KEYWORDS: Human vision and color perception, Visual optics, Imaging systems, Eye, Optical engineering, Medical research, Biomedical optics, Systems engineering, Visual system, Light emitting diodes
The human visual system is an exquisitely engineered system that can serve as a model and inspiration for the design of many imaging systems. Optics and optical engineering play a key role in developing new techniques and approaches for both the study of human vision and the design of novel imaging systems. For example, advances in optical sensing and imaging have led to important discoveries about retinal image processing, and optical design tools are necessary for improving vision in patients. While advances in optics are improving our understanding of the human visual system, this understanding has also led to improvements in artificial vision systems, image processing algorithms, visual displays, and even modern optical elements and systems.
The combination of air-puff systems with real-time corneal imaging (i.e. Optical Coherence Tomography (OCT), or Scheimpflug) is a promising approach to assess the dynamic biomechanical properties of the corneal tissue in vivo. In this study we present an experimental system which, together with finite element modeling, allows measurements of corneal biomechanical properties from corneal deformation imaging, both ex vivo and in vivo. A spectral OCT instrument combined with an air puff from a non-contact tonometer in a non-collinear configuration was used to image the corneal deformation over full corneal cross-sections, as well as to obtain high speed measurements of the temporal deformation of the corneal apex. Quantitative analysis allows direct extraction of several deformation parameters, such as apex indentation across time, maximal indentation depth, temporal symmetry and peak distance at maximal deformation. The potential of the technique is demonstrated and compared to air-puff imaging with Scheimpflug. Measurements ex vivo were performed on 14 freshly enucleated porcine eyes and five human donor eyes. Measurements in vivo were performed on nine human eyes. Corneal deformation was studied as a function of Intraocular Pressure (IOP, 15-45 mmHg), dehydration, changes in corneal rigidity (produced by UV corneal cross-linking, CXL), and different boundary conditions (sclera, ocular muscles). Geometrical deformation parameters were used as input for inverse finite element simulation to retrieve the corneal dynamic elastic and viscoelastic parameters. Temporal and spatial deformation profiles were very sensitive to the IOP. CXL produced a significant reduction of the cornea indentation (1.41x), and a change in the temporal symmetry of the corneal deformation profile (1.65x), indicating a change in the viscoelastic properties with treatment. Combining air-puff with dynamic imaging and finite element modeling allows characterizing the corneal biomechanics in-vivo.
Measurement of intraocular lens (IOL) alignment implanted in patients in cataract surgery is important to understand their optical performance. We present a method to estimate tilt and decentration of IOLs based on optical coherence tomography (OCT) images. En face OCT images show Purkinje-like images that correspond to the specular reflections from the corneal and IOL surfaces. Unlike in standard Purkinje-imaging, the tomographic nature of OCT allows unequivocal association of the reflection with the corresponding surface. The locations of the Purkinje-like images are linear combinations of IOL tilt, IOL decentration, and eye rotation. The weighting coefficients depend on the individual anterior segment geometry, obtained from the same OCT datasets. The methodology was demonstrated on an artificial model eye with set amounts of lens tilt and decentration and five pseudophakic eyes. Measured tilt and decentration in the artificial eye differed by 3.7% and 0.9%, respectively, from nominal values. In patients, average IOL tilt and decentration from Purkinje were 3.30±4.68 deg and 0.16±0.16 mm , respectively, and differed on average by 0.5 deg and 0.09 mm, respectively, from direct measurements on distortion-corrected OCT images. Purkinje-based methodology from anterior segment en face OCT imaging provided, therefore, reliable measurements of IOL tilt and decentration.
We have developed a new technique for the objective determination of the cone spacing in the living human fovea, and obtained direct measurements of the distance between cones at retinal eccentricities ranging from 0 to 1 deg. The method is similar to stellar speckle interferometry, and consists of recording a series of short-exposure images of small foveal patches, illuminated by a laser spot. Each individual image presents a speckle pattern, correlated with the topography of the cone mosaic, and contains spatial frequency information up to the diffraction limit of the optical system of the eye. The cone spacing is measured in the spatial domain, as the reciprocal of the radius of the ring present in the average power spectrum. The results obtained are in close agreement with estimates based on microscopy of excised fovea, with psychophysical measurements, and with recent results obtained with another high resolution imaging technique outside the center of the fovea.
A double-pass method is applied to determine the retinal image quality of eyes implanted with intraocular lenses (IOLs). The effect of focus on image quality was measured in two groups of patients that had been implanted with either monofocal or multifocal IOLs. The results show that the overall retinal image quality is reduced in eyes with multifocal lenses with respect to that implanted with monofocal IOLs. Although the depth of focus is larger in multifocal lOLs (4 to 5 D) than in the monofocal IOLs (2 to 3 D), some patients implanted with monofocal IOLs have higher image quality than those implanted with multifocal IOLs in a range of about 4 D around the best focus. In eyes implanted with monofocal IOLs, astigmatism plays a major role to reduce the retinal contrast, but also increases the depth of focus. These "in vivo" measurements show that there is considerable variability in image quality among individuals with the same type of monofocal IOLs. The main factors causing this variability seem to be age and astigmatism produced by surgery.
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