Clinical Applications of Retinal Imaging with Adaptive Optics

Before getting into the clinical utility of adaptive optics imaging technology, it is prudent to first review the basic principles of imaging with adaptive optics. With conventional optical imaging, the major factor limiting the achievable resolution is the eye’s monochromatic aberrations, which are due to imperfections in the optics of the eye. These wavefront aberrations can be separated mathematically into shapes described by low order polynomials (defocus and astigmatism) and higher order polynomials (e.g. coma and trefoil). Although lower order aberrations can be effectively corrected using spectacles or contact lenses, the higher order aberrations cannot over a large field of view. Their effect on visual function is not typically severe; however, higher order aberrations interfere with high-resolution retinal imaging. Ophthalmic adaptive optics systems are designed to measure and correct for these higher-order aberrations, and can provide image resolution that is limited only by the pupil diameter of the eye, the axial length of the eye, and the wavelength of light. As shown in Figure 1, ophthalmic adaptive optics imaging systems have three main components—a wavefront sensor (typically a Shack-Hartmann design, for measuring the eye’s aberrations), a corrective element (typically a deformable mirror, for correcting the aberrations), and an imaging device (typically a charge-coupled device [CCD] or photomultiplier tube). These design principles are not absolute, and alternative approaches that do not use a wavefront sensor or that use multiple corrective elements have been demonstrated. Nevertheless, the unifying feature of adaptive optics imaging systems is mitigation of the eye’s aberrations to achieve nearly diffraction-limited imaging. These imaging systems have so far taken the form of an adaptive optics fundus camera, an adaptive optics scanning laser ophthalmoscope, or an adaptive optics optical coherence tomograph (OCT).