Phase-sensitive multibeam OCT systems

Optical coherence tomography (OCT) has been introduced two decades ago as a non-invasive modality for imaging transparent and translucent tissues with high resolution. The dominating application field of OCT is ophthalmology, where OCT revolutionized retinal imaging and diagnostics. Since its introduction in 1991, OCT has been modified and improved in several ways. The enormous success of OCT in recent years can largely be attributed to a paradigm shift in OCT technology: The replacement of the first generation time domain OCT technique by the much faster and more sensitive spectral domain (SD) OCT technology. Commercial SD OCT systems have, however, still a major drawback: they acquire images based just on backscattered intensity. However, many biologic tissues yield only poor contrast if imaged on an intensity basis. These tissues may, however, change other properties of light, e.g., its polarization state, phase, or frequency. Measurements of these properties require phase information from the signals. Several such advanced OCT methods are presently under investigation, e.g. Doppler OCT for blood flow measurements, polarization sensitive (PS)-OCT for generating tissue specific contrast, and others. Most implementations of these techniques use a single sampling beam to scan the object. This poses several limitations, in terms of speed, sampling patterns, and accessible phase based information. This project shall explore multibeam (MB) OCT to overcome several of these limitations: The imaging speed can be increased by using several sampling beams simultaneously, enabling snap shot 3D imaging within fractions of a second. The phase information of two and more sampling beams can be combined to overcome the limits of accessible speed in Doppler OCT blood velocity measurements and can provide absolute flow velocity independent of vessel orientation. Other concepts use sampling beams of different polarization states or with slightly oblique angles to improve PS-OCT image quality and reduce speckle noise. In the course of this project, various new concepts of MB OCT shall be investigated. This shall be achieved by developing two MB OCT systems, a spectrometer based system at a wavelength of ~ 840 nm and a swept source based system at ~ 1060 nm. A flexible scan head providing various beam patterns shall be developed. The two MB OCT systems shall be evaluated in pilot studies for diagnostics of two major ocular diseases: diabetic retinopathy and glaucoma. In the first case, improved imaging of retinal capillary networks is of major interest, in the second case, absolute blood flow measurements in retinal vessels as well as birefringence measurements in retinal tissue are the target. The results of this project may pave the way for a new generation of advanced OCT systems, and, on a longer term, may improve diagnostics of ocular and other diseases.

The purpose of this project was to develop improved methods of optical coherence tomography (OCT) for retinal diagnostics, and to demonstrate their application for imaging and quantitative measurements of blood flow in the human retina. OCT has been developed since the early 1990s as a non-invasive modality for imaging transparent and translucent samples and tissues with a resolution of a few m. Since its first introduction, OCT has been modified, improved and extended in several ways and is now the gold-standard technique of ophthalmic imaging. However, commercially available OCT systems still had a major shortcoming at the time this project started: They acquired images based just on backscattered intensity, which provides structural, but no functional information. Advanced OCT methods can acquire additional information, leading to improved contrast and functional information, like polarization sensitive OCT and Doppler OCT. Most implementations of these techniques use a single sampling beam to raster scan the object to obtain 2D or 3D images. This poses several limitations, in terms of speed, contrast, and functional information. In this project, advanced OCT methods using multiple OCT beams simultaneously (MB-OCT) were developed in order to overcome several of these limitations: (i) The phase information of three sampling beams were combined to provide absolute blood velocity and blood flow measurements in retinal vessels and to measure the total blood flow in the human retina. Contrary to previous methods, the new scheme does not need any prior information on the vessel orientation and is therefore not affected by artefacts introduced by ocular motion during data acquisition. The method was successfully demonstrated to measure the total retinal blood flow in healthy subjects, with a precision of ~ 6%. In a first pilot study, we compared blood flow in patients with glaucoma (one of the leading causes of blindness) to that in healthy subjects and observed a strong reduction of blood flow in glaucoma patients. This indicates that MB Doppler OCT might be a useful method for glaucoma diagnostics. (ii) Another variant of MB Doppler OCT technology was used to analyze blood flow pulsations in retinal veins, a method possibly useful for measurements of intracranial pressure, a quantity of great interest in brain conditions that is presently only accessible by invasive methods. (iii) Another concept of MB-OCT exploits the anisotropic scattering of retinal tissues: some of the layers of the retina scatter the light with different intensities into different directions, which can be used by MB-OCT to generate additional and improved contrast in OCT images. We successfully demonstrated such directional scattering contrast in the retinal photoreceptor layer and in layers containing nerve tissue. These results may be useful for early diagnosis of diseases that change the microstructure of these tissues.