Photorefractive Keratectomy (PRK) is a widely used laser-assisted refractive surgical technique. While generally safe, in some cases it leads to subepithelial inflammation or fibrosis. We here present a robust, machine learning based algorithm for the detection of fibrosis based on Spectral Domain Optical Coherence Tomography (SD-OCT) images recorded in vivo on standard clinical devices. The images first undergo a treatment by a previously developed algorithm for standardisation. The analysis of the pre-treated images allow the extraction of quantitative parameters characterizing the transparency of human corneas. We here propose an extension of this work. Our model is based on 9 morphological quantifiers of the corneal epithelium and in particular of Bowman's layer. In a first step it is trained on SD-OCT images of corneas presenting Fuchs dystrophy, which causes similar symptoms of fibrosis. We trained a Random Forest model for the classification of corneas into "healthy" and "pathological" classes resulting in a classification accuracy (or success rate) of 97%. The transfer of this same model to images from patients who have undergone Photorefractive Keratectomy (PRK) surgery shows that the model output for probability of healthy classification provides a quantified indicator of corneal healing in the post-operative follow-up. The sensitivity of this probability was studied using repeatability data. We could therefore demonstrate the ability of artificial intelligence to detect sub-epithelial scars identified by clinicians as the origin of post-operative visual haze.
We describe an automated algorithm allowing extraction of quantitative corneal transparency parameters with clinical Spectral-Domain Optical Coherence Tomography (SD-OCT). Our algorithm employs a novel pre-processing procedure to standardize SD-OCT image analysis and to numerically correct common instrumental artifacts before extracting mean intensity stromal-depth (z) profiles over a 6-mm-wide corneal area. The z-profiles are analyzed using our previously developed objective method deriving quantitative transparency parameters which are directly related to the physics of light propagation in tissues. Tissular heterogeneity is quantified by the Birge ratio, Br; for homogeneous tissues (i.e., Br~1), the photon mean-free path (ls) may be determined. Images of 83 normal corneas (ages 22–50 years) from a standard SD-OCT device (RTVue-XR Avanti, Optovue Inc.) were processed to establish a normative dataset of transparency values. After confirming stromal homogeneity (Br⪅10), we measured a median ls of 570 μm (interdecile range: 270–2400 μm). Considering corneal thicknesses, this may be translated into a median fraction of transmitted (coherent) light Tcoh(stroma) of 51% (interdecile range: 22–83%). Excluding images with central saturation artifact raised our median Tcoh(stroma) to 73% (inter-decile range: 34–84%). These transparency values are slightly lower than previously reported, which we attribute to the detection configuration of SD-OCT with a relatively small and selective acceptance angle. No statistically significant correlation between transparency and age or thickness was found. Our algorithm provides robust and quantitative measurements of corneal transparency from standard SD-OCT images with sufficient quality and addresses the demand for such an objective means in the clinical setting.
KEYWORDS: Transparency, Cornea, Optical coherence tomography, Image segmentation, Principal component analysis, Algorithm development, In vivo imaging, Statistical analysis, Signal to noise ratio, Signal processing
We present an automated data analysis procedure for clinical SD-OCT images, capable of correcting hyperreflective artifacts due to the instrument. Quantitative parameters related to corneal transparency are extracted from n=85 normal corneas.
Lack of corneal transparency is a major cause of blindness worldwide. However, means to assess corneal transparency are limited and in current clinical and eye-bank practice usually involve a subjective and qualitative observation of opacities, sometimes with comparison against an arbitrary grading scale, by means of slit-lamp biomicroscopy. To address this unmet need, we have developed a method for corneal transparency assessment based on a new optical data analysis-based approach. Our method allows the objective extraction of quantitative parameters (including the scattering mean-free path, ls, a major indicator of scattering extent and thus of transparency of a medium) based on a physical model of corneal transparency and has been validated by laboratory experiments, using high-resolution, ex-vivo “fullfield” optical coherence tomography (FF-OCT). Here, we apply our algorithm to depth-resolved spectral domain OCT (SD-OCT) images of in-vivo corneas and demonstrate the feasibility of our approach by means of four representative clinical cases. Specifically, we illustrate its potential in discriminating between the four clinical cases and, if applicable, deriving the scattering mean-free path as a quantitative measure of corneal transparency from objective analysis of stromal light backscattering (attenuation of the coherent mean) with SD-OCT. This measure may be related to, or expressed as, Strehl ratio reduction and thus retinal PSF broadening. As such, our approach not only has the potential to supply the demand for an objective means to quantify corneal transparency in the clinical setting, but also to create an association with visual function.
We have determined the threshold radiant exposure for cell death in the endothelium of porcine cornea exposed to ultrashort laser pulses in the context of keratoplasty and the preparation of endothelial grafts. In this study, by progressively increasing the radiant threshold towards the higher values we have observed a decrease of living corneal endothelial cells. Further study will address the effect of dose and possible mechanism behind cell death.
Glaucoma is a disease of the optic nerve that is usually associated with an increased internal pressure of the eye and can
lead to a decreased vision and eventually blindness. It is the second leading cause of blindness worldwide with more than
80 million people affected and approximately 6 million blind. The standard clinical treatment for glaucoma, after
unsuccessful administration of eyedrops and other treatments, is performing incisional surgery. However, due to post-surgical
complications like scarring and wound healing, this conventional method has a global success rate of only about
60%. In comparison, as femtosecond laser surgery may be performed in volume and is a priori less invasive and less
susceptible of causing scarring, glaucoma laser surgery could be a novel technique to supplement the conventional
glaucoma surgery. We have been working on the development of a new tool for glaucoma treatment that uses an
optimized femtosecond laser source centered at 1.65 μm wavelength for making the surgery and an imaging system
based on optical coherence tomography (OCT) for guiding the laser surgery. In this proceeding, we present the results
obtained so far on the development and utilization of Fourier-domain OCT imaging system working at 1.3 μm center
wavelength for guiding the laser incision. Cross-sectional OCT image of pathological human cornea showing the
Schlemm's canal, where the surgery is intended to be done, is presented. By coupling OCT imaging system with the
laser incision system, we also demonstrate real-time imaging of femtosecond laser incision of cornea.
The optical properties of the cornea have been a research subject of great interest for many years. Several
early theories have been put forward to explain with more or less success the optical transparency of this
tissue, but it was not until Maurice demonstrated in a very elegant way during the 50s that this optical
transparency could be explained by the regular ultrastructure of the cornea. When becoming edematous, the
cornea's ultrastructure is perturbed and the tissue becomes a strongly scattering medium.
With the emergence of ophthalmologic surgery by ultrashort pulse lasers in recent years, a regain of interest in
the subject of corneal transparency arose. However, relatively little and no recent data of transparency spectra
measurements covering a large wavelength range is available in the literature. The purpose of this study is to
provide quantitative values for light scattering and its relation to the degree of edema by measuring the
spectrum of transmitted light through corneas presenting different degrees of edema. This paper focus on the
comparison of laboratory measurements published earlier with a new simple method we propose We also for
eye banks to quantitatively measure the degree of transparency of corneal grafts by measuring the modulation
transfer function of a Siemens star viewed through a corneal graft. Indeed, there is no current method to
determine the transparency of corneal graft but the subjectivity of the laboratory technician or the ophthalmic
surgeon.
Femtosecond laser surgery in the volume of corneal tissue is typically performed wavelengths of about 1 μm,
which gives excellent results on transparent corneas. However, the outcome is much worse in the case of
oedematous or pathological corneas as the laser beam propagation is disturbed by optical scattering. Our studies
suggest that this phenomenon can be greatly reduced by using a better suited laser wavelength. Best results are
obtained at 1.65 μm.
Currently, no compact femtosecond laser at this wavelength is commercially available. We have developed a
new simple, compact and stable laser source consisting of a non linear crystal pumped by a compact commercial
solid-state laser emitting at 1.03 μm in a configuration of an Optical Parametric Generation (OPG). The output
wavelength of this system can be tuned in the spectral range of 1.45 - 1.8 μm. A series of ex vivo penetrating
incisions using energies of the order of a few microjoules on corneal tissues have been performed while varying
the wavelengths from 1.45 μm to 1.7 μm. The results have been compared to experiments performed at 0.8 μm
and 1 μm. The use of longer infrared wavelengths around 1.65 μm for femtosecond laser keratoplasty
significantly improves the quality and the penetration depth of incision in case of pathological tissues, without
inducing any additional side effects.
The use of ultrashort pulse lasers is current in refractive surgery and has recently been extended to corneal grafting (keratoplasty). When performing keratoplasty, however, permanent degradation of the optical properties of the patient's cornea compromises the penetration depth of the laser and the quality of the incisions, therefore causing unwanted secondary effects. Additionally, corneal grafting needs considerably higher penetration depths than refractive surgery. Little data are available about the interaction processes of the femtosecond pulses in the volume of pathological corneas-i.e., in the presence of spherical aberrations and optical scattering. We investigate the influence of the focusing numerical aperture on the laser-tissue interaction. We point out that at low numerical apertures (NAs), tissue damage is produced below and above the focal region. We attribute this phenomenon to nonlinear self-focusing effects. On the other hand, at high NAs, spherical aberrations become significant when focusing at high depths for posterior surgeries, which also limit the cutting efficiency. As high NAs are advisable for reducing unwanted nonlinear effects and ensure accurate cutting, particular attention should be paid to aberration management when developing clinical femtosecond lasers.
The specular transmittance spectrum of human corneas is studied using a confocal geometry set-up. The comparison of the obtained spectrum with the total transmittance spectrum permits the determination of the stromal scattering spectrum of pathological corneas. The dependence of the scattering cross section on
wavelength dependence is analyzed.
Femtosecond laser surgery in the volume of corneal tissue is difficult in the case of oedematous or pathological corneas: in those corneas, the propagation of the laser beam is perturbed by the optical scattering. This phenomenon can be greatly reduced by using a better suited laser wavelength.
A series of ex vivo surgical experiments has been conducted at wavelengths around 1600 nm. The results have been compared to experiments performed at 800 nm and 1000 nm. We have compared penetration depth and incision quality as a function of wavelength and energy.
Nowadays, femtosecond lasers are routinely used in refractive eye surgery. Until recently, commercialised clinical systems
were exclusively based on ytterbium or neodymium-doped solid state lasers emitting sub-picosecond pulses at a
wavelength of about 1 μm and repetition rates of a few 10 kHz. These systems use pulse energies in the μJ range and
focussing optics of NA = 0.3 to 0.5. Recent developments have provided a variety of alternative and equally viable approaches:
systems are now available using nJ pulses at high numerical apertures and MHz repetition rates - an approach
so far only used for femtosecond cell surgery - and fibre laser technology is now being used for femtosecond laser corneal
surgery.
Recent research has also provided more insight in side effects occurring in present systems: self focusing phenomena and
so far unexplained periodical structures have been observed even at high numerical apertures (NA >> 0.5) and moderate
pulse energies. The interaction of femtosecond laser pulses with strongly scattering tissue has been studied in view of
extending the application of femtosecond lasers to keratoplasty for opaque corneas and to glaucoma surgery. The use of
new laser wavelengths and adaptive optics has been proposed.
Despite the reputation of femtosecond surgical systems for their precision, repeatability and the absence of secondary
effects or complications, a closer examination reveals the presence of subtle phenomena which merit further investigation.
We present three of these phenomena: the influence of optical aberration on the quality of the incision, the occurrence
of filamentation effects, and the deposit of microscopic glass fragments when performing penetrating incisions.
The application of femtosecond lasers in corneal transplant surgery requires high pulse energies to compensate for the strong optical scattering in pathological corneas. However, excessive energies deteriorate the quality of the incisions. The aim of this study is to demonstrate the dependence of side effects on local radiant exposure, numerical aperture, and tissue properties, to quantify the penetration depth of the laser for individual corneas, and to provide a method for optimizing the energy in the volume of the cornea. We examine histological and ultrastructural sections of clear and edematous corneas with perforating and lamellar incisions performed at different pulse energies. We demonstrate that the augmented energies in edematous corneas may result in unwanted side effects even when using high numerical apertures. The dependence of the laser beam penetration depth on pulse energy is evaluated by histology and an exponential decrease is observed. We show that the penetration length can be determined by evaluating the backscattered second-harmonic emission associated with the nonlinear optical properties of the tissue. This approach represents a noninvasive method for the in situ quantification of the laser beam attenuation, enabling us to adapt the pulse energy accordingly. Experiments using adapted energies show that the side effects are minimized.
Femtosecond lasers start to be routinely used in refractive eye surgery. Current research focuses on their application to
glaucoma and cataract surgery as well as cornea transplant procedures. To avoid unwanted tissue damage during the
surgical intervention it is of utmost importance to maintain a working energy just above the ablation threshold and
maintain the laser energy at this working point independently of the local and global tissue properties. To quantify the
attenuation of the laser power density in the tissue by absorption, scattering and modification of the point spread function
we monitor the second harmonic radiation generated in the collagen matrix of the cornea when exposed to ultrashort
laser pulses. We use a CPA system with a regenerative amplifier delivering pulses at a wavelength of 1.06 &mgr;m,
pulse durations of 400 fs and a maximum energy of 60 &mgr;J. The repetition rate is adjustable from single shot up to 10
kHz. The experiments are performed on human corneas provided by the French Eye bank. To capture the SHG radiation
we use a photomultiplier tube connected to a lockin amplifier tuned to the laser repetition rate. The measured data indicates
an exponential decay of the laser beam intensity in the volume of the sample and allows for the quantification of
the attenuation coefficient and its correlation with the optical properties of the cornea. Complementary analyses were
performed on the samples by ultrastructural histology.
We present a multiphoton imaging system mounted on a microsurgery experimental set-up using a Nd:glass femtosecond laser. The system permits to induce laser incisions in human cornea and sclera and to perform nonlinear imaging during the intervention.
The laser is a chirped pulse amplification (CPA) system with a regenerative amplifier delivering pulses at a wavelength of 1.06 μm, pulse durations of 400 fs and a maximum energy of 60 μJ at repetition rates up to 10 kHz. The delivery system provides spot sizes down to the micron range. The samples are human corneas retracted from the transplant circuit mounted on a moveable anterior chamber system. Photons generated by non-linear processes in the cornea travel backwards through the beam delivery optics and are captured by a photomultiplier tube behind a dichroic mirror. The signal is filtered by a lock-in amplifier tuned to the laser repetition rate. Scanning the sample permits the acquisition of three-dimensional microscopic images.
Above the incision threshold the set-up permits to induce laser cuts in human cornea following complex geometries. Below the threshold the laser pulses create secondary photons by the stimulation of non-linear optical processes in the samples which could be identified as being predominantly second harmonic generation (SHG). The in situ images obtained from the multi-photon module permit to control and optimise the surgical intervention.
The combination of multiphoton imaging and corneal surgery necessitates only minimal modifications of the optical system of a femtosecond surgical laser system. A combined system significantly improves parameter control and permits the monitoring of the surgical procedure.
We present a multiphoton microscope using a Cr4+:forsterite femtosecond laser with an emission wavelength of 1260 nm for the excitation of the multiphoton processes. This wavelength is well adapted to the "optical window" in biological tissues and permits to reach higher imaging depths than systems using more conventional titanium:sapphire laser sources. The paper describes the experimental set-up and reports on first results on human cornea and skin samples.
Diode pumped fiber lasers are compact reliable light sources and achieve today over 30 W fundamental mode power. Hence, they are attractive for the realization of portable laser Doppler anemometer (LDA) systems with high optical power. We present the application of fiber lasers together with diffraction optics for the realization of a miniaturized achromatic directional LDA system. Different methods for the realization of the directional discrimination of the scattering particle movement were investigated and compared. (1) The use of Bragg cells as frequency shifter and diffractive beam splitter for the realization of a heterodyne technique. (2) The application of a two- wavelength fiber laser and Fourier optical phase shifters for the realization of a homodyne technique. (3) The detection of the position of the scattering particle in the measuring volume by dual-fiber receiving techniques.
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