interferometer, and a photodetector. OCT is categorized into time domain OCT (TD-OCT) and frequency domain OCT (FD-OCT) as illustrated in Fig. 10.2 [11, 12]. TD-OCT employs a broadband light source such as a superluminescent diode and a scanning reference arm. The path length difference between the sample and reference arms of the interferometer is modulated by scanning a reference path length. The envelope of interference fringes is then extracted as a function of time, which means that image data is finally generated in a time domain. A single travel of the reference mirror creates a depth profile or an A-scan. Development of the high-speed scanning delay line of the reference arm allows OCT imaging speeds of several thousand axial profiles per second and video frame rate [13, 14]. FD-OCT additionally requires a spectrometer as a photodetector or wavelength-swept laser as light source without scanning a reference length. The former case is referred to as spectral domain OCT (SD-OCT), whereas the latter is termed swept-source OCT (SS-OCT) or alternatively optical frequency domain imaging (OFDI) [15–19]. SS-OCT has the advantages of easy implementation of polarization diverse detection as well as large depth range over SD-OCT. In the case of SS-OCT, a narrow instantaneous linewidth over a broad spectral range is tuned in wavelength as a function of time, and all echo signals from different depths are measured simultaneously. Thus, system sensitivity and imaging speeds can be dramatically improved [11, 19].

Intravascular OCT (IV-OCT) is a catheter-based imaging modality using a fiber-optic probe (Fig. 10.3). In general, in vivo intracoronary imaging is challenging because a suitable OCT catheter and contrast agent flushing protocols to remove blood need to be developed. IV-OCT additionally requires a catheter system that is composed of a rotary junction and a catheter as shown in Fig. 10.4 [20, 21]. A rotary junction plays the important role of pulling back and rotating a catheter as well as optically connecting an OCT system console with a catheter. A catheter consists of an optical probe and a protective sheath. An optical probe is composed of a single mode fiber, a small lens, and a hollow torque cable. The torque cable has multiple threads and layers to accurately transmit the proximal end rotation to the distal tip of a fiber probe in curved environments. As a rotating optical fiber is prone to fragility, it is inserted into a torque cable and fixed. To focus light on the vessel, an angle-polished ball lens or a gradient index (GRIN) lens is utilized. An inserted catheter is rotated to create two-dimensional cross-sectional images of coronary artery while it is pulled back to generate multiple frames (Fig. 10.5). IV-OCT creates original rectangular OCT images that are converted from polar to Cartesian coordinates for display as shown in Fig. 10.6. To examine the coronary artery by IV-OCT, blood in the artery must be removed to avoid massive optical scattering and attenuation by red blood cells. The first catheter-based imaging of a human artery ex vivo was conducted by Tearney et al. [22, 23]. This study reported that OCT images were capable of differentiating the intima, media, and adventitia of the artery. The first in vivo IV-OCT imaging in human patients was conducted by Jang et al. who demonstrated a comparison of OCT with IVUS images of tissue prolapse in a stent [24].

The first commercial IV-OCT product was the M2 OCT system (LightLab Imaging, Inc., Westford, MA, USA, now part of St. Jude Medical, Inc.) with regulatory approval in Europe and Japan in 2004, and the M3 system was launched 3 years later in Japan. Since both M2 and M3 systems were based on TD-OCT technology, the imaging speeds were limited (frame rate, 15.6/s for the M2 vs. 20/s for the M3, and pullback speed, 3.0 mm/s vs. 2.0 mm/s) [25, 26]. The balloon occlusion with lactated Ringer’s solution or normal flushing was performed from proximal location to the lesion during the OCT imaging. Dramatic advancements in the wavelength-swept source in FD-OCT enabled much faster frame rates (100 frames/s) and pullback speeds (5–20 mm/s), resulting in non-occlusive OCT imaging with flushing of viscous contrast [4, 27]. The commercial version of FD-OCT, a Dragonfly imaging catheter and C7-XR OCT system (St. Jude Medical/LightLab Imaging, Westford, MA, USA), was launched with the world’s first regulatory approval in 2010 [26]. The feasibility and safety of the Terumo OFDI system allowing 160 frames/s were evaluated in human patients in 2011 [28], and the first product of a LUNAWAVE OFDI system (Terumo Corporation, Tokyo, Japan) was then launched in more than 30 countries in Europe in 2013. The main specifications of TD-OCT and FD-OCT systems are summarized in Table 10.1 [4, 26, 28].

Since image quality is determined by imaging modality resolution, developing a high-resolution optical imaging system has been one of main research topics, involving high-speed and penetration depth imaging modalities. The image resolution of OCT is divided into axial resolution and lateral or transverse resolution. The axial resolution is the smallest distance between two objects that can be resolved along the axis of the incident beam. It is independent of the lens design and proportionally dependent on the center wavelength of light source and inversely proportional to the source bandwidth [29]. The lateral resolution in OCT imaging is the minimum resolvable distance between two objects which lie perpendicular to the OCT beam at the same depth position. It is mainly dependent on the focusing lens in the imaging core [30]. Another important parameter to determine the image quality is the depth of focus (DOF). DOF is twice the Rayleigh range, defined as the axial distance from the position of the minimum spot size (d) to the position of . A trade-off exists between lateral resolution and DOF. Thus, increasing the lateral resolution to acquire better image quality results in decreasing the DOF. Since the relative position of a catheter from the vessel wall significantly varies during IV-OCT imaging, great variations of the lateral resolution induce distortion of images. The axial resolution for IV-OCT is typically ~10 μm at a center wavelength (~1300 nm) in the light source, and the lateral resolution is between 20 and 40 μm and the DOF is ~1.3 mm [31, 32].