Researchers at the Massachusetts Institute of Technology (MIT; Cambridge, MA) have developed a new type of optical-coherence-tomography (OCT) system capable of taking high-resolution 3-D images of the retina that could greatly improve the diagnosis of critical eye diseases. With this technique, which uses a frequency-tunable ultrafast laser, they have obtained a 3-D retinal image in a human subject in less than a second (see figure).
At the CLEO/QELS meeting (May 6-11, Baltimore, MD), Robert Huber (a visiting scientist at MIT when the research was done; now at Ludwig Maximilian University in Munich, Germany) reported retinal scans at record speeds of up to 236,000 lines/s—a factor-of-10 improvement over current OCT technology. The current prototype is able to scan a 4 × 4 mm region of the retina in just 0.87 s, with an average depth resolution of between 10 and 15 µm.
Conventional OCT imaging typically yields a series of 2-D cross-sectional images of the retina that can be combined to form a 3-D image of its volume; however, limited imaging speeds and involuntary eye motion make it difficult to perform 3-D imaging of the retinal volume. An ophthalmological OCT system scans light back and forth across the eye, tracing thin, micrometer-scale lines that row-by-row build up high-resolution images. Commercial systems scan the eye at rates ranging from several hundred to several thousand lines per second. A typical patient can only keep the eye still for about one second, limiting the amount of 3-D data that can be acquired.
The MIT system relies on a Fourier-domain modelocked (FDML) laser source; earlier versions used broadband superluminescent diodes or femtosecond-laser sources in the 800 nm wavelength region to achieve 2 to 3 µm axial image resolutions with acquisition speeds of approximately 25,000 axial scans/s. Huber demonstrated the first FDML laser operating at 1300 nm in 2006; to perform retinal imaging, the device was modified to work over a wavelength range of 1040 to 1100 nm.
Huber collaborated with James Fujimoto, of MIT’s Department of Electrical Engineering and Computer Science and the Research Laboratory of Electronics, and graduate students Desmond Adler and Vivek Srinivasan to develop the prototype retinal-scanning OCT instrument, which is now in use at the New England Eye Center (Boston, MA). More than 1000 patients have been tested with the imaging system; for example, the prototype has been used to study acute macular neuroretinopathy, in which patients have red, wedge-shaped parafoveal lesions for which the retinal location is not clear. According to Huber, OCT enables enhanced imaging of intraretinal morphology, thus helping clinicians pinpoint the location of the lesions.
The MIT researchers hope that future clinical studies and further development of the system will allow ophthalmologists to obtain on-the-spot snapshots of the eye, containing comprehensive volumetric information about the microstructure of the retina, where most of the causes of blindness—including diabetic retinopathy, glaucoma, and age-related macular degeneration—reside.