Optical coherence tomography (OCT)
Optical coherence tomography (OCT) has much higher resolution compared to intravascular ultrasound. It has the resolution of the range of 10 – 20 microns, which is an order of magnitude better than that of intravascular ultrasound. But the limitation is the lower depth of tissue penetration which is only 2 to 3 millimeters, compared to the 10 millimeter tissue penetration of current intravascular ultrasound probes. Hence it has limitations while imaging large vessels like left main coronary artery and proximal left anterior descending coronary artery. The excellent resolution of optical coherence tomography is very useful in examining the coronary lesion characteristics like plaque rupture and also for monitoring the results of percutaneous interventions with stent deployment. OCT uses light waves instead of ultrasound for imaging. A light emitting source and system to collect the reflected light which checks the intensity of back scatter from internal microstructures with varying optical properties and the time delay of receiving the return signal are part of the OCT device. Since the speed of light is very much faster than that of sound, it is practically impossible to detect time delay of reflected waves electronically. A special device known as time domain OCT interferometer is used for this purpose. OCT is useful in plaque characterisation including the measurement of thickness of the fibrous cap which is an important determinant of plaque stability. The macrophage content of the plaque can also be estimated as the macrophages are of relatively large size (20 to 50 microns) and have a high degree of optical contrast. Hence plaques rich in macrophages have a high OCT signal variance. An excellent review on OCT is available at: Cardiac optical coherence tomography. Heart 2008;94:1200-1210, written by Raffel OC and colleagues, in the technology and guidelines section.