Murat V. Kalayoglu, M.D., Ph.D.
Advances in in vivo optical imaging have added several new diagnostic technologies to the Ophthalmologist’s armamentarium. One of the most exciting such technologies is optical coherence tomography (OCT), which is being developed and refined at a stunningly fast rate.
Optical coherence tomography (OCT) is analogous to ultrasound except that near-infrared light waves – instead of acoustic waves – are used to measure distances of specific structures. OCT depends on optical ranging; in other words, distances are measured by shining a beam of light onto the object, then recording the echo time delay of light. Since the velocity of light is so high, it is not possible to directly measure the echo time delay of reflections; therefore, a technique known as low-coherence interferometry compares reflected light from the eye to that reflected from a reference path of known length. Different internal structures produce different time delays, and cross-sectional images of the structures can be generated by scanning the incident optical beam. These two-dimensional scans are then displayed in a color scale where ‘warm’ colors (red to white) represent areas of high optical reflectivity, and ‘cool’ colors (blue to black) represent areas of low reflectivity.
The commercially available OCT machines (Humphrey Instruments) were originally developed by a team of bioengineers and Eye M.D.s at the Massachusetts Institute of Technology in Boston, MA. The original machine was brought to market in 1995 (OCT-1), and revised in 2001 to be more user- and patient- friendly (OCT-2). The 2002 release of the OCT-3 enabled in vivo imaging of the posterior segment at resolutions of <10 um. The current technology thus permits excellent resolution and has been used to help diagnose and guide treatment for macular edema, macular and lamellar holes, epiretinal membranes and pseudoholes, central serous chorioretinopathy, age-related macular degeneration and optic disc lesions.
Although the resolution power of the currently available OCT machines are remarkable, they are not sufficiently high to unequivocally identify all retinal sublayers and make ‘biopsy’-like diagnoses. Resolution is limited chiefly by the bandwidth of the light source, usually a superluminescent diode, and increased resolution will require higher bandwidth light sources. The emergence of ultrabroad bandwidth femtosecond laser technology has allowed the development of an ultra-high resolution OCT, currently being tested and refined in select laboratories. Indeed, ultrahigh resolution OCT has been demonstrated to achieve axial resolutions of 3 um during in vivo imaging of the human retina, which is two orders of magnitude higher than what can be achieved by conventional ultrasound imaging. The ultrahigh resolution OCT will in effect be a microscope capable of revealing certain histopathological aspects of macular disease in the living eye.
Such rapid advances in OCT imaging are likely to alter the practice of Ophthalmology dramatically in the next several years. Increased resolution and imaging speeds, wavefront correction, and the possibility of quantitative 3D modeling are just a few of the features to look for in the future. Further advances may transform the OCT from an ancillary procedure to a common and necessary “optical biopsy”. Indeed, future ophthalmologists using the next generation of OCT devices may diagnose macular disorders exclusively by digital imaging, without a funduscopic examination!