The cornea and the crystalline lens form the image on the retina of the human eye. Tilt and decentration of the components and the angle of the fovea (area of most acute vision) from the optical axis also contribute. The resulting image quality on the retina is influenced by diffraction through the circular pupil of the eye, by the scattering of light, by chromatic aberration and by the monochromatic aberrations of the system. In this presentation, we will concentrate on the monochromatic wavefront aberrations of the eye, their measurement, and their influence on vision and ophthalmic diagnosis.
We recently developed several versions of a multimodal adaptive optics (AO) retinal imager, which includes highresolution
scanning laser ophthalmoscopy (SLO) and Fourier domain optical coherence tomography (FDOCT) imaging
channels as well as an auxiliary wide-field line scanning ophthalmoscope (LSO). Some versions have also been
equipped with a fluorescence channel and a retinal tracker. We describe the performance of three key features of the
multimodal AO system including: simultaneous SLO/OCT imaging, which allows SLO/OCT co-registration; a small
animal imaging port, which adjusts the beam diameter at the pupil from 7.5 to 2.5 mm for use with small animals
ubiquitous in biological research or for extended depth-of-focus imaging in humans; and slow scan Doppler flowmetry
imaging using the wide field auxiliary LSO imaging channel. The systems are currently deployed in several
ophthalmology clinics and research laboratories and several investigations have commenced on patients with a variety
of retinal diseases and animals in vision research.
Adaptive Optics (AO) technology has been used in confocal scanning laser ophthalmoscopes (CSLO) which are
analogous to confocal scanning laser microscopes (CSLM) with advantages of real-time imaging, increased image
contrast, a resistance to image degradation by scattered light, and improved optical sectioning. With AO, the instrumenteye
system can have low enough aberrations for the optical quality to be limited primarily by diffraction. Diffraction-limited,
high resolution imaging would be beneficial in the understanding and early detection of eye diseases such as
diabetic retinopathy. However, to maintain diffraction-limited imaging, sufficient pixel sampling over the field of view
is required, resulting in the need for increased data acquisition rates for larger fields. Imaging over smaller fields may be
a disadvantage with clinical subjects because of fixation instability and the need to examine larger areas of the retina.
Reduction in field size also reduces the amount of light sampled per pixel, increasing photon noise. For these reasons,
we considered an instrument design with a larger field of view. When choosing scanners to be used in an AOCSLO, the
ideal frame rate should be above the flicker fusion rate for the human observer and would also allow user control of
targets projected onto the retina. In our AOCSLO design, we have studied the tradeoffs between field size, frame rate
and factors affecting resolution. We will outline optical approaches to overcome some of these tradeoffs and still allow
detection of the earliest changes in the fundus in diabetic retinopathy.
Ferrofluid mirrors have the potential to be an inexpensive adaptive optical element which can be used to improve images of structures at the rear of the eye. Their low cost could allow adaptive optics technology to find widespread use in clinical settings. As discussed elsewhere1, their stroke and speed are suitable for correcting the aberrations of the human eye. We present work on the static and dynamic responses of these mirrors using a Hartmann-Shack wavefront reconstruction technique. The displacement of the mirror versus the current in the magnetic field actuators has been measured, as well as actuator influence functions (including non-linearities). In addition, the real-time dynamics of the mirror have been characterized.
Photodynamic therapy (PDT) using verteporfin is widely used for treatment of age related macular degeneration (AMD).
Due to non-perfect selectivity of the drug accumulation in the neovasculature some collateral damage to healthy tissue
arises during the treatment. Damage to healthy structures in the eye is always a concern because of a high probability of
reducing visual acuity. Two-photon (2-&ggr;) photodynamic therapy potentially offers much higher treatment selectivity than
its one-photon (1-&ggr;) counterpart. By utilizing focused light for 2-&ggr; excitation, treatment volumes on the order of
microliters can be achieved thus maximizing localized insult to abnormal blood vessels and sparing healthy tissue. We
propose that 2-&ggr; photodynamic therapy will be valuable in the treatment of choroidal neovascularization secondary to
age related macular degeneration as well as other conditions. To ascertain feasibility of 2-&ggr; photodynamic therapy we
measured 2-&ggr; spectrum and cross sections of verteporfin (80 GM at 940 nm, 1 GM = 10-50 cm4s/photon), chlorin e6 (14
GM at 800 nm) and tetrasulfonated aluminum phthalocyanine (140 GM at 900 nm) and investigated their in vitro
efficiency under 2-&ggr; excitation. Only verteporfin demonstrated cell kill under the used irradiation parameters (average
light intensity 9.1 mW, wavelength 850 nm, total light dose 6900 J/cm2). Dorsal skinfold window chamber model in
mouse was used to test efficiency of 2-&ggr; PDT with verteporfin in vivo. Although we were able to induce photodynamic
damage to a blood vessel using 1-&ggr; excitation, 2-&ggr; excitation resulted in no visible damage to irradiated blood vessel. The most probable reason is low efficiency of verteporfin as a 2-&ggr; photosensitizer. We also report 2-&ggr; spectrum of new
photosensitizer, HCC4 (4300 GM at 830 nm), specifically designed for efficient 2-&ggr; excitation.
We wish to deliver Two-Photon Excitation (TPE) to the in vivo retina and to image its effects in order to understand and
treat eye disease. A schematic model of the rat eye with a gradient refractive index in the crystalline lens is reconstructed
in ZEMAXTM. This model predicts the monochromatic aberrations as a function of pupil size and field angle
and the change in the Point Spread Function (PSF) at best focus. A simple water model of the nonlinear pulse
broadening effect has been used to predict the minimal temporal pulse width that will propagate to the retina. In a rat
eye uncorrected for monochromatic aberrations, a pupil between 1mm and 1.8mm diameter delivers a peak intensity
acceptable for two-photon effects. A somewhat larger diameter pupil (1.35-2.0mm) gives an optimum optical quality for
imaging on the optical axis. Correction of the monochromatic aberrations with adaptive optics would improve both
imaging and peak intensity. The effect of second order dispersion is dependent on the form of the dispersion relation
used. Based on experimental results of second order dispersion, the minimum pulse width to reach the retina is
approximately 30fs for the rat eye and approximately 60fs for the human eye.
The University of Waterloo, partnered with key industry players, Photonics Research Ontario and the Ontario government, launched Ontario's first diploma-level photonics program to re-skill scientists and engineers. The Education Program for Photonics Professionals (EP3) offers the basics of a university level Optics education. The next step is to provide the courses at distance.
We previously developed a Mueller matrix formalism to improve confocal imaging in microscopes and ophthalmoscopes. Here we describe a procedure simplified by firstly introducing a generator of polarization states in the illumination pathway of a confocal scanning laser microscope and secondly computing just four elements of the Mueller matrix of any sample and instrument combination. Using a subset of Mueller matrix elements, the best images are reconstructed. The method was tested for samples with differing properties (specular, diffuse and partially depolarizing). Images were also studied of features at the rear of the eye. The best images obtained with this technique were compared to the original images and those obtained from frame averaging. Images corresponding to non-polarized incident light were also computed. For all cases, the best reconstructed images were of better quality than both the original and frame-averaged images. The best reconstructed images also showed an improvement compared with the images corresponding to non polarized light. This methodology will have broad application in biomedical imaging.
Optical aberrations reduce the imaging quality of the human eye. In addition to degrading vision, this limits our ability to illuminate small points of the retina for therapeutic, surgical or diagnostic purposes. When viewing the rear of the eye, aberrations cause structures in the fundus to appear blurred, limiting the resolution of ophthalmoscopes (diagnostic instruments used to image the eye). Adaptive optics, such as deformable mirrors may be used to compensate for aberrations, allowing the eye to work as a diffraction-limited optical element. Unfortunately, this type of correction has not been widely available for ophthalmic applications because of the expense and technical limitations of current deformable mirrors. We present preliminary design and characterisation of a deformable mirror suitable for ophthalmology. In this ferrofluidic mirror, wavefronts are reflected from a fluid whose surface shape is controlled by a magnetic field. Challenges in design are outlined, as are advantages over traditional deformable mirrors.
The University of Waterloo, partnered with key industry players, Photonics Research Ontario and the Ontario government, has launched Ontario’s first diploma-level photonics program to re-skill scientists and engineers. The Education Program for Photonics Professionals (EP3) offers the basics of a university level Optics education, but in a manageable timeframe and format for the working professional.
This paper demonstrates the application of a lensless fiber optic spectrometer (sensor) to study the onset of cataracts. This new miniaturized and rugged fiber optic probe is based upon dynamic light scattering (DLS) principles. It has no moving parts, no apertures, and requires no optical alignment. It is flexible and easy to use. Results are presented for cold-induced cataract in excised bovine eye lenses, and aging effects in excised human eye lenses. The device can be easily incorporated into a slit-lamp apparatus (ophthalmoscope) for complete eye diagnostics.
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