Optical Coherence Tomography (OCT)
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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.

The ability to detect the positional delay of light reflecting from a tissue sample is at the heart of OCT. Low-coherence interferometry provides just that. A low-coherence light source has the potential to produce interference fringes only when integrated with light from the same source that has traveled nearly exactly the same distance3. This means that if light from such a source is split by a beam splitter into two equal parts, both of them reflect off of different objects, and combine to form one beam again, they will produce an interference fringe pattern only if thedistance they traveled while split was exactly the same.
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26-03-2012, 02:57 PM

Optical Coherence Tomography (OCT)

.pdf   OCT_Schmitt.pdf (Size: 351.48 KB / Downloads: 47)


THE APPLICATION of optical technology in medicine
and biology has a long and distinguished history. Since
the 18th century, the microscope has been an indispensable
tool of biologists. With the invention of the laser in the
early 1960’s, physicians gained a new surgical instrument.
The development of fiber optics led to the manufacture of
endoscopes that permit direct viewing of internal organs deep
in the body. In the modern clinical laboratory, new optical
technologies facilitate the chemical analysis of tissue samples
and the counting and sizing of blood cells. In spite of these
and other advances, few of the optical instruments used in
medicine today take advantage of the coherent properties of
light. Even most instruments that employ lasers, the ultimate
generators of coherent light, can be classified as incoherent
optical systems because the focused laser beam serves mainly
as a source of illumination or concentrated heat. Perhaps one
of the reasons why optical coherence tomography (OCT) has
attracted the attention of engineers and scientists working in
the photonics field is that it has the potential to become the
first diagnostic imaging technology in which coherent optics
features prominently.

Fig. 1 shows the basic components of an OCT system. At
the heart of the system is an interferometer illuminated by a
broadband light source. In this section the interferometer is
first stripped to its bare essentials for analysis. Complications
introduced by scattering in a tissue sample are introduced later
in the section.

This section presents an overview of the issues involved
in the design of the optical components of the OCT system
(Fig. 1). Since most of the technology employed in OCT is
borrowed from the optical communications,

As a noninvasive imaging method, optical coherence tomography
must rely on the intrinsic variation of tissue properties to
differentiate tissue constituents. In the majority of applications
of OCT, the spatial variation of the coherent backscatter cross
section is the primary source of contrast. In principle, however,
any physical property that alters the amplitude, phase, or
polarization of the sample beam can be used to extract
information of diagnostic value. In this context, recent efforts
have turned to exploring ways of exploiting new contrast
mechanisms. In the last few years, four new OCT imaging
modes have been demonstrated: 1) polarization; 2) Doppler;
3) absorption; and 4) elasticity. A brief review of each mode
is given in the following paragraphs under separate headings.

Fueled by the explosion of innovations in the photonics
field, the development of optical coherence tomography (OCT)
has progressed rapidly over a period of less than 10 years.
Whether this pace of development can be sustained and
eventually lead to widespread use of OCT as a medical
diagnostic technique depends on the ability of researchers to
solve a number of tough problems that currently limit the
performance of OCT systems.
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28-03-2012, 11:09 AM

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