Angela Baumgartner

Polarization–Sensitive Optical Coherence Tomography of Dental Structures

Optical coherence tomography (OCT) has been developed during the last 10 years as a new noninvasive imaging tool and has been applied to diagnose different ocular and skin diseases. This technique has been modified for cross–sectional imaging of dental structures. In this first preliminary study the technique was applied to obtain tomographic images of extracted sound and decayed human teeth in order to evaluate its possible diagnostic potential for dental applications. Classical OCT images based on reflectivity measurements and phase retardation images using polarization–sensitive OCT were recorded. It was demonstrated that polarization–sensitive OCT can provide additional information which is probably related to the mineralization status and/or the scattering properties of the dental material. One of the attractive features of OCT is that it uses near–infrared light instead of ionizing radiation. Furthermore, high transversal and depth resolution on the order of 10 μm can be obtained. Present limitations, e.g. the limited penetration depth, and possible solutions are discussed.

Biometric investigation of changes in the anterior eye segment during accommodation

Non-invasive biometry of the anterior structures of the human eye can be performed with unprecedented precision of 8-10 microns and a resolution of approximately 9 microns by partial coherence interferometry, which has the potential to assess the effect of cycloplegia on the ocular components of the anterior eye segment, to further improve the precision to 1-2 microns by the use of these agents and to quantify the amount of residual accommodations in different states of cycloplegia. In addition, the anterior chamber depth, the thickness of the crystalline lens, their changes during accommodation, as well as the movement of the anterior and posterior lens pole during accommodation can be quantified objectively and accurately to investigate the mechanism of accommodation.

Interferometric Measurement of Corneal Thickness With Micrometer Precision

The recently developed partial coherence laser Doppler interferometry technique was improved to measure central and peripheral corneal thickness with high precision. Corneal thickness profiles were measured on 18 eyes of health, volunteer subjects. All of these eyes were measurable at angles (between visual axis and measuring direction) ranging from 20 degrees nasal to 25 degrees temporal. At larger angles (up to 35 degrees) only part of the eyes was measurable. The thickness profiles of the 18 corneas have a nearly perfectly parabolic shape within the measured region. The precision (standard deviation) was 1.6 microns for central measurements and decreased somewhat to about 3.5 microns at measuring angles in the range of 25 to 30 degrees. No significant interobserver variability was found on 14 eyes measured by three different observers. This study indicates that the new technique is likely to be superior to currently used ultrasound and conventional optical pachymetry techniques, especially for refractive procedures.

Dispersion effects in partial coherence interferometry: implications for intraocular ranging

In nondispersive media, the minimum distance that can be resolved by partial coherence interferometry (PCI) and optical coherence tomography (OCT) is inversely proportional to the source spectral bandwidth. Dispersion tends to increase the signal width and to degrade the resolution. We analyze the situation for PCI ranging and OCT imaging of ocular structures. It can be shown that for each ocular segment an optimum source bandwidth yielding optimum resolution exists. If the resolution is to be improved beyond this point, the group dispersion of the ocular media has to be compensated. With the use of a dispersion compensating element, and employing a broadband superluminescent diode, we demonstrate a resolution of 5 μm in the retina of both a model eye and a human eye in vivo. This is an improvement by a factor of 2-3 as compared to currently used instruments.

SIGNAL AND RESOLUTION ENHANCEMENTS IN DUAL BEAM OPTICAL COHERENCE TOMOGRAPHY OF THE HUMAN EYE

In the past 10 years, a dual beam version of partial coherence interferometry has been developed for measuring intraocular distances in vivo with a precision on the order of 0.3 to 3 μm. Two improvements of this technology are described. A special diffractive optical element allows matching of the wavefronts of the divergent beam reflected at the cornea and the parallel beam reflected at the retina and collimated by the optic system of the eye. In this way, the power of the light oscillations of the interfering beams incident on the photodetector is increased and the signal-to-noise ratio of in vivo measurements to the human retina is improved by 20 to 25 dB. By using a synthesized light source consisting of two spectrally displaced superluminescent diodes with an effective bandwidth of 50 nm, and by compensating for the dispersive effects of the ocular media, it was possible to record the first optical coherence tomogram of the retina of a human eye in vivo with an axial resolution of ∼6 to 7 μm. This is a twofold improvement over the current technology.

Dual-beam optical coherence tomography: signal identification for ophthalmologic diagnosis

The dual beam version of optical coherence topography can be used for noninvasive, high-resolution imaging of the human eye fundus, enabling in vivo visualization of retinal morphology as well as accurate quantification of the thickness profiles of its layers. Interferometric fundus signals-optical A-scans-and retinal tomograms of patients with glaucoma, diabetic retinopathy, and age-related macular degeneration are compared with those of healthy, normal subjects to elucidate the origin of the signal peaks detected and to investigate and interpret the retinal microstructures contained in the cross-sectional images.

Resolution-improved dual-beam and standard optical coherence tomography: a comparison.

Abstract Background: The purpose of the study was to demonstrate the improved axial resolution and longitudinal stability of dual-beam optical coherence tomography (OCT) in comparison to conventional OCT setups used in commercially available OCT instruments. Methods: The conventional OCT technique is based on an interferometric setup that is rather sensitive to axial eye motions. We have developed a special dual-beam OCT technique which eliminates the influence of axial eye motions. This is achieved by using the anterior corneal surface as the reference surface for the interferometric ranging. To improve the signal quality, the different wavefront curvatures of beams reflected at cornea and retina are matched by a diffractive optical element. To improve the axial resolution, a broadband synthesized light source with an effective bandwidth of 50 nm is used, and the group dispersion of the ocular media is compensated. Tomographic images were recorded in the fovea and the optic nerve head of healthy volunteers. For comparison purposes, approximately the same locations in the same eyes were imaged by a commercially available OCT instrument. Results: Compared to the standard OCT technique, the dual-beam OCT images show considerably improved axial resolution. Especially in tomograms recorded at the fovea, dual-beam OCT resolves microstructural details that are not visible in the standard OCT images. Furthermore, the axial stability of dual-beam OCT enables the recording of exact geometrical contours of fundus layers. Conclusions: Dual-beam OCT is able to provide structural information on the ocular fundus that is not obtained with standard OCT. The long recording times of our instrument limit the transverse resolution to 100–150 µm at present.

In-vivo intraocular ranging by wavelength tuning interferometry

Recently, wavelength tuning interferometry was suggested as an alternative technique for distance measurements. Compared to partial coherence interferometry, it has the advantages of needing no high precision mechanically moving components and the capability of measuring several distances simultaneously in very short time. We report on first measurements of intraocular distances in human eyes in vivo using a distributed Bragg reflector laser diode with a tuning range of 2 nm. We were able to measure the anterior chamber depth, the lens thickness, the vitreous depth, the axial eye length, and to estimate the thickness of the retina. The resolution is approximately 150 micrometer optical distance.

Measurements of the posterior structures of the human eye in vivo by partial-coherence interferometry using diffractive optics

In the past ten years, the dual beam version of partial coherence interferometry has been developed for measuring intraocular distances in vivo with a precision on the order of 0.3 to 3 micrometer. This technique has now been further improved by using diffractive optics. A special diffractive optical element focuses part of the laser beam on the vertex of the cornea and lets the other collimated parallel part of the beam pass through. The beams remitted from the eye will thereby be converted into parallel beams. The light power oscillations in the corresponding interferograms are much stronger than those of the narrow interference fringes obtained without that technique what significantly improves the signal to noise ratio. This makes it possible to clearly differentiate signals from different fundus layers. High precision in vivo fundus measurements have been performed at various positions on the human retina in order to obtain fundus profiles. These measurements have been synthesized to tomographic images of the human eye fundus. In order to localize the exact measurement point on the retina simultaneously to the fundus scans, a fundus camera has been implemented into the partial coherence interferometry system that allows a clear identification of the individual A-scan positions.

Optical coherence tomography of dental structures

In the past ten years Partial Coherence Interferometry (PCI) and Optical Coherence Tomography (OCT) have been successfully developed for high precision biometry and tomography of biological tissues. OCT employs the partial coherence properties of a superluminescent diode and the Doppler principle yielding resolution and precision figures of the order of a few microns. Presently, the main application fields of this technique are biometry and imaging of ocular structures in vivo, as well as its clinical use in dermatology and endoscopic applications. This well established length measuring and imaging technique has now been applied to dentistry. First in vitro OCT images of the cemento (dentine) enamel junction of extracted sound and decayed human teeth have been recorded. These images distinguish dentine and enamel structures that are important for assessing enamel thickness and diagnosing caries. Individual optical A-Scans show that the penetration depth into enamel is considerably larger than into dentine. First polarization sensitive OCT recordings show localized changes of the polarization state of the light backscattered by dental material. Two-dimensional maps of the magnitude of the interference intensity and of the total phase difference between two orthogonal polarization states as a function of depth can reveal important structural information.