Martin Villiger
Optical coherence tomography (OCT) is an interferometric imaging technique that can provide depth resolved cross-sectional views of biological tissue. OCT employs light with low temporal and yet high spatial coherence. The sample is illuminated point by point and raster scanned in the lateral directions. Part of the incident light is back-scattered by differences in the tissue refractive index and combined with a strong reference signal. The detection in the Fourier, or frequency domain (FDOCT), separates the interference signal spectrally and records the resulting pattern with a line camera. Processing of this pattern extracts the sample structure along the axial direction but without axial scanning and in parallel with high resolution of 2-3 µm. The interferometric detection conveys a very high sensitivity that allows imaging several hundred micrometers deep into the tissue. A single two dimensional scan thus can extract the three dimensional structure of the object, resulting in very high volume acquisition rates and offering an exceptional speed advantage over other optical imaging methods. Combined with its ability to measure an intrinsic sample property without the need of adding extrinsic labels or contrasting agents, FDOCT has become a confirmed tool for minimally invasive in vivo measurements and comprehensive volumetric imaging. While the axial resolution in OCT is defined by the coherence length of the employed light source, it is the optical focusing that specifies the resolution in the lateral direction, which reaches in general only approximately 10 µm. To improve the resolution, classical optical coherence microscopy (OCM) uses higher numerical apertures. However, this implies a reduction of the lateral dimension of the focal spot which leads to a dramatic decrease in the depth of field (DOF). This imposes a severe limitation on FDOCT's parallel depth extraction. Therefore, the motivation behind this thesis work was to circumvent the compromise between the lateral resolution and the depth of field by engineering an extended focal volume (xf). Combining FDOCT's assets of speed and sensitivity with the high lateral resolution of microscopy provided a very promising tool for rapid in vivo imaging at close to cellular resolution. To study the mechanisms limiting the DOF and to investigate what impact they have on the tomogram reconstruction process, we developed a model for image formation in FDOCT. Generally, the tomogram is reconstructed from the two dimensional interference patterns in the individual spectral channels of the detection. By making use of the coherent transfer functions (CTF), the spatial frequency content made accessible by each channel, was analyzed. This provided a novel perspective on the entire tomogram formation. We were able to show that the out-of-focus structures suffer from two signal degrading mechanisms. First, a de-focusing effect, induced by additional phase arguments in the spatial frequency domain provokes a lateral blurring. Second, the setup's reduced transmission efficiency for out-of-focus structures results in an amplitude scaling. The application of the CTF enabled us to illustrate the intrinsic link between these two effects and to develop optical design strategies and novel configurations to circumvent their detrimental impact on the quality of the tomogram. It is known that with the use of a conical lens, it is possible to produce a so-called diffraction-less beam. Close to the optical axis, the radial envelope of its field amplitude follows a Bessel function of order zero and is maintained upon propagation over a long axial range. We could show that a combination of such a Bessel-like beam with a Gaussian detection indeed limits both the de-focusing and amplitude scaling phenomenon, all the while improving the lateral resolution. The price to pay is a reduced signal amplitude also at the in-focus position, as compared to a Gaussian focal volume of a classical confocal setting. Another possibility is to combine the Bessel-like illumination with a Bessel-like detection, although with differently spaced side lobes in its envelope. These different combinations were evaluated both analytically and with computer simulations. Several of the proposed solutions were implemented in practice to confirm the theoretical predictions. Considering that the theoretical analysis was performed in the spatial frequency domain and in terms of the CTF, the corresponding system parameters had to be measured. Lacking an adequate technique, we developed and validated a method which derived characteristic parameters of the optical system's CTF from the measurement of a simple rubber surface. The tomogram of this sample consists of a fully developed speckle field. Although generally disfavored for degrading the tomogram quality, in this case the speckle shape provided information on the system characteristics because it was imposed by the illumination and detection apertures of the system in question. Since the principal interest behind the development of xfOCM was its amenability to live imaging, we used this novel concept to visualize individual islets of Langerhans in a mouse pancreas. These endocrine islets are assemblies of insulin-secreting beta cells contained within the exocrine tissue of the organ. They are of primary importance in the research on diabetes, but their large diversity in size, down to the micrometer scale, and their localization in the pancreas have so far hindered their observation in vivo. To meet the conditions for successful live imaging, we implemented the xfOCM into a specially designed setup that could provide enough mechanical robustness to allow for measurements over long periods. In addition, the set up was equipped with a fast beam scanning device to achieve high volume acquisition rates. The signal processing and the user interface were conceived in such a way that they offered a rapid visualization of the tomograms, providing an instantaneous feedback and facilitating sample handling. With the developed architecture we achieved label-free in vivo visualization of pancreatic lobules, ducts, blood vessels and individual islets of Langerhans and could demonstrate the potential of the xfOCM in diabetes research for high resolution studies on live animals. Besides the aim of an extended focal volume, the control over the design of the CTF also enabled the realization of a dark field configuration. Signals with low spatial frequencies are suppressed in this configuration, which enabled the measurement of the weak scattering signature of near transparent live cell samples. In conclusion, this thesis provides a better understanding of the image formation involved in the FDOCT. Several strategies to increase the trade-off between lateral resolution and DOF were proposed and validated experimentally. The suitability and potential of this approach for small animal in vivo imaging was demonstrated in the context of a relevant biological question.
EPFL2010
