Doppler holography, employing high-speed digital imaging and near-infrared light, maps blood flow in the eye's fundus. It can be exploited to estimate quantitatively the velocity and volume of blood flow by assessing the Doppler frequency broadening increase in retinal arteries with respect to local surrounding tissue. This technique enhances our ability to gauge hemodynamics within these arteries across the cardiac cycle, crucial for ocular disease diagnosis and management. Infrared radiation scatters and broadens within the retina's deeper layers, aiding the analysis of blood flow in superficial retinal vessels. Light interaction with blood scatterers is quantified to estimate flow velocity, using a model of forward scattering for momentum transfer. The root-mean-square velocity reflects the degree of Doppler broadening, allowing for a detailed assessment of retinal hemodynamics. This approach provides a valuable tool for analysing ocular vascular health.
Holographic retinal imaging can be affected by optical distortions from the eye and lenses. A digital Shack-Hartmann algorithm corrects this by splitting the Fourier plane into sub-apertures to measure local wavefront gradients via cross-correlation of sub-images. We examine wavefront regularization by Zernike polynomials for better aberration correction, and introduce a new method for calculating retinal image shifts. Using the entire computed image as a reference, rather than just the central sub-image, minimizes bias. Furthermore, we use a direct wavefront reconstruction approach, using overlapping sub-apertures and a 2D gradient integration algorithm to estimate the wavefront without regularization. Our findings show that this direct wavefront estimation enhances image resolution and contrast for Doppler holography of the eye fundus compared to wavefront regularization by Zernike polynomials.
Doppler holography uses high-speed imaging with near-infrared laser light to reveal blood flow contrasts in the eye fundus. We estimate the velocity of blood flow, blood volume rate and resistivity changes in in-plane retinal arteries by segmenting retinal vessels and calculating the local difference in root-mean-square Doppler frequency broadening compared to the background. Our approach allows for the estimation of hemodynamics in in-plane retinal arteries throughout the cardiac cycle, offering significant potential in the diagnosis and monitoring of ocular vascular conditions.
Laser Doppler holography is a technique developed to visualize blood vessels in human eye fundus and its dynamics. Quality of images obtained is compromised due to aberrations introduced by the anterior segment of the eye as well as by lenses used for viewing. As images calculated hold information about both amplitude and the phase of the field detected, the wavefront distortion can be estimated using the numerical Shack-Hartmann method in the off-line video rendering process. In result, resolution of images obtained is considerably increased and smaller blood vessels are revealed.
Diffuse illumination alleviates the issue of the presence of a laser hotspot at the waist of the incident beam in laser Doppler holography for ophthalmology. The image field of view can be increased because the focal point of the imaging lens doublet can be brought closer to the cornea without compromising safety. Under these conditions, compliance with European security safety limits guidelines for ophthalmic devices ISO 15004-2:2007 is guaranteed over the entire optical path of the incident beam, and no significant change in the calculated images is observed.
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