Researchers from the University of California, Davis (UC Davis) have developed an instrument that has measured tiny, light-evoked deformations in individual rods and cones in a living human eye. The approach may one day improve detection of macular degeneration, a leading cause of blindness in people over 55. The system is based on optical coherence tomography (OCT) and captures specialized OCT images simultaneously with scanning light ophthalmoscope images. This allows it to detect slight swelling in the outer segments of photoreceptors that occur as a side-effect of phototransduction — the process of sight. The technique is able to measure how individual rods and cones respond to light, and it could detect deformations significantly smaller than the wavelength of the imaging light source. Researchers have developed a unique synchronized high-speed OCT/scanning light ophthalmoscope (SLO) system that captures the function of the retina’s rods and cones. The OCT images are co-registered with SLO images to pinpoint the location and type of photoreceptors captured in the series of 3D OCT images. Courtesy of Mehdi Azimipour, UC Davis Eye Center. “Although imaging the swelling of rods and cones can reveal the dynamics of their response to light, until recently it was not known if these changes could be measured in vivo in the human eye,” Mehdi Azimipour, first author of the research paper, said. “This is because the size of the photoreceptors and the scale of the light-evoked deformations were well below the resolutions provided by retinal imaging systems.” The system has significant advantage over recent full-field OCT that has been used to visualize the light-evoked deformation of larger peripheral cones. The UC Davis device offers better confocality, which improves image quality by rejecting more scattered light and suppressing noise. Because the light-evoked deformation of photoreceptors can be very fast, the new system incorporates a high-speed Fourier domain mode-locked laser that enables fast imaging and can scan 16× faster than commercially available lasers used for swept-source OCT. The researchers also incorporated adaptive optics technology, which can measure the aberrations and correct them in real time. Even with adaptive optics, rod photoreceptors are too small to be imaged due to the system’s 1-µm-wavelength light source. To overcome that, the researchers added a scanning light ophthalmoscope imaging channel that uses a wavelength that is less than 1 µm to increase the imaging resolution and allow for the differentiation of rods and cones in co-registered OCT images. Hundreds of 3D OCT images are taken to pinpoint the location and type of photoreceptors, which produces enormous amounts of data. To accommodate that quantity of data and to make the technology practical for clinical use, software must be developed to allow for the processing of the data so larger areas of the retina can be scanned. The researchers’ next step is to use the instrument to measure photoreceptor light responses of patients with retinal diseases to see if new insights can be gained. The research was published in Optics Letters (www.doi.org/10.1364/OL.398868).