Abstract

Optical coherence tomography (OCT) is an imaging technique, based on near-infrared light, that allows cross-sectional imaging of biological systems. The technique is based on the measurement of optical reflections of biological tissues. An optical signal transmitted through or reflected from a biological tissue contains time-to-flight information which in turn yields spatial information about tissue microstructure (1). While first used in ophthalmology to assess non-invasively the human retina, it has been recently applied to cardiology to allow an invasive visualization of the vessel wall of the coronary arteries from inside the lumen, through a dedicated wire (where the OCT probe is located) tracked in the vessel of interest. The images are comparable in shape to those of intravascular ultrasound (IVUS). However, with respect to this well established technique, OCT offers at least a 10 times higher spatial resolution (from around 150-200 microns for IVUS to 15-20 microns for OCT), due to the different properties and wavelength of light versus ultrasound. This higher resolution translates in practice in the possibility to acquire much more detailed images and to visualize particulars on anatomy and devices, not visible with IVUS (2). A second advantage of OCT over IVUS is the faster acquisition time, so that the pullback speed of the probe to obtain sequential tomographic images of the coronary artery and thus a volumetric reconstruction of the segment of interest can be quicker than the IVUS one, which is usually 0.5-1 mm per second (3). On the other hand, the penetration power of OCT is lower compared to IVUS. In coronary arteries, with OCT it is possible to visualize around 2 millimeters thickness of the vessel wall starting from the lumen border, thus it is almost always impossible to visualize the whole vessel contour up to the adventitia, consequently missing phenomena such as remodeling, complete plaque assessment, aneurysm formation. Furthermore, a second disadvantage of OCT versus IVUS is the need to eliminate the presence of blood (which absorbs part of the light and thus blocks its passage) between the probe and the vessel wall, ad to substitute it with solutions that do not impact the passage of light (such as contrast media). While in the beginning of the clinical development of OCT, this was achieved by proximal occlusion of the vessel of interest with a compliant balloon inflated at low pressure, nowadays with the last generation OCT device, which allows a pullback speed of the probe of 20 mm per second, it is enough to inject manually a full syringe of contrast, obtaining an effective washout of blood from the coronary artery for a few seconds, sufficient to visualize the segment of interest. Notwithstanding these limitations, OCT has recently taken the lead over IVUS in the assessment of intracoronary stents, mainly due to its higher resolution. Indeed, it has been soon evident that several phenomena occurring after stent implantation could be easily missed by IVUS but recognized and described by OCT, such as discrete tissue prolapse, focal dissections, localized thrombus formation, minimal strut malapposition and detailed neointimal hyperplasia growth. Basically, it has been possible to proceed from an IVUS-based “stent-level” analysis to an OCT-based “strut-level” analysis. However, optical coherence tomography is still a relatively “young” and not completely standardized technique thus new data applying this technique to the clinical arena are more than welcome.