Optical Coherence Tomography

OCT is analogous to ultrasound as it measures the backreflection of infrared light rather than sound.1-3  OCT’s greatest advantage over any other currently available imaging technology is its high resolution.  Resolutions of catheter-based systems are in the range of 10 to 20 μm. With more sophisticated sources, resolutions as high as 4 μm can be achieved, which may eventually be applied to catheter-based approaches.13  One of OCT’s several other advantages for intracoronary imaging is that it’s acquisition rates are now well in excess of video rate.14,15, 97  Another benefit of OCT is that typical catheters contain no transducers within their frame, which enables them to be small and inexpensive. The smallest catheter to date is the 0.017” OCT imaging wire from LightLab Imaging (Westford, MA).16  Additionally, the OCT system is compact and portable, smaller than the size of an ultrasound unit. Figure 1 shows images of a Lighlab imaging engine, 2.7 French catheter, and a 0.019” imaging wire.  The fact that OCT uses light to create an image also allows the opportunity for the system to use a variety of spectroscopic techniques such as polarization spectroscopy (PS-OCT) for collagen, elastography for tensile strength, OCT Doppler for angiogenesis, phase sensitive OCT for assessing tissue composition, image analysis (such as wavelet analysis) for tissue composition, and potentially absorption spectroscopy/dispersion analysis for lipid composition (which includes second order correlation assessments of lipid).11, 17-21, 33-35, 82-84, 79 This techniques, and their minimal current clinical use, will be raised several times through out these web pages.  It may also be useful for assessing inflammatory cell concentration, but the potential limitations of the technique for this as well as measuring inflammation in general, is discussed (macrophage page).

1.B OCT Theory

There are several embodiments of OCT, which are differentiated in more detail in the supplemental.2, 28, 29  The time domain approach (TD-OCT) will be described here. The original form of OCT works by directing ultrashort light pulses or more commonly low coherent light at a sample. The time it takes for the light to be reflected back to the detector, or the echo delay time, is used to measure distance. The intensity of backreflection is plotted as a function of depth, allowing the creation of both two and three dimensional images. Due to the high speed associated with the propagation of light the echo delay time cannot be measured electronically. Consequentially, a technique known as low coherence interferometry is utilized as shown in figure 2 and discussed in the supplemental.5, 6

Essentially, the lateral resolution is determined by the lens system used, which is typically in the range of 20-30 μm. Higher lateral resolutions deteriorate more rapidly off the focus.  Penetration is normally within 2-3 mm, about the length of a standard biopsy and generally sufficient for imaging coronary arteries but the latter statement will be discussed in detail.3, 4   Techniques being pursed by our group to increase penetration is the use of a parallel ultrasound beam to reduce multiple scattering and use of a log amplifier with SS-OCT.  Appropriate speckle reduction and image process techniques also offer potential.