Volker Westphal

Correction of geometric and refractive image distortions in optical coherence tomography applying Fermat’s principle

We describe a methodology for quantitative image correction in OCT which includes procedures for correction of nonlinear axial scanning and non-telecentric scan patterns, as well as a novel approach for refraction correction in layered media based on Fermats principle. The residual spatial error obtained in layered media with a fan-beam hand-held probe was reduced from several hundred micrometers to near the diffraction and coherence-length limits.

Real-time, high velocity-resolution color Doppler optical coherence tomography

Color Doppler optical coherence tomography (CDOCT) is a noninvasive optical imaging technique for micrometer-scale physiological flow mapping simultaneously with morphological optical coherence tomography imaging. We have developed a novel CDOCT signal-processing strategy capable of imaging physiological flow rates at 8 frames/s. Our new strategy features hardware-implemented digital autocorrelation across subsequent scans, permitting us to measure 300-Hz-8-kHz Doppler shifts upon signals of 0.6-MHz bandwidth. The performance of the CDOCT system was demonstrated in a flow phantom and in vivo in Xenopus laevis.

Photothermal coagulation of blood vessels: A comparison of high-speed optical coherence tomography and numerical modelling

Optical-thermal models that can accurately predict temperature rise and damage in blood vessels and surrounding tissue may be used to improve the treatment of vascular disorders. Verification of these models has been hampered by the lack of time- and depth-resolved experimental data. In this preliminary study, an optical coherence tomography system operating at 4-30 frames per second was used to visualize laser irradiation of cutaneous (hamster dorsal skin flap) blood vessels. An argon laser was utilized with the following parameters: pulse duration 0.1-2.0 s, spot size 0.1-1.0 mm, power 100-400 mW. Video microscopy images were obtained before and after irradiations, and optical-thermal modelling was performed on two irradiation cases. Time-resolved optical coherence tomography and still images were compared with predictions of temperature rise and damage using Monte Carlo and finite difference techniques. In general, predicted damage agreed with the actual blood vessel and surrounding tissue coagulation seen in images. However, limitations of current optical-thermal models were identified, such as the inability to model the dynamic changes in blood vessel diameter that were seen in the optical coherence tomography images.

Correlation of endoscopic optical coherence tomography with histology in the lower-GI tract.

BACKGROUND Optical coherence tomography (OCT), a noninvasive optical imaging technique, provides high-resolution cross-sectional images of tissue microstructure. We developed a system for real-time endoscopic OCT (EOCT) of the human GI tract. During clinical trials, the structure of mucosa and submucosa, glands, blood vessels, pits, villi, and crypts was observed in a range of GI organs. Although EOCT images are thought to accurately depict actual histologic features, there are few data to support this assumption. Therefore, the present study correlated images acquired with an EOCT imaging system in vitro to corresponding histologic sections. METHODS EOCT images were obtained of fresh specimens of ileum, colon, and rectum that then were fixed in formalin and were processed for microscopic evaluation by using standard methods. The thickness of mucosa and of submucosa was determined for both EOCT images and histologic slides. RESULTS The first hyper-reflective layer in the EOCT images was identified as mucosa. A close correlation (R 2=0.84) was observed between EOCT and histology. Furthermore, the submucosa and the muscularis propria could be identified as the next deepest hyporeflective band and a hyper-reflective layer, respectively, in EOCT images. The submucosa was found to be more compressible than mucosa, and its EOCT appearance was dependent on its content of adipose tissue. CONCLUSIONS EOCT provides images that precisely correlate with the histologic structure of the mucosa and the submucosa of the GI tract.

Phase-referenced Doppler optical coherence tomography in scattering media

We present a fiber-based, low-coherence interferometer that significantly reduces phase noise by incorporating a second, narrowband, continuous-wave light source as a phase reference. By incorporating this interferometer into a Doppler OCT system, we demonstrate significant velocity noise reduction in reflective and scattering samples using processing techniques amenable to real-time implementation. We also demonstrate 90% suppression of velocity noise in a flow phantom.

⁎3442 Role of high resolution endoscopic imaging using optical coherence tomography (oct) in patients with barrett's esophagus (be).

OCT, a novel technique for high resolution endoscopic imaging clearly delineates microscopic mucosal and submucosal structures of the GI tract. Aim: To study the ability of OCT to identify and characterize BE. Methods: OCT images were obtained using an endoscopic OCT probe (2.4 mm dia, 0.5 mm focal distance)in patients undergoing surveillance EGD for BE. Images were captured digitally and were objectively rated in terms of clarity, resolution, ease of identification of BE and to determine whether areas of dysplasia could be identified. Four quadrant large particle biopsies were taken from the area of BE at 2 cm intervals and were correlated with OCT images. Results:8 patients with BE (mean age 62.5 years, 6 men) were studied. Mean length of BE was 7.7 (SE 2.2) cms; 3 patients had focal areas of high grade dysplasia. During OCT imaging squamous epithelium and Barretts epithelium could be readily distinguished. Also, areas with Barretts epithelium lacked the characteristic thick, dark band which is seen with stratified squamous epithelium. The esophageal mucosa was significantly thicker in the areas of Barretts epithelium (mean thickness 0.6 mm vs. 0.4 mm, p

Real-time optical coherence tomography of the anterior segment using hand-held and slit-lamp adapted systems

Real-time optical coherence tomography (OCT) was used to visualize and quantify structures in the anterior segment of the eye. Results obtained with hand-held and slit-lamp adapted OCT systems are presented. Preliminary data indicates strong potential for the use of real-time OCT in anterior segment biometry and in non-invasive assessment of normal and pathological anterior segment anatomy.

Real-time in vivo optical coherence microscopy

Summary form only given. In-vivo confocal microscopy has been proposed/sup 1/ and demonstrated as a powerful tool for noninvasive imaging permitting optical sectioning in bulk biological samples due to its strong axial discrimination. Depending on the numerical aperture (NA), axial and lateral resolutions of several micrometers are obtainable. The maximal depth of imaging is limited to several hundred microns in highly scattering tissues such as the skin, however, by multiple scattered photons arising from out-of-focus-regions of the sample. Optical coherence microscopy (OCM) is a combination of confocal microscopy and optical coherence tomography (OCT). In OCM, the coherence gate of OCT is overlapped with the confocal gate of confocal microscopy to provide increased rejection of multiply scattered photons and therefore extend the usable depth range.

Quantitative OCT image correction using Fermat's principle and mapping arrays

Optical coherence tomography (OCT) is a relatively new developed technique to image tissue microstructure in vivo with a resolution of about 10 micrometers . So far, the research has focused on increasing the resolution, increasing the acquisition rate, developing new sample arm scanning techniques, or functional imaging like color Doppler OCT. But one of the main advantages of OCT compared to ultrasound, non-contact imaging, also results in a mayor image distortion: refraction at the air-tissue interface. Also, applied scanning configurations can lead to deformed images. Both errors prevent accurate distance and angle measurements on OCT images, necessary e.g. for Glaucoma diagnosis in the anterior segment of the eye. We describe a methodology for quantitative image correction in OCT which includes procedures for correction of arbitrary spatial warping caused by non-uniform axial reference and lateral sample scan patterns, as well as a novel approach for refraction correction in layered media based on Fermats principle. The de-warping corrections are implemented in real-time by use of pointer (mapping) arrays, while the refraction correction algorithm is more computationally intensive and is performed off-line.

Noninvasive characterization of anterior segment structures using real-time optical coherence tomography at 1310 nm

Real-time optical coherence tomography (OCT) was used to visualize and quantify structures in the anterior segment of the eye. Current results of ongoing clinical trials are presented. Preliminary data indicates strong potential for the use of real time OCT as a tool for noninvasive characterization of the anterior chamber angle and for anterior segment biometry.