Many technology devotees are familiar with the writings of Arthur C. Clarke, one of the most prolific science fiction writers of all time. One may also be familiar with his third law, “Any sufficiently advanced technology is indistinguishable from magic”. This frequently seems to be the case when I attempt to explain the fundamental principles of Optical Coherence Tomography or OCT. If you have a firm grip on how one obtains an optical image from below the surface of some material, then read no further. If sub-surface imaging sounds somewhat magical, then read on. Through our long-standing relationships in Academia and significant investments in research and development, Benchmark can leverage OCT technology in new and existing diagnostic applications.
the light we see
It is quite easy to demonstrate how light can penetrate many materials. If one shines a flashlight through the web of ones fingers, you will see a reddish glow. Light passes through many biological tissues as long as the thickness is not too great. Most of the light that penetrates the tissue is reflected or scattered in random directions which means it cannot be captured by a typical imaging system. A typical imaging system only captures light reflected from the surface and into the numerical aperture of the lens system. We will ignore the finer details of photography and imaging.
the power of interference
OCT allows us to capture the light reflected from below the surface using a technique called interferometry. Unlike standard imaging where pixels are captured simultaneously, interferometry generates an image one pixel at a time.
A properly constructed interferometer will detect light in a very small area (~10x10 microns) when it interferes with the light in an optical reference path. To do this, the interferometer splits the light from a short coherence length source into two paths, the sample path and the reference path.
The reflected light from each path is then recombined to interfere at a detector. Interference will only occur if the optical path lengths are the same and the light has a short coherence length. If the sample path is constructed such that the optical path length for interference is below the surface, then light can be detected as it is reflected from below the surface, allowing the capture of a pixel.
Next the interferometer sweeps the reference path length to capture pixels at multiple depths and raster scans the sample path in x and y to generate a 3D point cloud. The point cloud is then post-processed by the imaging analysis software to display a 2D image in any image plane of interest, typically parallel to the optical axis. 3D imaging software can also be used to allow a clinician to see into a structure from many different viewing angles.
what does the future hold
Through Benchmark's continued investment in new research, designs, techniques and processes, we are able to enable our customers with the latest technology platforms. Sign up to receive notice of my next article where I’ll be exploring the current and future applications of OCT. This revolutionary imaging technique is shining a light into worlds we have yet to explore.