Optical Coherence Tomography
Published on Feb 20, 2020
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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