In an effort to provide more detailed information about epileptic lesions in neural tissuefor the purpose of efficient removal, an Optical Coherence Tomography system was built to provide non-destructive imaging. The device is based upon the principle of a low-coherenceinterferometer. Such an instrument can detect if a particular point within a sample of tissue reflects light; an integration and interpretation of many scans of many points provides a highresolution image. The device built for this investigation has been successful in scanning semi-reflectiveobjects such as glass and glossy plastic. Objects with low reflectivity or high degrees of scattering did not produce strong enough signals to be detected successfully. One-dimensional scans were accomplished with reasonable speed and under computer control. Limited twodimensional scans were also taken with much lower speed and with constant human intervention. Optical Coherence Tomography (OCT) is an imaging technique that is similar in principle to ultrasound, but with superior resolution. It relies on exposing a sample to a burst of light and then measuring the reflective response from different depths and is therefore capable of scanning non-invasively beneath the surface of the sample. In ultrasound imaging, it is relatively easy to measure the time delay of each reflected packet. However, for light pulses, interferometry must be used to measure the displacement with meaningful accuracy. The amount of light reflected from each point within the scanning window in the sample is plotted graphically as an OCT image. The goal of this investigation is to use Optical Coherence Tomography to image epileptic lesions on cortical tissue from rats. Such images would be immensely useful for surgical purposes. They would detail how deep the lesion is, allowing for precise removal that neither removes an insufficient amount of damaged tissue nor extracts too much healthy tissue. Though commerical OCT systems already exist, they typically do not scan very deeply beneath sample surfaces. For the purpose of this study, a system must be constructed that scans up to 2 millimeters into tissue1. Unfortunately, an increase in axial depth necessitates a decrease in transverse (along the surface of the sample) resolution due to focal restrictions of the objective lenses2. However, this loss is acceptable for this investigation, as the main goal is to determine lesion depth and not to achieve perfect image clarity.

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Optical coherence tomography (OCT) is an optical signal acquisition and processing method allowing extremely high-quality, micrometer-resolution, three-dimensional images from within optical scattering media (e.g., biological tissue) to be obtained. In contrast to other optical methods, OCT, an interferometric technique typically employing near-infrared light, is able to penetrate significantly deeper into the scattering medium, for example ~3× deeper than its nearest competitor, Confocal microscopy. Depending on the use of high-brightness and wide-spectrum light sources such as superluminescent diodes or ultrashort pulse lasers, OCT has achieved sub-micrometer resolution (with very wide-spectrum sources emitting over a ~100 nm wavelength range). It is one of a class of optical tomographic techniques. A relatively recent implementation of OCT, frequency-domain OCT, provides advantages in signal-to-noise ratio and therefore faster signal acquisition. OCT systems, which are commercially available, are finding diverse application in areas such as art conservation and diagnostic medicine (notably in ophthalmology where it permits remarkable noninvasive images to be obtained from within the retina).



Optical Coherence Tomography, or ËœOCTâ„¢, is a technique for obtaining sub-surface images of translucent or opaque materials at a resolution equivalent to a low-power microscope. It is effectively Ëœoptical ultrasoundâ„¢, imaging reflections from within tissue to provide cross-sectional images.

OCT is attracting interest among the medical community, because it provides tissue morphology imagery at much higher resolution (better than 10 µm) than other imaging modalities such as MRI or ultrasound.

The key benefits of OCT are:
Live sub-surface images at near-microscopic resolution
Instant, direct imaging of tissue morphology
No preparation of the sample or subject
No ionizing radiation

OCT delivers high resolution because it is based on light, rather than sound or radio frequency. An optical beam is directed at the tissue, and a small portion of this light that reflects from sub-surface features is collected. Note that most light is not reflected but, rather, scatters. The scattered light has lost its original direction and does not contribute to forming an image but rather contributes to glare. The glare of scattered light causes optically scattering materials (e.g., biological tissue, candle wax, or certain plastics) to appear opaque or translucent even while they do not strongly absorb light (as can be ascertained through a simple experiment ” e.g., shining a red laser pointer through one's finger). Using the OCT technique, scattered light can be filtered out, completely removing the glare. Even the very tiny proportion of reflected light that is not scattered can then be detected and used to form the image in, e.g., a scanning OCT system employing a microscope.

The physics principle allowing the filtering of scattered light is optical coherence. Only the reflected (non-scattered) light is coherent (i.e., retains the optical phase that causes light rays to propagate in one or another direction). In the OCT instrument, an optical interferometer is used in such a manner as to detect only coherent light. Essentially, the interferometer strips off scattered light from the reflected light needed to generate an image. In the process depth and intensity of light reflected from a sub-surface feature is obtained. A three-dimensional image can be built up by scanning, as in a sonar or radar system.

Within the range of noninvasive three-dimensional imaging techniques that have been introduced to the medical research community, OCT as an echo technique is similar to ultrasound imaging. Other medical imaging techniques such as computerized axial tomography, magnetic resonance imaging, or positron emission tomography do not utilize the echo-location principle.

The technique is limited to imaging 1 to 2 mm below the surface in biological tissue, because at greater depths the proportion of light that escapes without scattering is too small to be detected. No special preparation of a biological specimen is required, and images can be obtained ˜non-contact™ or through a transparent window or membrane. It is also important to note that the laser output from the instruments is low “ eye-safe near-infra-red light is used “ and no damage to the sample is therefore likely.


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http://agingeyeglaucoma/OCT.pdf
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Optical Coherence Tomography (OCT) is an imaging technique that is similar
in principle to ultrasound, but with superior resolution. The principle OCT is white
light or low coherence interferometry. It relies on exposing a sample to a burst of
light and then measuring the reflective response from different depths and is
therefore capable of scanning non-invasively beneath the surface of the sample. In
ultrasound imaging, it is relatively easy to measure the time delay of each reflected
packet. However, for light pulses, interferometry must be used to measure the
displacement with meaningful accuracy. The amount of light reflected from each
point within the scanning window in the sample is plotted graphically as an OCT
image.
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
Full-field OCT optical setup
.
. The optical setup typically consists of an interferometer ( typically Michelson
type) with a low coherence, broad bandwidth light source. Light is split into and
recombined from reference and sample arm, respectively Components include:
super-luminescent diode (SLD), convex lens (L1), 50/50 beam splitter (BS), camera
objective (CO), CMOS-DSP camera (CAM), reference (REF) and sample (SMP).
Applications
1. Ophthalmology is a main field of OCT application. The first commercial
instrument too has been introduced for ophthalmic diagnostics (Carl Zeiss Meditec
AG).
2. 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.
3. 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.
4. 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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