Innovative experimental concepts for optical coherence tomography

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.

Multiple scattering in wide-field optical coherence tomography

Boris Karamata

Optical Coherence Tomography (OCT), a well-established imaging method based on low-coherence interferometry, provides cross-sectional images of the internal structure of biological samples with a resolution in the micrometer range. OCT was successfully applied on various tissues such as for instance the retina, the skin or a tooth. In highly scattering tissues like the skin, probing depth is limited to approximately 2mm, mainly due to insufficient rejection of multiply scattered light. Presently, the contribution of multiple scattering in OCT is not fully understood. Therefore, there is a strong and urgent need to develop models allowing a reliable evaluation of the system's limitations as well as the improvement of the imaging capabilities. It is generally believed that a relevant model should account for loss of correlation between the reference and the sample field due to multiple scattering. We developed a new comprehensive model of OCT. Our preliminary study revealed that the reference and sample fields are actually fully correlated. This important result allowed us to model the OCT signal as a sum of stationary random phasors and treated it as a statistical signal. The mean of this signal can be calculated thanks to classical results of statistical optics and to a Monte Carlo simulation. Unlike other existing models, our model accounts for the source autocorrelation function. The model proved to be in excellent agreement with a whole range of experimental data gathered in a comprehensive study of cross-talk in wide-field OCT. Moreover, our results put in question the applicability of widely used models of OCT based on the "extended Huygens-Fresnel principle", which assume a partial correlation between interfering fields due to multiple scattering. The construction of conventional OCT images is based on lateral scanning of a beam focused within the sample. To increase image acquisition speed and eliminate the need for lateral scanning, wide-field OCT was recently developed. Our experimental and theoretical investigations of the potential and limitations of wide-field OCT revealed the crucial role played by the spatial coherence of the light source. Spatially coherent illumination generates considerable coherent optical cross-talk, which prevents shot-noise-limited detection and diffraction-limited imaging in scattering samples. The dependence on several parameters of the optical system and of the sample properties was investigated in a comprehensive study. Cross-talk increases with the wide-field diameter, numerical aperture, source coherence length, and sample optical density; and strongly depends on sample anisotropy. We showed that spatially incoherent illumination realized with a thermal light source permits cross-talk suppression in wide-field OCT, i.e. rejection of multiply scattered light to a level comparable to that of point scanning OCT. We performed a theoretical study which revealed that the power per spatial mode radiated by thermal light sources is too low to permit a high signal-to-noise ratio while maintaining a fast acquisition speed. Therefore, wide-field OCT realized with either spatially coherent or spatially incoherent illumination suffers from inherent fundamental limitation. This led us to investigate the possibility of exploiting a spatially incoherent light source brighter than a thermal light source. We came to the conclusion that such a "pseudothermal" light source can potentially lead to wide-field OCT systems devoid of cross-talk and an image acquisition speed higher than that of a point scanning OCT system. However, the attractive properties of pseudothermal light sources could be gained at the expense of the simplicity and the economical advantages offered by thermal light sources. Furthermore, fast acquisition speed also relies on a performing "smart pixel detector array". Presently, such detectors do not have sufficient sensitivity and their frequency readout is too low as shown in our feasibility study. We performed a theoretical investigation of the potential of thermal light sources in terms of axial resolution and power per mode. The former revealed that the maximum power per mode is radiated at a wavelength higher than the spectral peak of a blackbody radiator. This led to the important practical conclusion that, at 6000 K, the maximum power is collected in the therapeutic spectral window in OCT (600 - 1300 nm), while at 3000 K this peak is shifted out of the therapeutic window leading to significant power losses. More generally, our work provides a design tool for choosing the optimal thermal light source for a given therapeutic window in terms of signal-to-noise ratio. Currently available sources at 6000 K consist of high pressure gas arc lamps providing a spectrum endowed with spectral lines deleterious for OCT. By suppressing a portion of the spectrum devoid of spectral lines of a mercury arc lamp, we obtained amongst the highest axial resolution so far reported in OCT. Furthermore, the importance of the speckle statistics in OCT incited us to clarify the origin of a difference between two theoretical results reported in the literature. Indeed, two calculations of the amplitude distribution of speckles in OCT, each of them based on a different mathematical formulation, yield different results. We showed that a modification of an initial assumption in one of the formulation leads to equivalent results. In conclusion, this thesis provides a deeper understanding of the potential and limitations of widefield OCT, leading to important design rules. Moreover, it presents a new comprehensive model of OCT putting in question other widely used models.

EPFL2004

Self-referenced method for optical path difference calibration in low-coherence interferometry

Boris Karamata, Theo Lasser, Markus Laubscher, René Salathé

A simple method for the calibration of optical path difference modulation in low-coherence interferometry is presented. Spectrally filtering a part of the detected interference signal results in a high-coherence signal that encodes the scan imperfections and permits their correction. The method is self-referenced in the sense that no secondary high-coherence light source is necessary. Using a spectrometer setup for spectral filtering allows for flexibility in both the choice of calibration wavelength and the maximum scan range. To demonstrate the method's usefulness, it is combined with a recently published digital spectral shaping technique to measure the thickness of a pellicle beam splitter with a white-light source. (C) 2003 Optical Society of America.

2003

Endoscopic low coherence interferometry applied to tri-dimensional contouring of upper airways

Yves Delacrétaz

This thesis presents a novel approach for optical tri-dimensional contouring, requiring only one acquisition to extract a contour depth. It is based on an interferometric setup, using a reduced coherence light source, and is fully adaptable to endoscopic procedure commonly in used in ear-nose-throat (ENT) medicine. It has been demonstrated that the method can be successfully applied to imaging of porcine upper airways. Although this method uses interferometry to obtain depth range measurement, the phase signal is not directly evaluated, rather the location of the coherent superposition of two waves is extracted, and thus provides a robust technique even in harsh environment. The key feature is to extract the coherent area on the image by locating intentionally induced fringes created by off-axis superposition of a smooth reference wave and an object wave coming from a rough or diffusing sample through an optical system with limited aperture. A model for the propagation of the light through the optical system has been developed. It takes into account the broadened emission spectrum of the light source, as well as the statistical speckle effect induced by the illumination of a rough surface with partially coherent light. Simulated results have been carefully compared with measurements, both in the spatial domain and in the Fourier domain (spatial frequencies response), which has permitted to better understand the processes involved in the creation of the interference fringes, and improve the design of the device. Different filters have been investigated in order to extract the depth information from acquired interferograms. By using Gabor filterbanks, which do not need any a priori knowledge of the signal, selectivity of the extracted signal has been enhanced by a factor of 1.3 compared to a simple local Fourier spectrum evaluation, and time of the extraction procedure divided by 6, even when compared to fast iterative calculation in the Fourier spectrum. Moreover, by imposing the frequency and/or orientation of the signal to detect, which can be known by a simple calibration procedure, it has been proved that the selectivity can be further enhanced by a factor of 2.5, keeping the calculation time as short or even shorter. A compact prototype has been designed and built, with a specific multiple fibers arrangement for illumination. The system is separated in two parts: the first one is directly attached to the rigid endoscope, and contains imaging as well as recombining object/reference wave optics, and the detector. It fits into a box with overall dimensions of 115 × 83 × 94 mm. The second part contains the laser source, the scanning arm and the injection optics for the fibered system. It is designed to be left on a small cart. This device can potentially be brought to the bedside, and used during routine endoscopic check-up. Associated with the device, a complete framework has been elaborated in order to simplify the acquisition procedure as well as the signal processing. This results in a fully automated environment taking in charge hardware control, data storage and signal processing, up to the tri-dimensional scene rendering.