[In vivo imaging of the corneal nerve plexus : From single image to large scale map]

The sub-basal nerve plexus (SNP) of the cornea provides the possibility of in vivo and non-invasive examination of peripheral nerve structures by corneal confocal microscopy (CCM). Thus morphological alterations of the SNP can be directly detected and quantified. A single CCM image is insufficient for a well-founded diagnosis because of the inhomogeneous distribution of the nerve fibers; therefore, there is a demand for techniques for large area imaging of the SNP. This article provides an overview of published approaches to the problem. Current developmental work at the Karlsruhe Institute of Technology and the University of Rostock Eye Clinic is expected to lead to a simplified handling of the technology and a further improvement in the image quality.

[Morphometric characterization of the subbasal nerve plexus : Detection and analysis of networks of nerve fibers]

Confocal laser scanning microscopy is a versatile tool in medical research and enables noninvasive in vivo imaging of the corneal subbasal nerve plexus. The aim of this work is to provide a structured overview about the detection and quantification of nerve fibers of the subbasal nerve plexus from images acquired by confocal laser scanning microscopy. Relevant steps are explained and potential factors influencing the quality of the results are pointed out. Information obtained from the quantification of subbasal nerve fiber structure can be potentially used as clinical parameters in the context of diagnostics and therapy control of diabetic neuropathy.

[Magnetic Resonance Microscopy of the Accommodative Apparatus]

Magnetic resonance microscopy (MRM) at ultra-high magnetic fields allows acquisition of high resolution MR images in the micrometre range. The use of ultra-high magnetic fields opens the possibility of user-independent and artefact-free detailed characterisation of the anatomical tissue of the human eye, which is not achievable with classical imaging techniques. This article correlates MRM of the anterior eye segment and the accommodative apparatus at 9.4 Tesla with conventional histology.

[Finite Element Modelling of the Eye for the Investigation of Accommodation]

Background: Accommodation research increasingly uses engineering methods. This article presents the use of the finite element method in accommodation research. Material and Methods: Geometry, material data and boundary conditions are prerequisites for the application of the finite element method. Published data on geometry and materials are reviewed. It is shown how boundary conditions are important and how they influence the results. Results: Two dimensional and three dimensional models of the anterior chamber of the eye are presented. With simple two dimensional models, it is shown that realistic results for the accommodation amplitude can always be achieved. More complex three dimensional models of the accommodation mechanism - including the ciliary muscle - require further investigations of the material data and of the morphology of the ciliary muscle, if they are to achieve realistic results for accommodation. Discussion and Conclusion: The efficiency and the limitations of the finite element method are especially clear for accommodation. Application of the method requires extensive preparation, including acquisition of geometric and material data and experimental validation. However, a validated model can be used as a basis for parametric studies, by systematically varying material data and geometric dimensions. This allows systematic investigation of how essential input parameters influence the results.

Quantitative assessment of central and limbal epithelium after long-term wear of soft contact lenses and in patients with dry eyes: a pilot study

PurposeAnalysis of microstructural alterations of corneal and limbal epithelial cells in healthy human corneas and in other ocular conditions.Patients and methodsUnilateral eyes of three groups of subjects include healthy volunteers (G1, n=5), contact lens wearers (G2, n=5), and patients with dry eyes (G3, n=5) were studied. Imaging of basal (BC) and intermediate (IC) epithelial cells from central cornea (CC), corneal limbus (CL) and scleral limbus (SL) was obtained by in vivo confocal microscopy (IVCM). An appropriate image analysis algorithm was used to quantify morphometric parameters including mean cell area, compactness, solidity, major and minor diameter, and maximum boundary distance.ResultsThe morphometric parameters of BC and IC demonstrated no significant differences (P>0.05) between groups. Comparison between three corneal locations (CC, CL, and SL) within the groups showed significant differences (P<0.05) with mean values of cell area, compactness, solidity, and major and minor diameter of BC that increase from CC to limbus. The BC were round and regular in the central cornea (P<0.05) compared with CL and SL.ConclusionsIVCM enables high-quality confocal images from central corneal and limbal epithelium. This quantitative study demonstrated morphological differences in the basal and intermediate epithelium between limbus and central cornea, and found no differences between contact lens wearers, dry eyes, and normal subjects.

[Diffusion Weighted Magnetic Resonance Imaging and its Application in Ophthalmology]

The value of diffusion-weighted magnet resonance imaging (DWI-MRI) has been demonstrated for an ever growing range of clinical indications. DWI is sensitive to the diffusion of water molecules and probes their random displacement within tissue. DWI provides both qualitative and quantitative information on tissue characteristics, e.g. tissue cellularity. This review provides an overview of diffusion-weighted imaging and its emerging applications in ophthalmology. The basic physics and technical foundations of DWI are introduced. The emerging applications of DWI are surveyed, particularly in diseases of the eye, orbit and optical nerve.

[Implant Design by Means of Multiphoton Polymerization]

BACKGROUND

Additive manufacturing and 3D printing create new paths for the design and manufacturing of implants. Technologies with high resolution are required for the development of microstructured eye implants. In the present study, we demonstrate how these technologies can be used during the design development and manufacturing of a multifocal diffractive aspheric intraocular lens.

MATERIAL AND METHODS

Multiphoton polymerisation (MPP) is used to manufacture a diffractive relief with resolution in the sub-micrometer range. The relief is applied to the moulded body of a refractive lens, forming a trifocal lens. Propagation of light behind the lens is visualised in water with fluorescein.

RESULTS

Multifocal lenses were successfully manufactured with this approach. The optical design with three foci is confirmed by the light propagation images. The images even clearly demonstrate the impact of the refractive and diffractive elements and may provide information on artefacts and aberrations.

CONCLUSIONS

Multiphoton polymerisation is an interesting tool for the flexible manufacturing of complex multifocal lenses. With future technological progress in 3D printing with MPP, this is a promising method for on-demand manufacturing of patient individual intraocular lenses.

Adopting oculopressure tonometry as a transient in vivo rabbit glaucoma model

Glaucoma represents a group of eye disorders partly related to raised intraocular pressure (IOP) leading to progressive optic nerve damage resulting in impaired vision and possibly blindness. To assess the suitability of new IOP lowering therapeutic strategies, such as the implantation of glaucoma drainage devices, appropriate animal models have to be used. Currently, a number of rodent glaucoma models are available [1], however, especially for surgical interventions rodent eyes are too small. Rabbits are much more suitable with respect to dimension. Unfortunately, rabbit glaucoma model systems described in literature are difficult to reproduce or fail totally, associated with a high level of discomfort and pain for treated animals. Therefore, development of an in vivo rabbit glaucoma model is one of the most important goals in glaucoma research. Here, we describe the adaptation of the oculopressure tonometry, an existing method to quantify the outflow of aqueous humor in humans, to generate a transient glaucoma model in rabbits. The existing suction-cup oculopressor (SCOP) is extended with newly designed suction-cups, which are adjusted to the anatomy of the rabbit eye. The modification of the oculopressure tonometry method facilitates an increase in IOP over a time frame of 9 minutes by vacuum induced deformation of the rabbit eye. This method can be used to test functionality of fistulating glaucoma surgeries or implanted drainage devices in a long term follow-up without any side effects and suffering of the animals.

[Visual analysis for an early detection of diabetic neuropathy]

Diabetic neuropathy is the most common long-term complication of diabetes mellitus. It comes along with significant nerve dysfunction, which is not reversible. Hence, it is essential to detect nerve fibre abnormalities as early as possible. In this paper, we investigate markers describing degradation of corneal nerves. We apply statistical computations and visual analysis to identify those variables of two clinical studies that separate DN patients from a control group. In this way, the diagnosis of DN patients is supported. The visual analysis is based on different representations visualizing both the statistical results and the gathered multi-variate data. The user can interactively manipulate the views, or select data that will be shown by further displays. In this way, the understanding of the data and its classification is supported. Ambiguous categorisations can be identified and grouped into a so-called "fuzzy group". For this group, further investigations are needed to decide about diabetic neuropathy.

[Optimisation of the visualisation technique for optical paths through intraocular lenses for characterisation of multifocal imaging properties of Fresnel-zone plates]

The utilisation of the diffractive properties of Fresnel zone plates offers the possibility of intraocular lens designs with multiple foci. Such intraocular lenses can be manufactured by two-photon polymerisation (2PP). This paper explains the underlying concept and shows the principles for visualisation of the focus properties of such implants.

[Current progress of the artificial accommodation system]

In case of presbyopia or cataract the "artificial accommodation system" represents one future possibility to durably restore the ability to accommodate. The work presented describes recent progress in the development of the artificial accommodation system. Major advances were achieved in the fields of the actuator system for the active optics, the pupil near reflex sensor, the communication system, the power supply system as well as in system integration. Beside the technical advances, first trials were performed to implant the artificial accommodation system into animals. These trials showed that the new lens shaped design and the C-shaped haptics are beneficial for implantation and secure fixation of the implant inside the capsular bag.

[Effect of pseudophacic mini-monovision as an option for independence of spectacles in everyday life]

BACKGROUND

Modern day patients who undergo bilateral cataract surgery expect to be spectacles-independent after surgery. The options that are available to attain this goal are pseudophakic monovision, implanting multifocal IOLs or alternatively accommodative IOLs. The aim of this study is to establish the suitability in daily life when attaining minor monovision, named hereafter mini-monovision.

MATERIAL/METHODS

In this clinical retrospective study 30 patients were examined. These subjects where bilaterally pseudophake and received either AcrySof®-IOL or Acreos® Adapt-IOL with a slight anisometropia of between 0.5 and 1.75 D and with a maximal astigmatism up to 1.5 D. Visual acuity (VA) for distance (ETDRS), intermediate (Jaeger 80 cm) and near (Jaeger 40 cm) as well as defocus curves were assessed. Furthermore were examined: the reading speed using the font size of the Radner reading chart, anisoconia (FA Oculus according to Esser), stereopsis (Titmus), subjective patient satisfaction (adapted VF-14-questionnaire) and the subjective dependence on having to wear spectacles.

RESULTS

The median anisometropia was at 0.75 D; the age was between 61 and 80 (median being at 73 years). 12 patients were male and 18 female. When examined with binocular vision 100 % could attain distance VA of at least decimal 1.0 (LogMAR 0) and an intermediate VA of at least decimal 0.8 (LogMAR 0.1), up to 86.57 % a near VA of at least Jaeger 5, (LogMAR 0.4); up to 63.33 % even managed at least Jaeger 3 (LogMAR 0.3). The median average reading speed for binocular uncorrected reading under photopic conditions was 145 words/minute and under mesopic conditions 117 words/minute. The critical font size was LogRAD 0.60 (Jaeger 5-6), the anisoconia at 2 % and stereopsis at 80 arc seconds. The general dependence on glasses was reduced from 100 % preoperatively to 13 %, postoperatively. Simultaneously a high patient satisfaction score of 93.18 in the VF-14 questionnaire was attained. The result is comparable to those quoted in the literature on full monovision and mulitifocal IOLs.

CONCLUSION

To attain spectacles independency, pseudophake mini-monovision is a cost efficient alternative to multifocal lenses for well selected patients. A relative reduction in near VA must be taken into account. To establish the refractive target further examinations are required. Establishing the accurate depth of vision will be helpful in attaining this.

[In vivo MR microscopy of the human eye]

MR microscopy using an ultra high-field MR system is a novel non-invasive imaging technique to explore the human eye without optical distortions. This review aims to provide an insight into the technique. Normal MR microscopic anatomy of the human eye in vivo is demonstrated and clinical applications of MR microscopy are discussed.

[Comparison of dysphotopsia effects in phakic and pseudophakic eyes using Rostock glare perimeter]

BACKGROUND

Pseudophakic dysphotopsia as unwanted side effect after cataract surgery are becoming increasingly important. The so-called glare perimetry allows a realistic quantification of these phenomena. The article presents the method on the example of healthy subjects and pseudophakic patients.

PATIENTS AND METHODS

Using glare perimetry phakic and pseudophakic subjects were examined for differences in disability glare. For this, data from 60 phakic persons of different ages (45 ± 17.1 years) were used. As pseudophakic subjects 31 carriers of monofocal lenses (70 ± 6.7 years) and 25 carriers of multifocal lenses (71 ± 8.5 years) were tested.

RESULTS

Disability glare was significantly smaller in the phakic group (1.00 ± 0.336°) than in the pseudophakic group (1.56 ± 0.622°). Among the pseudophakic eyes those with a multifocal lens (1.69 ± 0.367°) were significantly more sensitive to glare than those with a monofocal lens (1.43 ± 0.492°).

CONCLUSION

Glare perimetry allows an objective quantification of effects of dysphotopsia under realistic conditions. Pseudophakic eyes show a higher sensitivity to glare than eyes with the natural clear lens. Here, eyes with multifocal lenses prove to be particularly sensitive to glare.

[Characterisation of tear film dynamics after application of trehalose for treatment of dry eye]

The implementation of additional modalities for tear film break-up time characterisation expands the application range of the Oculus® Keratograph 5M. The aim of the present study was to evaluate the possibilities for non-invasive break-up time analysis using this device. Furthermore we applied the Oculus® Keratograph 5M to characterise possible modifications of tear break-up time after application of Thealoz® eye drops (Théa Pharma). The device allowed for a precise and solid topographical analysis of tear film dynamics. We could show that at four weeks after treatment, trehalose solution was a better treatment for dry eye compared with saline. These results are in agreement with our previous in vitro findings concerning the protective role of trehalose on desiccation-caused cell death in a corneal epithelial model.

[Pathological conditions of the ocular surface -- a clinical and confocal laser-scanning microscopy study]

Confocal in vivo laser scanning microscopy is an established technique to visualise morphology of the cornea and conjunctiva, whereby the image interpretation needs experience. We report about changes of the ocular surface in the pathological conditions of infectious, metabolic and traumatic genesis and discuss their relevance. The micromorphology of the corneal epithelium and stroma in respect to pathogens (bacterial, fungal) is discussed. Metabolic disease induces multifaceted corneal alterations which can be visualised and used for assessment of the disease progression. Follow-up microscopic investigations allow for an assessment of the wound healing dynamics and enable a prognosis to be made for corneal recurrence. Taken together, confocal in vivo microscopy allows a non-invasive microscopy on the cellular level and thus complements clinical diagnostics.

[In vivo imaging of limbal epithelium and palisades of Vogt]

Limbal disorders can lead to loss of stem cell population and deficiency in corneal integrity. The aim of this study is to describe the morphological features of this region by using confocal laser scanning microscopy (CLSM) and spectral domain optical coherence tomography (SD-OCT) in healthy subjects, contact lens wearers and in patients with surface disorders. On examination hyper- and hyporeflective finger-like structures were found with numerous basal epithelial cells within and in between crypts. In contrast, detailed imaging of limbal palisades of Vogt was not possible on SD-OCT, the corresponding hyperreflective line was identified. These typical crypts were less pronounced or absent in cases with corneal alterations as has been demonstrated. Additionally morphological changes at the level of intermediate and basal cell layers were discussed. A better understanding of the corneal limbus can provide valuable clues on understanding cell integrity and wound-healing processes. In future, quantitative analyses of limbal epithelium in different corneal disorders will be preferable.

Ex vivo measurement of postmortem tissue changes in the crystalline lens by Brillouin spectroscopy and confocal reflectance microscopy

Use of Brillouin spectroscopy in ophthalmology enables noninvasive, spatially resolved determination of the rheological properties of crystalline lens tissue. Furthermore, the Brillouin shift correlates with the protein concentration inside the lens. In vitro measurements on extracted porcine lenses demonstrate that results obtained with Brillouin spectroscopy depend strongly on time after death. The intensity of the Brillouin signal decreases significantly as early as 5 h postmortem. Moreover, the fluctuation of the Brillouin frequency shift inside the lens increases with postmortem time. Images of lens tissue taken with a confocal reflectance microscope between measurements reveal a degenerative aging process. These tissue changes correlate with our results from Brillouin spectroscopy. It is concluded that only in vivo measurements appropriately reflect the rheological properties of the eye lens and its protein concentration.

[Diagnosis of conjunctival neoplastic lesions by confocal in-vivo microscopy]

BACKGROUND

There is a wide spectrum of benign and malignant conjunctival neoplastic lesions that are often impossible to distinguish clinically by slit-lamp microscopy. The current study was undertaken to compare in-vivo confocal laser scanning microscopy (CLSM) and histology for the preoperative assessment of benign or malignant status.

CASE REPORTS

We present the clinical details of three patients. In two cases the neoplastic lesions were classified as benign (actinic keratosis). In-vivo CLSM revealed densely layered, sometimes hyperreflective conjunctival epithelial cells, together with multiple inflammatory cells and microcysts. Correlated findings on histology showed keratinisation with inflammatory infiltrates and intracellular oedema formation. In-vivo CLSM images in the third patient revealed interruptions of the layered epithelial structure with regular conjunctival epithelium co-existing with complexes of enlarged cells with polymorphic nuclei. Histology also showed an abrupt transition from regular squamous epithelium to hyperplastic, dysplastic squamous epithelium. In this case the neoplastic lesion was classified as carcinoma in situ.

DISCUSSION

The in-vivo CLSM images correlated positively with histology findings. Although in-vivo CLSM offers the capability to perform non-invasive examinations over time, associated histological assessment (because of its more precise detail and additional staining techniques) remains indispensable for planning further action and determining the prognosis.

[Non-invasive, spatially resolved determination of tissue properties of the crystalline lens with regard to rheology, refractive index, density and protein concentration by using Brillouin spectroscopy]

The confocal Brillouin spectroscopy is an innovative measurement method that allows the non-invasive determination of the rheological properties of materials. Its application in ophthalmology can offer the possibility to determine in-vivo the deformation properties of sections of transparent biological tissue such as the cornea or eye lens with spatial resolution. This seems to be a promising approach concerning current presbyopia research. Due to the spatially resolved detection of the viscoelastic lens properties, a better understanding of the natural aging process of the lens and the influences of different lens opacities on the stiffness is expected. From the obtained spectral data the relative protein levels, the relative refractive index profile and the relative density profile within the lens tissue can be derived in addition. A measurement set-up for confocal Brillouin microscopy based on spectral analysis of spontaneous Brillouin scattering signals by using a high-resolution dispersive device is presented. First in-vitro test results on animal and human lenses are presented and evaluated concerning their rheological significance. These data are compared with known research results.

[Image reconstruction of the corneal subbasal nerve plexus with extended field of view from focus image stacks of a confocal laser scanning microscope]

BACKGROUND

Confocal laser scanning microscopy (CLSM) allows the in vivo analysis of nerve structures of the human cornea. In this way, pathological alterations of the peripheral nervous system that also affect the corneal subbasal nerve plexus (SNP) can be diagnosed non-invasively and possibly earlier than with other methods. The field of view of in vivo CLSM images of the cornea (ca. 0.4 × 0.4 mm²) is not sufficient for a reliable assessment. Two phenomena make the image assessment difficult: the presence of ridge-like tissue deformations in the neighbourhood of the SNP and image distortions that are induced by involuntary and unavoidable eye movements during image acquisition. This paper presents an image processing method for generating undistorted images of the SNP with an extended field of view.

METHODS

The presented method has been tested on five volunteers. Eight focus image stacks have been taken and processed from each subject using a Heidelberg Retina Tomograph with Rostock Cornea Module (HRT). An image registration scheme specifically adapted to the image acquisition system corrects the non-linear motion-induced image distortions and reconstructs a volume from each focus image stack. The epithelial basal boundary surface including the SNP appears as a distinctive hyper-reflective layer inside the reconstructed volume. Extracting this continuous layer generates a depth map and finally a two-dimensional image of the SNP. A final fusion step of the single reconstructed SNP images leads to laterally extended images.

RESULTS

Out of 40 focus image stacks, 34 have been fully processed into two-dimensional SNP reconstruction images. Six focus image stacks could not be transformed into volumes because of extremely fast eye movements during the image acquisition that prevented the complete image registration of the stacks. The 34 SNP reconstruction images depict an average area of 94.7 % ( ± 6.2 %) with respect to the field of view of a single HRT image. The final fusion of the reconstructed images resulted in an average increase of the image area by a factor of 2.6 (ranging from 2.2 to 3.1).

CONCLUSION

The presented image processing algorithms are capable of correcting the motion-induced image distortions and of generating larger two-dimensional images of the SNP even in presence of severe tissue deformations. These images provide the basis for a more reliable assessment of the corneal nerve fibres.

[Quantitative analysis of corneal subbasal nerve plexus with in vivo confocal laser scanning microscopy]

BACKGROUND

An analysis of the corneal subbasal nerve plexus (SNP) allows an evaluation of the peripheral neuropathy in cases of degenerative diseases. In order to study the SNP structures quantitatively the automatically calculated morphological and topological parameters are required.

METHODS

In vivo confocal laser scanning microscopy (Heidelberg Retina Tomograph II/Rostock Cornea Module) was performed in healthy volunteers as well as patients with severe diabetic neuropathy. An adapted image processing algorithm was used to preprocess, segment and evaluate quantitatively the nerve fibers of the SNP. Data sets were analysed statistically.

RESULTS

The developed algorithm allows an automated detection of SNP structures. Furthermore, it allows the collection of data based on morphological and topological parameters. The main parameters that show significant differences between healthy cornea and cases of diabetic neuropathy are nerve fibre density and length, number of branching, tortuosity and number of terminal and crossing points. All parameters of the measurements can be used isolated, combined or weighted for quantification of the SNP networks.

CONCLUSION

The presented fully automated preprocessing eliminates a large number of motion-induced artefacts. The quality of the resulting pictures allows an automated quantification using characteristic measurements. This represents an in vivo, non-invasive technology analysing degenerative changes of SNP especially in the course of diabetes mellitus.

[Energy and memory efficient calculation of the accommodation demand in the artificial accommodation system]

Presbyopia and cataract are gaining more and more importance in the ageing society. Both age-related complaints are accompanied with a loss of the eye's ability to accommodate. A new approach to restore accommodation is the Artificial Accommodation System, an autonomous micro system, which will be implanted into the capsular bag instead of a rigid intraocular lens. The Artificial Accommodation System will, depending on the actual demand for accommodation, autonomously adapt the refractive power of its integrated optical element. One possibility to measure the demand for accommodation non-intrusively is to analyse eye movements. We present an efficient algorithm, based on the CORDIC technique, to calculate the demand for accommodation from magnetic field sensor data. It can be shown that specialised algorithms significantly shorten calculation time without violating precision requirements. Additionally, a communication strategy for the wireless exchange of sensor data between the implants of the left and right eye is introduced. The strategy allows for a one-sided calculation of the demand for accommodation, resulting in an overall reduction of calculation time by 50 %. The presented methods enable autonomous microsystems, such as the Artificial Accommodation System, to save significant amounts of energy, leading to extended autonomous run-times.

[A new mechatronic system to restore the accommodation ability of the human eye]

In this paper a new approach for restoring human accommodation is presented. It is an artificial accommodation system based on the latest results of microtechnology, electronics and automatic control. The concept of the overall system is described and the results obtained for the different subsystems are discussed.

[On-line mapping of corneal structures with in vivo laser scanning microscopy]

PURPOSE

The aim of this study was to design and evaluate online mapping of human corneal structures by in vivo laser scanning confocal microscopy.

METHOD

Six human corneae (four from healthy volunteers, two after LASIK,) were examined with confocal microscopy based on the confocal laser scanning microscope type Heidelberg Retina Tomograph II and Rostock cornea module. In each case an on-line mapping with max. size up to 3.2 x 3.2 mm (3072 x 3072 pixel) was performed.

RESULTS

On-line mapping was performed to demonstrate the structures of the healthy cornea (epithelium, subepithelial nerve plexus, endothelium) as well as postoperative wound healing processes after LASIK. The quality and size of the image are considerably influenced by compliance of the patient and experience of the investigator.

CONCLUSIONS

On-line mapping of cornea with in vivo confocal microscopy allows one to perform a large area 2D reconstruction and analyses of normal and pathological cornea. The presented method is considerably better than existing off-line reconstruction possibilities in terms of image quality and time consumption and affords an opportunity for further experimental and clinical studies.

[Two-photon microscopy of the cornea using intrinsic contrast]

BACKGROUND

Three-dimensional imaging of the cornea under physiological conditions is best performed with intrinsic contrast mechanisms for the visualisation of cells and extracellular matrix. However, the unique transparency of the cornea goes along with a lack of contrast for the extracellular matrix (ECM) in reflective mode microscopy and optical coherence tomography.

METHODS

Femtosecond laser-based non-linear microscopy provides novel contrast mechanisms for the visualisation of ECM. The confinement of the non-linear contrast to the focus volume provides an intrinsic sectioning property for 3D imaging. Further advantages of the infrared light are lower phototoxicity and higher penetration depth into the tissue. For the visualisation of the cornea and its layered substructures two non-linear contrast mechanisms are of main interest: Two-photon excited autofluorescence of NAD(P)H in the cytoplasma and second harmonic generation (SHG) in the collagen-I fibres of the stroma. Ex-vivo corneas of the rabbit were imaged to demonstrate the abilities of non-linear microscopy.

RESULTS

Using the autofluorescence of NAD(P)H the corneal epithelium with squamous cells, wing cells and basal cells is visualised in three dimensions without additional exogenoeus staining. Stromal keratocytes are also imaged using the NAD(P)H autofluoresecence. The layered structure of lamella in the stroma is visible after virtual resclicing of the 3D volume data. The en-face SHG images detected through the transparent cornea in forward direction show areas of parallel streaks, which increase in size and periodically alter in orientation (90 degrees , 45 degrees) with increasing depth from anterior to posterior. These streaks are not visible in the backward SHG signal. First results on rabbit corneas, which were cross-linked with Rivoflavin and UV application showed a signature of treatment five weeks post treatment. There were zones in the stroma totally lacking NAD(P)H autofluorescence and the abundance of keratocytes was less homogeneous than in control corneas.

CONCLUSION

These results and current reports on applications in the literature show that femtosecond laser-based non-linear microscopy is an emerging imaging modality which provides dye-free imaging of the corneal ECM and therefore complements scattering imaging modalities such as optical coherence tomography and confocal laser scanning microscopy in the reflective mode.