Imaging the living retina at the cellular level with AO parallel spectral-domain optical coherence tomography

We investigate a novel camera that incorporates adaptive optics (AO) and optical coherence tomography (OCT) to determine if it can achieve the necessary 3-D resolution, sensitivity, and speed for imaging individual cells in the living human retina. An AO spectral-domain OCT system was constructed that is based on a free-space Michelson interferometer design. The OCT sub-system consists of a broadband superluminescent diode whose beam passes through an astigmatic lens to form a line illumination pattern on the retina, which is then imaged onto the slit of an imaging spectrometer. The detector of the spectrometer is a scientific-grade areal CCD. Conventional flood illumination, also with AO, was integrated into the camera and provided confirmation of the focus position in the retina. Short bursts of narrow B-scans (100x560 microns) of the living retina were subsequently acquired at 500 Hz during dynamic compensation that corrected the most significant ocular aberrations across a dilated 6 mm pupil. Camera sensitivity (up to 94 dB) was sufficient for observing reflections from essentially all neural layers of the retina. The 3-D resolution of the B-scans (3.0x3.0x5.7 microns) is the highest reported to date in the living human eye. It was sufficient to observe the interface between the inner and outer segments of individual photoreceptor cells, resolved in both lateral and axial dimensions. The waveguiding nature of the photoreceptors is suggestive at multiple reflective sites. Micro-movements of the retina during short burst imaging allow averaging to reduce speckle contrast, but they appear insufficient for significant speckle reduction.