Innovative experimental concepts for optical coherence tomography

Abstract

Optical Coherence Tomography (OCT) is a recent biomedical imaging technique based on low-coherence interferometry that is capable of acquiring depth-resolved reflectivity maps of scattering tissues with high sensitivity. The conventionally employed imaging mode is to build up a cross sectional image by scanning the sample surface with a point illumination. In order to increase imaging speed, the concept of parallel detection has been introduced to OCT. Here, a whole sample line or even surface is imaged directly onto an array of photodetectors, making the lateral scanning motion obsolete. A customized detector array has been developed in our institute for parallel OCT imaging based on complimentary metal-oxide-semiconductor (CMOS) technology. It associates a signal processing circuit to each photosensitive area of the array, capable of demodulating the interferometric signal on-chip. In this manner, only the signal envelop has to be read out, allowing for a high dynamic range and high frame rates. At the beginning of the thesis, this device had only been tested for topographic measurements on reflective surfaces. As a direct continuation of this previous work, the initial objective of this thesis has been to extend its application to parallel OCT in scattering samples and to identify other uses of this technology in the field of OCT. Accordingly, the first part of this work is dedicated to parallel detection in OCT, using this customized CMOS detector array. The feasibility of the approach is shown experimentally on scattering samples. Reflective surfaces covered with scattering solutions of varying concentration and onion samples are studied. Furthermore, the initial goal of fast OCT imaging is pursued by using the detector at its technological limits and realizing video-rate, three-dimensional OCT acquisitions of a dynamically changing sample. The thermal deformations induced by the probing beam on a dark strand of human hair are imaged at 25 volume acquisitions per second. Identified as a possible new application for the CMOS detector array, the concept of wavelength de-multiplexing for spectroscopic OCT is investigated in the second part of this thesis. Wavelength de-multiplexing is an experimental method for realizing spectroscopically resolved, time-domain OCT measurements. In contrast to the currently employed numerical post-processing methods for extracting spectroscopic information, wavelength de-multiplexing relies on optical wavelength separation in the interferometer detection arm and acquisition of independent wavelength channels using a detector array. The method itself is first studied using a laterally translated photodiode for detection, in order to compare it to the conventional spectroscopic OCT approach. The absorption characteristics of a glass filter and a Nd-doped crystal are measured in this manner. Then, associated to the CMOS detector array in a proof-of-principle experiment, the dynamically changing, spatially resolved absorption of a dye mixing process is measured online. The experimental setup used for wavelength de-multiplexing has lead to an innovative technique that is of interest to low coherence interferometry in general. The axial depth scan employed in all time-domain low coherence interferometry setups should ideally introduce a linear change with time of optical path difference between the interferometer's sample and reference arms. However, depending on the scan method used, the path difference variation is only approximatively linear and can even vary randomly from scan to scan. For precise, phase-sensitive and repetitive measurements the scans have to be calibrated. By increasing the coherence length of part of the detected radiation in the detection arm of the interferometer through narrow spectral filtering, a calibration signal is created that permits the precise measurement of the optical path difference variation and thus its calibration without the need for a secondary interferometer. The usefulness of the approach is demonstrated by combining it with a recently published spectral shaping technique for sidelobe suppression in OCT when using non-Gaussian source spectra. In order to be beneficial, such a technique requires highly repetitive measurements that can only be obtained with particularly stable scanning devices or well calibrated ones. In summary, this thesis studies three innovative experimental concepts that aim to improve OCT imaging speed and imaging quality and proposes them as interesting new tools to the OCT community.