Optical coherence tomography: Applications expand as technology matures 21 March 2013, SPIE Newsroom. DOI: 10.1117/2.3201303.07
Optical Coherence Tomography (OCT) is an emerging imaging method to record cross sectional views of semitransparent samples like biological tissue using near infrared light(NIR) . Due it`s unique properties it has found many applications in biomedical imaging and is nowadays a standard diagnostic too e.g. in ophthalmology. These unique properties include high resolution, which is typically in the range of a few micrometers and fast image acquisition with a large field of view of several millimeters with video speed frame rate. Additionally the measurement is contact free, which is an advantage in clinical settings. As a drawback, in comparison to other tomographic imaging methods like ultrasound or magnetic resonance tomography, the imaging depth is limited to a few millimeters.
The physical principle behind OCT is based on low coherence interferometry. The light which is backscattered from the sample is overlaid with reference light in an interferometer. A detector is used to record the interference between both beams and analysis of the interference fringes yields a depth profile at one point of the sample. By moving the spot across the sample a full cross sectional image or a three dimensional volume can be recorded. There are many different detecting and processing schemes, from which Time Domain OCT (TD OCT) and Frequency Domain OCT (FD OCT) are the most important. In TD OCT the mirror in the reference arm is moved and the interference is recorded with a broad band detector. Due to the low coherence length of the light source interference can only occur when the path length difference between the reference arm and the structure of the sample is matched. Because of this coherence filter the light backscattered from other depths in the sample is effectively suppressed. In FD OCT the reference mirror position is fixed and the signal at the interferometer exit is recorded with a spectrometer. Taking the long coherence length of each spectrometer channel into account, the sample information can be recorded across the whole depth without moving the mirror. Due to interference each scatterer in the sample produces a modulation across the spectrum. Eventually a Fourier Transform yields the same depth profile as in TD OCT. Beside of this general classification several related techniques exist, like Swept Source OCT or Full Field OCT.
The research at the PTT group compromises three areas: (1) the combination of spectroscopy and OCT called Spectroscopic OCT (SOCT) , (2) the translation of image processing concepts known from Digital Holography to OCT and (3) eventually the application of OCT and these extensions for imaging of biological tissue and technical samples .
In SOCT (1) depth resolved spectra can be calculated by using post processing methods like time frequency distributions. The detection of the sample`s spectral features is challenging because of the limited spectroscopic resolution and bandwidth of OCT systems. Additionally the wavelength and spatial dependent transfer function of the OCT system and speckle like noise disturb the signal. Furthermore there are only a few absorbing chromophores in the NIR, which can be detected by SOCT, and spectral changes due to scattering are mostly relatively feature less. Therefore we use modern signal processing tools like pattern recognition and other techniques like concepts from hyperspectral imaging for signal analysis in SOCT. The information of the analysis is presented as “digital staining” of the intensity based OCT images.
Applications of this concept compromise the analysis of cartilage under compression and brain tumors.
In Digital Holography (2) amplitude and phase of a sample can be reconstructed from the recorded interference pattern. This allows the measurement of a topographic height map of the sample with an accuracy which can be better than 1nm in axial direction. Typically only a monochromatic laser is used in DH. By using wavelength scanning holography, which is closely connected to Swept Source Full Field OCT we have introduced a new imaging concept called “Depth Filtered Digital Holography”. Here multiply holograms are recorded with different wavelengths and depth profiles are calculated using the concept of FD-OCT. After a depth of interest is identified, which can be for instance a buried layer in the sample, this depth region is transformed back to obtain again multiple holograms for different wavelengths. This approach reduces spurious reflections and makes thus quantitative phase imaging in semitransparent samples feasible. We extended this approach also to scanning spot FD-OCT systems. For this purpose we developed a method to reduce sample and system introduced aberrations and perform multi wavelength phase unwrapping .
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