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Gang Huang, Xiaofeng Qi, Thomas Gast, Ting Luo, Stephen A Burns; In-vivo measurement of retinal blood cell velocity using a dual-beam adaptive optics scanning laser ophthalmoscopy (AOSLO). Invest. Ophthalmol. Vis. Sci. 2014;55(13):5193.
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Retinal blood cells can be visualized in-vivo with an AOSLO. However, the measurement of retinal blood cell flow, especially at arteriole and capillary levels, can be challenging. The normal frame rate of an AOSLO system is not fast enough for a direct analysis of cell motion between adjacent frames. In this study, we develop a novel technique that is based on a dual-beam AOSLO to measure blood cell velocities in human retinal vessels in-vivo.
We developed a dual-beam AOSLO system by modifying the optical configuration of a steerable wide-field single-beam AOSLO. We added a second optical channel, which carries a different wavelength from the original channel. The two channels were aligned by adjusting a dichroic mirror and steering mirrors at the system entrance. For detection, the two beams were separated by another dichroic mirror and recorded with separate detectors. The frame rate of this system is 30fps. We then angularly separated the two beams in the system and thus produced two imaging fields that project simultaneously on the retina but that are offset in both spatial and time space. The offset distance and direction are adjustable and were calibrated on a model eye. The offset between the two channels from zero (no offset) to relatively large delays (for instance 3 ms). Images of blood cells acquired from the two beams are paired and therefore we can measure the distance individual cells travelled between the two images and thus compute the cell velocity.
Two young healthy subjects were imaged. We measured velocities in arterioles and capillaries in various regions of the retina. By displacing the two images by 0.2 degree, which is equivalent to approximately 3 ms, we were still able to image a region of 1.8 degree by 1.8 degree with both imaging fields, and achieved an effective frame rate of 300 Hz.
We successfully developed a dual-beam AOSLO to measure retinal blood cell velocities. The use of two beams, slightly misaligned allows construction of an AOSLO with a tunable velocity sensitivity that overcomes the limitation provided by the inherent frame rate of the AOSLO. It should provide a new tool to investigate the role of capillary blood flow in diabetes, hypertension, glaucoma, and many retinal vasculature diseases with fewer frames.
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