Presbyopia is an age related, gradual loss of accommodation, mainly due to changes in the crystalline lens. As part of research efforts to understand and cure this condition, ex vivo, cross-sectional optical coherence tomography images of crystalline lenses were obtained by using the Ex-Vivo Accommodation Simulator (EVAS II) instrument and analyzed to extract their physical and optical properties. Various filters and edge detection methods were applied to isolate the edge contour. An ellipse is fitted to the lens outline to obtain central reference point for transforming the pixel data into the analysis coordinate system. This allows for the fitting of a high order equation to obtain a mathematical description of the edge contour, which obeys constraints of continuity as well as zero to infinite surface slopes from apex to equator. Geometrical parameters of the lens were determined for the lens images captured at different accommodative states. Various curve fitting functions were developed to mathematically describe the anterior and posterior surfaces of the lens. Their differences were evaluated and their suitability for extracting optical performance of the lens was assessed. The robustness of these algorithms was tested by analyzing the same images repeated times.
A custom-built OCT system was used to obtain images of the whole mouse eye. We developed a semi-automated segmentation method to detect the boundaries of the anterior and posterior corneal, lens and retinal surfaces as well as the anterior surface of the iris. The radii of curvature of the surfaces were calculated using a conic section fit of each boundary. Image distortions due to refraction of the OCT beam at the successive boundaries were corrected
using a ray-tracing algorithm. Corrected ocular distances, radii of curvature of the cornea and lens surfaces, and anterior chamber angle were obtained on 3 C57BL/6J mice. In vivo imaging of the whole eye, segmentation, conic function fits and correction were successful in all three animals. The posterior lens surface of one mouse could not be fit accurately with a conic section. Biometric parameters of C57BL/6J mice compared well with previous published data obtained from histological sections. The study demonstrates the feasibility of quantitative in vivo biometry of mouse models.
Presbyopia is the age related, gradual loss of accommodation, mainly due to changes in the crystalline lens.
As part of research efforts to understand and cure this condition, ex vivo, cross-sectional OCT images of
crystalline lenses were obtained and analyzed to extract their physical and optical properties. The raw OCT
images are distorted, as the probing beam passing through media of different refractive indices and
refraction on curved surfaces. In a first step, various filters, edge detection and pattern matching methods
are applied to isolate the edge contour. An ellipse is fitted to the lens outline to obtain central reference
point for transforming the pixel data into the analysis coordinate system. This allows for the fitting of high
order equation to obtain a mathematical description of the edge contour, which obeys constraints of
continuity as well as zero to infinite surface slopes from apex to equator. Robustness of these algorithms
are tested by analyzing the images at various contrast levels. Gradient refractive index of the lens is
determined and the physical shape is reconstructed. In a further refinement, the refraction on the curved
anterior surface is compensated to obtain the actual shape of the posterior surface. Once the physical shape
is fully reconstructed, the optical properties are determined by fitting conic sections to both surfaces and
calculating the power profile across the lens. The relative contribution of each of these refinement steps is
investigated by comparing their influence on the effective power of the lens.
Purpose: To design a non-contact optical system to measure lens capsule thickness in cadaver eyes. Methods: The optical system uses a 670nm laser beam delivered to a single-mode fiber coupler. The output of the fiber coupler is focused onto the tissue using an aspheric lens (NA=0.68) mounted on a motorized translation stage. Light reflected from the sample is collected by the fiber coupler and sent to a silicon photodiode connected to a power meter. Peaks in the power signal are detected when the focal point of the aspheric lens coincides with the capsule boundaries. The capsule thickness is proportional to the distance between successive peaks. Anterior and posterior lens capsule thickness measurements were performed on 13 human, 10 monkey, and 34 New Zealand white rabbit lenses. The cadaver eyes were prepared for optical measurements by bonding a PMMA ring on the sclera. The posterior pole was sectioned, excess vitreous was removed, and the eye was placed on a Teflon slide. The cornea and iris were then sectioned. After the experiments, the lenses were excised, placed in 10% buffered formalin, and prepared for histology. Results: Central anterior lens capsule thickness was 9.4±2.9μm (human), 11.2±6.6μm (monkey), and 10.3±3.6μm (rabbit) optically and 14.9±1.6μm (human), 17.7±4.9μm (monkey), and 12.6±2.3μm (rabbit) histologically. The values for the central posterior capsule were 9.4±2.9μm (human), 6.6±2.5μm (monkey), and 7.9±2.3μm (rabbit) optically and 4.6±1.4μm (human), 4.5±1.2μm (monkey), and 5.7±1.7μm (rabbit) histologically. Conclusions: This study demonstrates that a non-contact optical system can successfully measure lens capsule thickness in cadaver eyes.
The effects of wavelength on infrared (IR) laser ablation with a free electron laser were studied by analyzing the acoustic signals produced during ablation of gelatin and tissue samples. The acoustic signals resulting from surface ablation of the samples were recorded with a piezoelectric microphone and the acoustic energy contained in the signal was calculated for samples of varying mechanical strength. Gelatin samples of different mechanical strengths were made by varying the water concentration in the gels to 70% and 90% wt./vol. The gels were irradiated at wavelengths of λ = 2.94, 2.80, and 6.45 μm with the measured acoustic energy normalized to the incident laser pulse energy. The results showed that while there was a statistically significant difference in the average acoustic energy measured for both concentrations of gelatin at λ = 2.94 and 2.80 μm, there was no difference in the average acoustic energy for the two concentrations of gelatin at λ = 6.45 micrometers . This supports the model of mechanical weakening of the sample by breaking the amide II molecular bonds in proteins, originally proposed by Edwards et al.
The Vanderbilt University free-electron laser (FEL) provides a continuously tunable ((lambda) equals 2 - 10 micrometer) source of pulsed IR radiation with a pulse structure unlike those of conventional lasers (a macropulse of 5 microseconds consisting of a train of 1 ps micropulses at a frequency of 3 GHz). A numerical hydrodynamic code at Lawrence Livermore National Laboratory, known as LASTIS3D, was used to model the ablation of tissue using the FEL. This study investigates the role of the FEL pulse structure by comparing the results from simulations using a time-averaged energy deposition and a pulsetrain energy deposition.
The Vanderbilt free-electron laser provides a continuously tunable ((lambda) equals 2 - 10 micrometer) source of pulsed IR radiation with a pulse structure unlike those of conventional lasers (a macropulse of 4.5 ms consisting of a micropulse train of 1 ps pulses separated by 350 ps). Tuned to the vibrational mode of the amide-II band ((lambda) equals 6.45 micrometer), the laser is well suited for precise tissue ablation with reported minimal thermal and collateral tissue damage. However, the relative influences of the pulse structure and wavelength on tissue ablation is still not clear. The effects of different wavelengths ((lambda) equals 2.94, 3.36, and 6.45 micrometer) on tissue ablation were compared using pump-probe imaging of tissue phantoms while simultaneously laser-induced pressures were measured with a piezoelectric needle hydrophone. Bovine heart tissue was ablated in vitro using clinically relevant parameters and laser induced damage was examined histologically. The results of these experiments, and their implications will be discussed.
The design of a handheld hollow waveguide-based probe is presented to deliver the unique output of the Vanderbilt Free Electron Laser (FEL) for medical applications. The neurosurgical hand piece incorporates a hollow waveguide, a focusing microlens, an optical fiber to deliver an aiming beam ((lambda) equals 632 nm) and a channel for purging the probe with nitrogen. The hollow waveguide is preferentially used to transmit a wavelength of lambda equals 6.45 micrometer (vibration mode of the amide-II band) and is able to tolerate the high peak intensity (greater than 1014 W/m2) of the picosecond micropulses of the FEL. The calcium fluoride lens focuses the beam to a spot diameter of 400 micrometer at a working distance of 20 mm. To maximize the transmission of the probe, the hollow waveguide is purged with nitrogen to prevent atmospheric absorption at 6.45 micrometer. Temporal broadening of the micro pulses that propagate in the hollow wave guide was studied using intensity autocorrelation measurements and beam profile measurements with a pyroelectric camera. Design consequences and application of the probe for FEL neurosurgery are discussed.
Purpose: The goal of the study is to measure the group refractive index of the human cornea in vitro to improve the accuracy of corneal thickness measurements. Methods: Corneal buttons were trephined from 23 human cadaver eyes and the group refractive index of the cornea was measured at lambda equals 840 nm using a low-coherence Michelson interferometer and the technique proposed by Sorin and Gray (Phot. Tech. Lett. 4:105 - 107, 1992). The effect of dehydration on the measurement was studied by measuring the corneal optical thickness as a function of time. Results: Preliminary measurements of the group refractive index at 840 nm gave ng equals 1.450 plus or minus 0.024 for the human cornea, which is much higher than a calculated group refractive index of ng equals 1.387. Because of dehydration, the optical thickness of the cornea decreased at a rate of 5.5 micrometer/minute which led to an artificially high value for the group refractive index. Conclusion: The calculated group refractive index of ng equals 1.387 appears to be an accurate value for the purpose of corneal thickness measurements using low-coherence interferometry, and corneal group refractive index measurements can be performed in vitro if the measurements are performed rapidly to avoid the effect of dehydration.
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