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Monday, December 27, 2010

A glimpse (or primer) of the Plenary Session -- "Medical Imaging Using Optical Coherence Tomography"

          Optical coherence tomography (OCT) has been growing exponentially over the past two decades. It is not difficult to see why -- First of all, its resolution (~10^-6 meter) and the strength of penetration depth (a few millimeters) fill the gap between different imaging methods. Figure 1 shows this clearly. In specific, it comes right between the commercial medical devices and instruments that are very popular in the research communities. Due to this fact, it is not hard to realize why it attracts the attention both from industry and the academia. The development is accelerating by the boost of both ends for sure. Secondly, OCT uses such low energy (~10^-6 to 10^-3 W ) that the tissues are hardly harmed. In fact, "in vivo" imaging is readily achievable and has been demonstrated on the cross-sectioning of several skin tissues. Diagnostics of the eye diseases, in particular, has been a major applications of OCT too. Thirdly, the sensitivity of the OCT is well above 80dB, and this is a great news to fellows of imaging sciences since signal to noise ratio is always a persistent enemy when they are pushing the frontiers in optical science. These fascinating features of OCT are based on its principle -- light interference -- a phenomenon that was observed first in 17th century. Of course, it comes with a twist.

Figure 1. Comparison of different imaging methods. AFM: atomic force microscopy. TIRF: total internal reflection Fluorescence. MPM: multi-photon microscopy. OCT: optical coherence tomography. US: ultrasound. MRI: magnetic resonance imaging. PET: positron emission tomography

          Most people are familiar with the phenomenon of light interference, especially through the fringe pattern that is usually demonstrated by laser in high school or college. In fact, if we split the light into two and recombine them, they do not always interfere to each other unless the distances traveled by light along two paths are similar. The tolerance on the distance difference has a formal name -- coherence length (lc). It is inversely proportional to the bandwidth of the light. This is the reason that interference is easily shown by laser with single frequency since with single frequency, the lc is easily extended to meters! On the other hand, in OCT, researchers use super broad band light source to decrease lc to micrometers to fulfill the depth resolution. This twist makes OCT a high resolution imaging method. Several light sources and their resolutions are described in figure 2.
Figure 2. Different light sources of OCT. This table is courtesy of A. Fercher, W. Drexler, C Hitzenberger and T. Lasser in Reports on Progress in Physics 66 (2003) 239-303

          The way OCT hinges on the interference principle is like the following: the light from the source is divided into two; one being sent to the sample and the other to an adjustable delay line. The light reflected from the sample is recombined with the light from the delay line, and the measurements are based on the interference strength. As aforementioned, the light has very short lc, so only the light that reflects from a small section of the depth of the sample can interfere with the light from the delay line. By changing the delay line, we are actually able to do the imaging, slice by slice through the thickness of the sample. And then, combining with the lateral scanning, a 3D image is resulted.
          Two branches of OCT are often mentioned. One is time domain OCT and the other is called Fourier Domain (FD) OCT. The main difference is that, in time domain OCT, an adjustable delay line is required. However, in FD-OCT, this delay line is replaced by scanning the frequency of the light source, or using a grating to disperse the reflected light. Both branches are extending the applications through researchers' efforts.
          This is merely a silhouette of OCT, or actually a glimpse of it.  In CLEO, we have the chance to hear it from one of its most important pioneers -- Professor James Fujimoto. He will present great insight on where OCT is now and where it will go in the future. I am very excited and looking forward to his presentation!
The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.