Harald Sattmann

Submicrometer axial resolution optical coherence tomography.

Optical coherence tomography (OCT) with unprecedented submicrometer axial resolution achieved by use of a photonic crystal fiber in combination with a compact sub-10-fs Ti:sapphire laser (Femtolasers Produktions) is demonstrated for what the authors believe is the first time. The emission spectrum ranges from 550 to 950 nm (lambda(c)=725 nm , P(out)=27 mW) , resulting in a free-space axial OCT resolution of ~0.75 mum , corresponding to ~0.5 mum in biological tissue. Submicrometer-resolution OCT is demonstrated in vitro on human colorectal adenocarcinoma cells HT-29. This novel light source has great potential for development of spectroscopic OCT because its spectrum covers the absorption bands of several biological chromophores.

Adaptive-optics ultrahigh-resolution optical coherence tomography.

Merging of ultrahigh-resolution optical coherence tomography (UHR OCT) and adaptive optics (AO), resulting in high axial (3 microm) and improved transverse resolution (5-10 microm) is demonstrated for the first time to our knowledge in in vivo retinal imaging. A compact (300 mm x 300 mm) closed-loop AO system, based on a real-time Hartmann-Shack wave-front sensor operating at 30 Hz and a 37-actuator membrane deformable mirror, is interfaced to an UHR OCT system, based on a commercial OCT instrument, employing a compact Ti:sapphire laser with 130-nm bandwidth. Closed-loop correction of both ocular and system aberrations results in a residual uncorrected wave-front rms of 0.1 microm for a 3.68-mm pupil diameter. When this level of correction is achieved, OCT images are obtained under a static mirror configuration. By use of AO, an improvement of the transverse resolution of two to three times, compared with UHR OCT systems used so far, is obtained. A significant signal-to-noise ratio improvement of up to 9 dB in corrected compared with uncorrected OCT tomograms is also achieved.

In vivo retinal optical coherence tomography at 1040 nm-enhanced penetration into the choroid

For the first time in vivo retinal imaging has been performed with a new compact, low noise Yb-based ASE source operating in the 1 microm range (NP Photonics, lambdac = 1040 nm, Deltalambda = 50 nm, Pout = 30 mW) at the dispersion minimum of water with ~7 microm axial resolution. OCT tomograms acquired at 800 nm are compared to those achieved at 1040 nm showing about 200 microm deeper penetration into the choroid below the retinal pigment epithelium. Retinal OCT at longer wavelengths significantly improves the visualization of the retinal pigment epithelium/choriocapillaris/choroids interface and superficial choroidal layers as well as reduces the scattering through turbid media and therefore might provide a better diagnosis tool for early stages of retinal pathologies such as age related macular degeneration which is accompanied by choroidal neovascularization, i.e., extensive growth of new blood vessels in the choroid and retina.

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.

Imaging of polarization properties of human retina in vivo with phase resolved transversal PS-OCT

Recently, we developed a phase resolved polarization sensitive OCT system based on transversal scanning. This system was now improved and adapted for retinal imaging in vivo. We accelerated the image acquisition speed by a factor of 10 and adapted the system for light sources emitting at 820nm. The improved instrument records 1000 transversal lines per second. Two different scanning modes enable either the acquisition of high resolution B-scan images containing 1600x500 pixels in 500ms or the recording of 3D data sets by C-scan mode imaging. This allows acquiring a 3D-data set containing 1000x100x100 pixels in 10 seconds. We present polarization sensitive B-scan images and to the best of our knowledge, the first 3D-data sets of retardation and fast axis orientation of fovea and optic nerve head region in vivo. The polarizing and birefringence properties of different retinal layers: retinal pigment epithelium, Henles fiber layer, and retinal nerve fiber layer are studied.

Dynamic coherent focus OCT with depth-independent transversal resolution

Abstract We present a new OCT technique which renders the transversal resolution depth independent. This is achieved by an optical setup which shifts the focus of the beam illuminating the object through the object depth without changing the path length in the corresponding interferometer arm. Therefore, the coherence gate remains at the beam focus without any readjustment of the reference arm. Depth resolution was tested with the help of microscopy cover-plates and transversal resolution was tested with the help of Ronchi rulings. Resolution was 100 lines mm−1 over an object depth of 430 μm. For a first demonstration of the properties of this dynamic coherent focus scheme in a biologic system a section of a human cornea was used. We expect that this technique can further be improved to obtain transversal resolution down to the 1-μm range

Measurement of the thickness of fundus layers by partial coherence tomography

In the past few years, a new noninvasive optical ranging technique, partial coherence interferometry, has been developed to measure various intraocular distances. A dual-beam version of this method offers high longitudinal resolution by using laser light with high spatial coherence but short coherence length-15 μm (full width at half maximum)-emitted by a special super luminescent diode. This technique is extended to obtain measurements not only parallel to the vision axis but at arbitrary horizontal and vertical angles to it. This is achieved by a new instrument, a fully computer controlled scanning partial coherence interferometer. By tilting the laser beam in horizontal and vertical directions, this scanning partial coherence interferometer measures the distance from the anterior corneal surface to different points of the retina. These results are then plotted to form topographic images containing information about the contour and the thickness profile of different retinal structures, e.g., the retinal thickness and the retinal nerve fiber layer thickness. Provided that there is no other strong reflection nearby, the absolute position of these retinal layers (respective to the cornea as a reference surface) can be determined in vivo with a precision of 5 μm. Furthermore, the intensities of multiple longitudinal scans at different angles between vision axis and measurement direction can be converted into pixel colors and mounted to form a 2-D false color image. These tomograms show the contour and the structure of different retinal layers.

A thermal light source technique for optical coherence tomography

A new technique for optical coherence tomography imaging with spatially low-coherent light sources is presented. In this technique the low coherence interferometry (LCI) depth-scan is performed by the image of the light source, and, therefore, simultaneously by a multitude of mutually incoherent LCI channels, to increase the probe beam power. Thermal light sources have the advantage of extremely low time-coherence with coherence lengths in the 1 μm range. The performance of a tungsten halogen lamp with a thermal spectrum and a xenon arc lamp with broadened spectral lines superimposed on a thermal continuum are compared.

Enhanced visualization of choroidal vessels using ultrahigh resolution ophthalmic OCT at 1050 nm

In this article the ability of ultrahigh resolution ophthalmic optical coherence tomography (OCT) to image small choroidal blood vessels below the highly reflective and absorbing retinal pigment epithelium is demonstrated for the first time. A new light source (lambdac= 1050 nm, Deltalambda = 165 nm, Pout= 10 mW), based on a photonic crystal fiber pumped by a compact, self-starting Ti:Al2O3 laser has therefore been developed. Ex-vivo ultrahigh resolution OCT images of freshly excised pig retinas acquired with this light source demonstrate enhanced penetration into the choroid and better visualization of choroidal vessels as compared to tomograms acquired with a state-of-the art Ti:Al2O3 laser (Femtolasers Compact Pro, lc= 780 nm, Deltalambda= 160 nm, Pout= 400 mW), normally used in clinical studies for in vivo ultrahigh resolution ophthalmic OCT imaging. These results were also compared with retinal tomograms acquired with a novel, spectrally broadened fiber laser (MenloSystems, lambdac= 1350 nm, Deltalambda= 470 nm, Pout = 4 mW) permitting even greater penetration in the choroid. Due to high water absorption at longer wavelengths retinal OCT imaging at ~1300 nm may find applications in animal ophthalmic studies. Detection and follow-up of choroidal neovascularization improves early diagnosis of many retinal pathologies, e.g. age-related macular degeneration or diabetic retinopathy and can aid development of novel therapy approaches.

Three-dimensional optical coherence tomography at 1050 nm versus 800 nm in retinal pathologies: enhanced performance and choroidal penetration in cataract patients.

Frequency domain optical coherence tomography (FD-OCT), based on an all-reflective high-speed InGaAs spectrometer, operating in the 1050 nm wavelength region for retinal diagnostics, enables high-speed, volumetric imaging of retinal pathologies with greater penetration into choroidal tissue is compared to conventional 800 nm three-dimensional (3-D) ophthalmic FD-OCT systems. Furthermore, the lower scattering at this wavelength significantly improves imaging performance in cataract patients, thereby widening the clinical applicability of ophthalmic OCT. The clinical performance of two spectrometer-based ophthalmic 3-D OCT systems compared in respect to their clinical performance, one operating at 800 nm with 150 nm bandwidth (approximately 3 microm effective axial resolution) and the other at 1050 nm with 70 nm bandwidth (approximately 7 microm effective axial resolution). Results achieved with 3-D OCT at 1050 nm reveal, for the first time, decisive improvements in image quality for patients with retinal pathologies and clinically significant cataract.