OCT delivers high resolution because it utilizes light, rather than sound or radio frequencies. In coronary applications, OCT involves directing an optical beam from a catheter extended into the artery of interest. This light penetrates with a depth of 1 mm to 2 mm into the tissue and a small portion of the light that reflects from the sub-surface features is collected. The majority of this collected light is not reflected, but scattered in a way that causes an effect similar to glare. However, the OCT technique can filter out this reflected light, allowing it to produce an image based only on the small proportion of the collected light that is reflected by the tissues.
Because OCT can only image tissues at a depth of a few millimeters, it must be performed endovascularly in cardiology applications. As a result, OCT is most often compared to another imaging technology, intravascular ultrasound (IVUS). Compared to IVUS, OCT produces much higher resolution images, but it cannot produce as broad an image of the cardiovascular system. The main drawback of first generation OCT systems compared to IVUS is the fact that blood flow through the artery must be stopped (for a short time) with a specialized catheter prior to OCT imaging. This is due to the fact that blood can attenuate the OCT optical beam. While this drawback existed in the majority of first generation OCT systems used in Europe and Japan, second generation systems operating on the concept of optical frequency domain imaging (OFDI) are expected to mitigate this problem. OFDI is capable of acquiring images at much higher frame rates than standard OCT, allowing for the imaging of coronary arteries following a brief flush of the artery with saline. Please note that OFDI is sometimes referred to as Fourier domain optical coherence tomography (FD-OCT).