There have been three basic approaches to optical tomography since the early 1980s: diffraction tomography, diffuse optical tomography and optical coherence tomography (OCT). Optical techniques are of particular importance in the medical field, because these techniques promise to be safe and cheap and, in addition, offer a therapeutic potential. Advances in OCT technology have made it possible to apply OCT in a wide variety of applications but medical applications are still dominating. Specific advantages of OCT are its high depth and transversal resolution, the fact, that its depth resolution is decoupled from transverse resolution, high probing depth in scattering media, contact-free and non-invasive operation, and the possibility to create various function dependent image contrasting methods. This report presents the principles of OCT and the state of important OCT applications. OCT synthesises cross-sectional images from a series of laterally adjacent depth-scans. At present OCT is used in three different fields of optical imaging, in macroscopic imaging of structures which can be seen by the naked eye or using weak magnifications, in microscopic imaging using magnifications up to the classical limit of microscopic resolution and in endoscopic imaging, using low and medium magnification. First, OCT techniques, like the reflectometry technique and the dual beam technique were based on time-domain low coherence interferometry depth-scans. Later, Fourier-domain techniques have been developed and led to new imaging schemes. Recently developed parallel OCT schemes eliminate the need for lateral scanning and, therefore, dramatically increase the imaging rate. These schemes use CCD cameras and CMOS detector arrays as photodetectors. Video-rate three-dimensional OCT pictures have been obtained. Modifying interference microscopy techniques has led to high-resolution optical coherence microscopy that achieved sub-micrometre resolution. This report is concluded with a short presentation of important OCT applications. Ophthalmology is, due to the transparent ocular structures, still the main field of OCT application. The first commercial instrument too has been introduced for ophthalmic diagnostics (Carl Zeiss Meditec AG). Advances in using near-infrared light, however, opened the path for OCT imaging in strongly scattering tissues. Today, optical in vivo biopsy is one of the most challenging fields of OCT application. High resolution, high penetration depth, and its potential for functional imaging attribute to OCT an optical biopsy quality, which can be used to assess tissue and cell function and morphology in situ. OCT can already clarify the relevant architectural tissue morphology. For many diseases, however, including cancer in its early stages, higher resolution is necessary. New broad-bandwidth light sources, like photonic crystal fibres and superfluorescent fibre sources, and new contrasting techniques, give access to new sample properties and unmatched sensitivity and resolution.

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Optical coherence tomography - principles and applications

Current medical imaging technologies allow visualization of tissue anatomy in the human body at resolutions ranging from 100 micrometers to 1 millimeter. These technologies are generally not sensitive enough to detect early-stage tissue abnormalities associated with diseases such as cancer and atherosclerosis, which require micrometer-scale resolution. Here, optical coherence tomography was adapted to allow high-speed visualization of tissue in a living animal with a catheter-endoscope 1 millimeter in diameter. This method, referred to as "optical biopsy," was used to obtain cross-sectional images of the rabbit gastrointestinal and respiratory tracts at 10-micrometer resolution.

In Vivo Endoscopic Optical Biopsy with Optical Coherence Tomography

Diffuse optical tomography (DOT) is an ongoing medical imaging modality in which tissue is illuminated by near-infrared light from an array of sources, the multiply-scattered light which emerges is observed with an array of detectors, and then a model of the propagation physics is used to infer the localized optical properties of the illuminated tissue. The three primary absorbers at these wavelengths, water and both oxygenated and deoxygenated hemoglobin, all have relatively weak absorption. This fortuitous fact provides a spectral window through which we can attempt to localize absorption (primarily by the two forms of hemoglobin) and scattering in the tissue. The most important current applications of DOT are detecting tumors in the breast and imaging the brain. We introduce the basic idea of DOT and review the history of optical methods in medicine as relevant to the development of DOT. We then detail the concept of DOT, including a review of the tissue's optical properties, modes of operation for DOT, and the challenges which the development of DOT must overcome. The basics of modelling the DOT forward problem and some critical issues among the numerous implementations that have been investigated for the DOT inverse problem, with an emphasis on signal processing. We summarize with some specific results as examples of the current state of DOT research.

Imaging the body with diffuse optical tomography

We describe an original microscope for high-resolution optical coherence tomography applications. Our system is based on a Linnik interference microscope with high-numerical-aperture objectives. Lock-in detection of the interference signal is achieved in parallel on a CCD by use of a photoelastic birefringence modulator and full-field stroboscopic illumination with an infrared LED. Transverse cross-section (en-face, or XY) images can be obtained in real time with better than 1-microm axial (Z) resolution and 0.5-microm transverse (XY) resolution. A sensitivity of approximately 80 dB is reached at a 1-image/s acquisition rate, which allows tomography in scattering media such as biological tissues.

High-resolution full-field optical coherence tomography with a Linnik microscope

In this paper, we review the developments in optical coherence tomography (OCT) for three-dimensional non-invasive imaging. A number of different OCT techniques are discussed in some detail including time-domain, frequency-domain, full-field, quantum and Doppler OCT. A theoretical treatment is given and some relevant comparisons made between various implementations. The current and potential applications of OCT are discussed, with close attention paid to biomedical imaging and its metrological issues.

https://iopscience.iop.org/article/10.1088/0022-3727/38/15/002/pdf

Theory, developments and applications of optical coherence tomography

We present a new microscopy system for imaging in turbid media that is based on the spatial coherence gate principle and generates in parallel a complete two-dimensional head-on image without scanning. This system has been implemented in a commercial microscope and preserves the lateral resolution of the optics used. With a spatially incoherent source, speckle-free images with diffraction-limited resolution are recorded at successive depths with shot-noise-limited detection. The setup comprises a photoelastic modulator for path difference modulation and a two-dimensional CCD array and uses a multiplexed lock-in detection scheme.

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Full-field optical coherence microscopy.

Tagging of photon trajectories in scattering media is possible by application of a localized ultrasonic field to the sample and by measurement of the induced speckle modulation. Instead of using a single optical detector, which integrates the signal of many speckle grains, we propose a more efficient detection scheme that uses a CCD camera and parallel lock-in detection to record the full modulation of the speckle. The advantage of this multiplex detection is demonstrated, as well as the imaging capabilities of the process for biological tissues.

Ultrasonic tagging of photon paths in scattering media: parallel speckle modulation processing.

We describe the design of a versatile electronic system performing a lock-in detection in parallel on every pixel of a 2D CCD camera. The system is based on a multiplexed lock- in detection method that requires accurate synchronization of the camera, the excitation signal and the processing computer. This device has been incorporated in an imaging setup based on the optical coherence tomography principle, enabling to acquire a full 2D head-on image without scanning. The imaging experiment is implemented on a modified commercial microscope. Lateral resolution is on the order of 2 micrometers , and the coherence length of the light source defines an axial resolution of approximately 8 micrometers . Images of onion cells a few hundred microns deep into the sample are obtained with 100 dB sensitivity.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Low-coherence in-depth microscopy for biological tissue imaging: design of a real-time control system

We present a new microscopy system based on the OCT principle, that uses a multiplexed lock-in detection scheme to generate a 2D head-on image in parallel without lateral scanning. Our 'full-field optical coherence microscopy' comprises a Michelson interferometer built with a polarizing beam splitter, and uses a photoelastic birefringence modulator to modulate the optical path difference between the two orthogonal polarizations. A novel signal processing method is used to achieve a demodulation in parallel on every pixel of a 256 X 256 CCD camera. A 840 nm electroluminescent diode with 20 micrometers coherence length is used to illuminate the field of view through the microscope objective lens. In-depth exploration of the sample is realized by changing the plane of focus. The lateral resolution of the images is limited by the camera pixel size and is 2 micrometers . The axial sectioning ability is approximately 8 micrometers . Having validated our setup on model samples, we now evaluate its performance on biological structures. As an example, images of onion cells from 50-400 micrometers below the surface are obtained in 1 s with 100 dB sensitivity.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Optical coherence microscopy for the in-depth study of biological structures: system based on a parallel detection scheme

We have constructed an interference microscope that produces, in real time, reflectivity and topography images of surfaces with depth discrimination better than 1 ?m. Intensity and phase images are obtained at the rate of 50??per second by use of a multiplexed lock-in detection and MMX assembler-optimized calculation routines. With a wavelength of 0.84 ?m, depth discrimination of 0.7 ?m and lateral resolution of 0.3 ?m were demonstrated, in good agreement with theory. Two-dimensional cross-sectional reflectivity and topography images taken at different depths in an integrated circuit are presented.

Martial Lebec

Papers

Full-field optical coherence microscopy.

Ultrasonic tagging of photon paths in scattering media: parallel speckle modulation processing.

Low-coherence in-depth microscopy for biological tissue imaging: design of a real-time control system

Optical coherence microscopy for the in-depth study of biological structures: system based on a parallel detection scheme

Real-time reflectivity and topography imagery of depth-resolved microscopic surfaces