Ralf Engelhardt

Optical coherence tomography of the human skin

BACKGROUND Optical coherence tomography (OCT) is a new diagnostic method for tissue characterization. OBJECTIVE We investigated normal and pathologic structures in human skin in several locations to evaluate the potential application of this technique to dermatology. METHODS Based on the principle of low-coherence interferometry, cross-sectional images of the human skin can be obtained in vivo with a high spatial resolution of about 15 microns. Within a penetration depth of 0.5 to 1.5 mm, structures of the stratum corneum, the living epidermis, and the papillary dermis can be distinguished. RESULTS Different layers could be detected that were differentiated by induction of experimental blisters and by comparison with corresponding histologic sections. Furthermore, OCT images of several skin diseases and tumors were obtained. CONCLUSION OCT is a promising new imaging method for visualization of morphologic changes of superficial layers of the human skin. It may be useful for noninvasive diagnosis of bullous skin diseases, skin tumors, and in vivo investigation of pharmacologic effects.

Low-coherence optical tomography in turbid tissue: theoretical analysis

On the basis of white-light interferometry and statistical optics, a theoretical model for low-coherence optical tomography is presented that establishes the relation of interference modulation with path-length-resolved reflectance and that can provide analytical expressions and numerical solutions by means of a Fourier transform. The Monte Carlo technique is used to simulate the path-length-resolved reflectance from different multilayer tissue phantoms. Theoretical analyses and preliminary experimental results suggest that, unlike time-resolved spectroscopy, low-coherence optical tomography detects the local relative variations of path-length-resolved reflectance from the turbid tissues.

Slit-lamp-adapted optical coherence tomography of the anterior segment

Abstract · Purpose: To evaluate the diagnostic potential of a slit-lamp-adapted optical coherence tomography (OCT) system as an in vivo imaging device for routine clinical examination of the anterior segment of the eye. · Patients and methods: In a pilot study, healthy volunteers and patients with different pathologies of the anterior segment were examined with a slit-lamp-adapted OCT system using 100–200 axial scans with 100-Hz line-scan frequency. The scan length is variable up to 7 mm, and the axial depth is 1.5 mm in tissue. · Results: The slit-lamp-adapted OCT system allowed direct biomicroscopic imaging of the measured area. Anatomic structures and morphological changes anterior to the attenuating iris pigment epithelium could be visualized with high accuracy. Biometric analyses of the cornea, the chamber angle, the iris and secondary cataract were possible. Complete demonstration of the chamber angle was difficult due to the backscattering properties of the anterior part of the sclera and the consequent shadowing of the most peripheral part of the iris. · Conclusions: Slit-lamp-adapted OCT is a useful diagnostic tool which allows in vivo microscopic cross-sectional imaging of the anterior segment and precise measurement of ocular structures.

Corneal Optical Coherence Tomography Before and Immediately After Excimer Laser Photorefractive Keratectomy

PURPOSE To investigate the representation of the corneal structure with optical coherence tomography before and immediately after excimer laser photorefractive keratectomy. METHODS Twenty-four eyes of 24 patients with myopia and myopic astigmatism were prospectively studied. The corneal thickness and the corneal profile were assessed with slit-lamp-adapted optical coherence tomography preoperatively and immediately after excimer laser photorefractive keratectomy. RESULTS The attempted mean spherical equivalent of the refractive corrections was -6.7 +/- 3.6 (mean +/- SD) diopters with a mean calculated stromal ablation depth of 91 +/- 38 microm. The corneal optical coherence tomography was reproducible in all patients, demonstrating a mean decrease of central corneal thickness after epithelial debridement and excimer laser photorefractive keratectomy of 118 +/- 45 microm. The comparison of the calculated stromal ablation depth and the corneal thickness changes determined by corneal optical coherence tomography revealed a significant linear relationship with a correlation coefficient of 0.88 (P <.001). The flattening of the corneal curvature was confirmed in all patients with the optical coherence tomography system and correlated with the attempted refractive correction (r =.82, P <.001). CONCLUSIONS The slit-lamp-adapted optical coherence tomography system presented in this study allowed noncontact, cross-sectional, and high-resolution imaging of the corneal configuration. This initial clinical evaluation demonstrated that corneal optical coherence tomography could be a promising diagnostic modality to monitor corneal changes of thickness and curvature before and after excimer laser photorefractive keratectomy.

Contrast limits of coherence-gated imaging in scattering media

The fundamental difference between time-resolved and coherence-gated imaging modalities in scattering media is analyzed in terms of their optical transfer functions. The effectiveness of coherence gating for multiple-scattering rejection is shown by imaging a 100-mum-thick razor blade hidden in the scattering phantoms (i.e., Intralipid suspensions) with different scattering coefficients. We found that the imaging contrast is limited by multiple scattering and speckle effects in high-scattering media, and the measured effective penetration depth of optical coherence tomography is approximately equal to six mean free paths under the experimental conditions of a numerical aperture of less than 0.1 and a scattering anisotropy of approximately 0.8.

Optical coherence-gated imaging of biological tissues

We present optical coherence-gated tomography (OCT) in turbid biological tissues. The fast OCT system that is used in this study is a single-mode fiber-optic interferometer with low-coherence light at 830 nm which can perform a cross-sectional image of 250-600 pixels in a few seconds. Preliminary results suggest that OCT can provide high-resolution imaging in low-scattering and high-scattering superficial tissues. In vitro imaging of porcine cornea after being coagulated with a laser shows that OCT is a promising tool for the evaluation of pathological structures in low-scattering tissue. This technique can also be used to diagnose disease in high-scattering tissues like bladder and living skin. In addition, 50 MHz ultrasound images and histological pictures are presented for comparison with OCT.

First Experimental and Clinical Results With Transscleral Optical Coherence Tomography

BACKGROUND AND OBJECTIVE To evaluate the potentials of optical coherence tomagraphy (OCT) using long wavelength to penetrate highly scattering tissues of the eye and visualize the anterior chamber angle and the ciliary body. METHODS OCT images were generated by an experimental prototype in enucleated porcine eyes using as light source a superluminiscent diode with a wavelength of 1310 nm and a scan frequency of 60 Hz. The number of lateral scans was variable in a range from 100 to 400. RESULTS Infrared OCT was able to penetrate the sclera. The anterior chamber angle could be visualized completely and the ciliary body could be identified. However, it was not possible to penetrate the highly reflective iris pigment epithelium. CONCLUSION The use of infrared OCT allows penetration of the sclera, thus, providing complete visualization of the anterior chamber angle and limited demonstration of the ciliary body. Because of its higher resolution, it may represent an interesting noninvasive alternative to ultrasound biomicroscopy.

Optische Kohärenztomographie der Kornea und des vorderen Augenabschnitts

Ziel: Die Methode der optischen Kohärenztomographie (OCT) wurde bezüglich ihrer Eignung zur Vermessung des vorderen Augenabschnitts, der Kornea und thermischer Gewebeschädigungen untersucht. Beispielhaft wurden hierzu im Rahmen der Laserthermokeratoplastik (LTK) die Veränderungen der Hornhautdicke durch die Bestrahlung sowie die induzierten thermischen Läsionen vermessen.Material und Methoden: Mit dem von uns entwickelten experimentellen Scanning-OCT wurden mit einer optischen Auflösung von ca. 20 µm axial und 25 µm lateral x-z-Schnitte des vorderen Augenabschnitts vorgenommen. Als Modellauge dienten frisch enukleierte, tonisierte Schweineaugen. Thermische Läsionen wurden mit einer kontinuierlich emittierenden Laserdiode (λ = 1,86 µm) und verschiedenen Bestrahlungsparametern appliziert. Vor und nach Koagulation wurden die Hornhaut von Limbus zu Limbus in einem zentralen OCT-Schnittbild dargestellt sowie die einzelnen Koagulationsherde vermessen.Ergebnisse: Globale und lokale Hornhautdickenänderungen sowie der Abstand Hornhaut-Linse lassen sich mit hoher Genauigkeit bestimmen. Thermische Läsionen sind in ihren Ausdehnungen klar darstellbar und zeigen gute Übereinstimmungen mit histologischen Schnittbildern, sind jedoch aufgrund der optischen Auflösung an ihren Rändern nicht so scharf abgegrenzt, wie von histologischen Präparaten bekannt.Schlußfolgerung: Mit der Methode der OCT lassen sich in vitro und, mit reduzierter Auflösung, auch in vivo quantitative Vermessungen des vorderen Augenabschnitts vornehmen. Aufgrund der qualitativ guten Übereinstimmung des Ausmaßes thermischer Schädigungen der Hornhaut mit der histologischen Morphometrie läßt sich dieses Meßverfahren auch klinisch nach LTK zur Kontrolle der Ausdehnung und Denaturierungstiefen von Koagulationen sowie zur Untersuchung des individuellen Heilungsverlaufs verwenden.Target: The method of optical coherence tomography (OCT) was investigated regarding its suitability and limits for measuring the cornea and the anterior segment of the eye. Furthermore, the stromal expansion of thermally induced lesions in the cornea directly after irradiation was determined within the scope of the laser thermokeratoplasty (LTK).Material and methods: With the experimental scanning OCT system, x-z sections of the anterior eye segment were made with an optical resolution of about 20 µm axially and 25 µm laterally. Freshly enucleated, tonicized porcine eyes were used as model eyes. Thermal lesions were applied with a continuously emitting laser diode (λ = 1.86 µm) and various radiation parameters. Before and after coagulation, the cornea was viewed from limbus to limbus in a central OCT scan and the individual coagulation source was measured.Results: Global and local changes of the thickness of the cornea as well as the distance between cornea and lens were measured with high precision. Thermal lesions in their expansion can be clearly presented and matching well with the histologically stained sections, but are not as exactly defined at the edges due to the limited optical resolution, as known from histological preparations.Conclusion: With the OCT method quantitative measuring of the anterior eye segment can be performed in vitro and with reduced resolutions also in vivo. Due to the qualitatively good correspondence regarding the dimensions of thermal damage of the cornea with histologically obtained morphometric results, this method can be used for supervision of coagulation directly after LTK as well as for examination of the individual healing process.

Untersuchungen der Hornhaut mittels optischer Kohärenztomographie

ZusammenfassungZielsetzung. In der vorliegenden Untersuchung wurde der klinische Einsatz der optischen Kohärenztomographie (OCT) bei der zweidimensionalen Darstellung der Hornhaut überprüft. Patienten und Methoden. Die kontaktfreie spaltlampenadaptierte OCT wurde zur klinischen Evaluierung von pathologisch veränderten Hornhäuten, der Vermessung der zentralen Hornhautdicke sowie zur Überprüfung der kornealen Kurvatur eingesetzt. Ergebnisse. Die OCT ermöglichte eine Korrelation zwischen Reflexionsunterschieden und morphologischen Veränderungen sowie eine präzise kontaktfreie Biomorphometrie intrastromaler und retrokornealer Umbauvorgänge. Narbiges Gewebe zeigte sich als hyperreflektive Lichtstreuung, während zystische Veränderungen hyporeflektiv erschienen. Mit der OCT konnte außerdem eine reproduzierbare Messung der zentralen Hornhautdicke vorgenommen werden. Eine Bestimmung der Hornhautkurvatur aus den Oberflächensignalen scheint möglich. Schlussfolgerung. Aufgrund der kontaktfreien, einfachen und raschen Untersuchung an der Spaltlampe stellt die korneale OCT eine Ergänzung des diagnostischen Spektrums zur hochauflösenden Beurteilung der kornealen Morphologie, der Hornhautdicke sowie der Kurvatur dar.AbstractIntroduction. This study evaluated the clinical use of optical coherence tomography (OCT) for two-dimensional representation of the cornea. Patients and methods. Noncontact slitlamp-adapted OCT was used in selected cases to evaluate pathologically altered corneas and to measure the central corneal thickness and curvature. Results. OCT provided correlation between differences in reflection and morphological changes. Scar tissue resulted in hyperreflective light scattering, wheras cystic lesions were hyporeflective. Precise biomorphometry also allowed representation of intrastromal and retrocorneal changes. Central corneal thickness measured by OCT yielded reproducible values and corneal curvature could be calculated from the optical signals of the corneal surface. Conclusions. OCT provides high-resolution representation of the cornea and exact evaluation of its morphology, thickness, and curvature. Due to the noncontact, simple, and rapid examination using the slitlamp the corneal OCT method is a promising additional diagnostic modality.

Minimization of cavitation effects in pulsed laser ablation illustrated on laser angioplasty

Cavitation effects in pulsed laser ablation can cause severe deformation of tissue near the ablation site. In angioplasty, they result in a harmful dilatation and invagination of the vessel walls. We suggest to reduce cavitation effects by dividing the laser pulse energy into a pre-pulse with low and an ablation pulse with high energy. The pre-pulse creates a small cavitation bubble which can be filled by the ablation products of the main pulse. For suitable energy ratios between the pulses, this bubble will not be enlarged by the ablation products, and the maximal bubble size remains much smaller than after a single ablation pulse. The concept was analyzed by numerical calculations based on the Gilmore model of cavitation dynamics and by high-speed photography of the effects of single and double pulses performed with a silicone tube as vessel model. The use of double pulses prevents the deformation of the vessel walls. The concept works with an energy ratio of up to about 1:30 between the pulses. For the calculated optimal ratio of 1:14.6, the bubble volume is reduced by a factor of 17.7. The ablation pulse is best applied when the pre-pulse bubble is maximally expanded, but the timing is not very critical.