Summary
Optical coherence tomography is a new endocoronary imaging modality employing near infrared light, with very high axial resolution. We will review the physical principles, including the old time domain and newer Fourier domain generations, clinical applications, controversies and perspectives of optical coherence tomography.
Résumé
La tomographie par cohérence optique est une modalité d’imagerie récente endocoronaire utilisant la lumière infrarouge, caractérisée par une haute résolution. Dans cet article, on discute les principes physiques en discutant l’ancienne et la nouvelle génération de tomographie par cohérence optique, time domain et Fourier domain respectivement.
Introduction
Optical coherence tomography (OCT) is a new imaging modality, used for the first time by Huang et al. in 1991 in vitro on the human peripapillary region of the retina and coronary arteries . OCT is based on near infrared light; an optical beam is directed at the tissues, most of the light scatters and only the small portion of this light that reflects from subsurface features is collected and forms the image by yielding spatial information about tissue microstructure. The critical advantage of OCT over ultrasonography and magnetic resonance imaging is due to its micrometer resolution (about 10–15 μm of tissue axial resolution) .
Physical principles and acquisition systems
OCT uses low coherent near infrared light. The wavelength used is around 1300 nm to minimize energy absorption in the light beam caused by protein, water, haemoglobin and lipids . The physics principle that allows the filtering of scattered light is optical coherence . A light source emits a low-coherence, laser light wave. The light wave reaches a beam splitter or a partial mirror, which splits the light wave in half. One part of the light wave travels to a reference mirror, where it reflects directly back towards the beam splitter. The second part travels to the sample tissue. Depending on the optical properties of the tissue, some amount of light may be absorbed, refracted or reflected . Reflection occurs when there is a region of sharp refractive index mismatch; therefore the velocity of light is not considered constant when it passes through different media. Light travels faster in a medium of low refractive index compared to a medium of high refractive index. The amount of reflection depends on the level of mismatch, the angle and the polarization of the incident angle. The reflected portion of the light travels back towards the beam splitter, where it meets with the reference light wave. The interaction between these two light waves is the basis on which OCT produces images . When two light waves of the same wavelength and constant phase difference meet, they are combined through superposition; this phenomenon is called interference. If the light waves are in phase, they add together in constructive interference; if they are out of phase, they cancel each other out in destructive interference . When the sample and reference light waves meet, they either intensify or diminish depending on how the sample light interacts with the tissue . A detector uses the light or dark pattern produced to create a pixel for that specific region . OCT cross-sectional imaging is achieved by performing successive axial measurements of back-reflected light at different transverse positions. After scanning a whole area, a full image of the tissue may be produced.
The major limitation of intracoronary OCT is blood attenuation due to the backscattering properties of red blood cells, thus we need to displace blood from the field of view.
There are two OCT systems: the first-generation system or time domain OCT and the new-generation system or Fourier domain OCT.
Time domain OCT
Time domain OCT (TD-OCT) uses an occlusive technique that requires stopping of the coronary blood flow by soft balloon inflation . The pullback speed of TD-OCT ranges between 1 and 5 mm/s . TD-OCT uses a broadband light source containing a moving mirror that allows scanning of each depth position in the image, pixel by pixel. This mechanical scanning process limits the rate at which images can be acquired .
TD-OCT is limited by the risk of balloon injury, a balloon-vessel size mismatch, a long diseased lesion exceeding 30 mm, the inability to visualize ostial or very proximal lesions and the inability to study the left main coronary artery.
Fourier domain OCT
The development of the new-generation or Fourier domain OCT (FD-OCT) enables high-speed pullbacks (10–25 mm/second) during image acquisition, allowing the visualization of long coronary segments in a much reduced acquisition time and without the need for transient occlusion of the coronary artery. The non-occlusive technique requires simultaneous flushing with a viscous iso-osmolar solution through the guiding catheter . The fluid infused requires a viscosity higher than that of blood; non-occlusive OCT image acquisition using iodixanol 320 is the standard flushing solution . The amount of iodixanol 320 used for OCT pull-back is usually 3-fold greater than that required for standard coronary iodixanol 320.
FD-OCT uses a wavelength-swept laser as the light source and the reference mirror is fixed. This change in technology results in a better signal-to-noise ratio and faster sweeps, allowing a dramatically faster image acquisition and pullback speed than TD-OCT . Presently, the maximum imaging speed that can be achieved with FD-OCT is limited by digital data transfer and storage .
OCT versus intravascular ultrasound
Many trials have compared OCT with intravascular ultrasound (IVUS) for tissue characterization of human coronary plaques. OCT is mainly limited by its penetration depth. Within its penetration depth OCT has much higher sensitivity and specificity for characterizing calcification, fibrosis, lipid pool intimal hyperplasia , fibrous cap erosion and rupture, intracoronary thrombus and thin cap fibroatheroma ( Fig. 1 ), for the detection of stent endothelialization, strut coverage and stent apposition and expansion, and for lumen border visualization and measurement of correct lumen area . As for IVUS, the critical lumen area for intermediate lesions is 4 mm 2 . Measurements of lumen diameter and lumen area obtained with OCT and IVUS were highly correlated, although OCT measurements were found to be 7% smaller ; these findings may be more relevant in small vessels. Compared with OCT, IVUS tends to underestimate stent tissue coverage . Table 1 shows the physical properties of IVUS and OCT.
| IVUS | OCT | |
|---|---|---|
| Wavelength (μm) | 35–80 | 1.3 |
| Energy source | Ultrasound | Infrared |
| Penetration (mm) | 10 | 1–2.5 |
| Axial resolution (μm) | 100–200 | 15–20 |
| Lateral resolution (μm) | 200–300 | 20–40 |
Clinical applications
Coronary plaque classification
OCT was validated in vitro for atherosclerotic plaque characterization on a large post-mortem specimen in 2002 and later in vivo human studies confirmed the ability of OCT to characterize the plaque : fibrous plaques are characterized by a homogeneous rich signal; fibrocalcific plaques reveal signal-poor regions with sharply delineated borders; lipid-rich plaques show diffusely bordered signal-poor regions (lipid is present in two quadrants in any of the images within a plaque); vulnerable plaques are characterized by a thin-capped fibroatheroma, defined as a fibrous cap thickness < 70 μm ( Fig. 1 ), within a lipid-rich plaque; microchannels are defined as no-signal tubuloluminal structures without a connection to the vessel lumen, recognized on three consecutive cross-sectional OCT images , and are seen with increased neovascularization of atherosclerotic plaque ( Fig. 2 ). Fig. 3 shows a typically stable and calcified coronary plaque with thick fibrous cap.
