Coherence Tomography (OCT)
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.
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.
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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