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

This paper reviews the state of the art of optical coherence tomography (OCT), an interferometric imaging technique that provides cross-sectional views of the subsurface microstructure of biological tissue. Following a discussion of the basic theory of OCT, an overview of the issues involved in the design of the main components of OCT systems is presented. The review concludes by introducing new imaging modes being developed to extract additional diagnostic information.

Optical coherence tomography (OCT): a review

A micron-resolution particle image velocimetry (micro-PIV) system has been developed to measure instantaneous and ensemble-averaged flow fields in micron-scale fluidic devices. The system utilizes an epifluorescent microscope, 100–300 nm diameter seed particles, and an intensified CCD camera to record high-resolution particle-image fields. Velocity vector fields can be measured with spatial resolutions down to 6.9×6.9×1.5 μm. The vector fields are analyzed using a double-frame cross-correlation algorithm. In this technique, the spatial resolution and the accuracy of the velocity measurements is limited by the diffraction limit of the recording optics, noise in the particle image field, and the interaction of the fluid with the finite-sized seed particles. The stochastic influence of Brownian motion plays a significant role in the accuracy of instantaneous velocity measurements. The micro-PIV technique is applied to measure velocities in a Hele–Shaw flow around a 30 μm (major diameter) elliptical cylinder, with a bulk velocity of approximately 50 μm s-1.

http://microfluidics.stanford.edu/Publications/Publications/ParticleTracking_Diagnostics/Santiago%20Micro%20PIV%20ExperFluids.pdf

A particle image velocimetry system for microfluidics

Optical coherence tomography (OCT) is an emerging biomedical optical imaging technique that performs high-resolution, cross-sectional tomographic imaging of microstructure in biological systems. OCT can achieve image resolutions of 1-15 microm, one to two orders of magnitude finer than standard ultrasound. The image penetration depth of OCT is determined by the optical scattering and is up to 2-3 mm in tissue. OCT functions as a type of 'optical biopsy' to provide cross-sectional images of tissue structure on the micron scale. It is a promising imaging technology because it can provide images of tissue in situ and in real time, without the need for excision and processing of specimens.

Optical coherence tomography for ultrahigh resolution in vivo imaging

Using a low-coherence Michelson interferometer, we measure two-dimensional images of optical birefringence in bovine tendon as a function of depth. Polarization-sensitive detection of the signal formed by interference of backscattered light from the sample and a mirror in the reference arm give the optical phase delay between light that is propagating along the fast and slow axes of the birefringent tendon. Images showing the change in birefringence in response to laser irradiation are presented. The technique permits rapid noncontact investigation of tissue structural properties through two-dimensional imaging of birefringence.

/pdf/two-dimensional-birefringence-imaging-in-biological-tissue-4i9dovihva.pdf

Two-dimensional birefringence imaging in biological tissue by polarization-sensitive optical coherence tomography.

An optical Doppler tomography (ODT) system that permits imaging of fluid flow velocity in highly scattering media is described. ODT combines Doppler velocimetry with the high spatial resolution of low-coherence optical interferometry to measure fluid flow velocity at discrete spatial locations. Tomographic imaging of particle flow velocity within a circular conduit submerged 1 mm below the surface in a highly scattering phantom of Intralipid is demonstrated.

/pdf/optical-doppler-tomographic-imaging-of-fluid-flow-velocity-2fidjptvq9.pdf

Optical Doppler tomographic imaging of fluid flow velocity in highly scattering media.

We report the development of an optical technique for noninvasive in vivo imaging of tissue structure and blood flow dynamics with high spatial resolution (2 - 15 micrometer) in biological systems. The technique is based on coherence optical Doppler tomography (ODT), which combines Doppler velocimetry with optical coherence tomography to measure blood flow velocity at discrete spatial locations. The exceptionally high resolution of ODT allows noninvasive in vivo imaging of both blood microcirculation and tissue structures surrounding the vessel, which has significance for biomedical research and clinical applications. Tomographic velocity imaging of in vivo blood flow in a rat mesentery is demonstrated.

/pdf/optical-doppler-tomography-for-noninvasive-imaging-of-in-4leq59zqt3.pdf

Optical Doppler tomography for noninvasive imaging of in vivo blood flow

Optical methods, devices, and systems for non-invasively detecting transient surface displacements in a neuron are disclosed. Methods, devices, and systems provided may employ a phase-sensitive optical low coherence reflectometer. In addition, surface displacements due to action potential propagation in neural tissues may be detected in some embodiments using back-reflected light. According to some embodiments, exogenous chemicals or reflection coatings are not required. Transient neural surface displacement of less than 1 nm in amplitude and 1 ms in duration may be detected and may be generally coincident with action potential arrival to the optical measurement site. The systems and methods may be used for noninvasive detection of various neuropathies such as retinal neuropathies. They may also be useful in detecting the effects of various pharmacological agents.

Measurement of neural functionality using phase sensitive optical reflectometry

In vivo blood flow imaging with use of coherence optical Doppler tomography

Laser‐induced mechanical deformations in materials can be detected without contact using various optical methods. We have developed a low‐coherence optical probe to monitor thermal elastic deformations and acoustic transients in materials exposed to laser excitation. Our approach utilizes principles from low‐coherence interferometry with phase‐sensitive detection of the coherence function of backscattered light from two spatially separated sites in a test material. High spatial resolution and sensitivity of the optical probe may be used to identify subsurface light‐absorbing structures in turbid media and determine optical properties non‐invasively. The low‐coherence optical sensor may prove useful for non‐contact studies of tissue‐like materials in biomedical engineering.

D. Dave

Papers

Optical Doppler tomographic imaging of fluid flow velocity in highly scattering media.

Optical Doppler tomography for noninvasive imaging of in vivo blood flow

Measurement of neural functionality using phase sensitive optical reflectometry

In vivo blood flow imaging with use of coherence optical Doppler tomography

Low‐Coherence Optical Probe for Non‐Contact Detection of Photothermal and Photoacoustic Phenomena in Biomaterials