Essay: Optical coherence tomography

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Optical coherence tomography is a technique that generates slice images of a three-dimensional object (Fercher, Drexler, Hitzenberger, & Lasser, 2003). Compare to other tomographic techniques, for example CT scan, OCT is a non-invasive and it is particular importance in the medical field. Optical coherence tomography is physically founded on optical diffraction tomography (ODT) (Fercher, Drexler, Hitzenberger, & Lasser, 2003).
Optical coherence tomography is based on the principle of Michelson interferometry (Jaffe & Caprioli, 2004). Figure 1 below shows a basic schemes of an OCT. A commercial OCT setup consist and SLD (SuperĀ Luminescent Diode) light source, a beam splitter, a reference mirror, a detector and the object of interest. The low coherence light is coupled to a fiberoptic travels to a beam splitter and is directed through the ocular media to the retina and to the reference mirror (Jaffe & Caprioli, 2004). Light that passes through the eye is reflected by structure in different retinal tissue layers. When the distance between the light source and the retinal tissue is not equal the distance between the light source and the reference mirror, the interference patterns are not aligned which result a low amplitude signal. The distance between the reference mirror and the beam splitter is continuously varied. At the point where the reference arm and the retinal arm are matched, the interference patterns are aligned and a maximum signal is detected (Jaffe & Caprioli, 2004). The scan procedures for OCT consist of two parts. First, a depth-scan is performed and the detail of a depth-scan is mentioned previously. It is also analogous to the A-scan in ultrasonography using light instead of sound. The second part of the OCT scan is the lateral-scan. It is performed by moving the sample or by scanning the probe beam illuminating the sample (Fercher, Drexler, Hitzenberger, & Lasser, 2003) and a 2 dimensional cross-sectional image will be obtained. To obtain a 3 dimensional image, a series of adjacent depth-scan followed by a lateral-scan is performed.
Figure 1 : Standard OCT scheme based on low time coherence Michelson interferometry.
Unlike x-ray, PET or SPECT which use unscattered photons to obtain tomographic, OCT uses backscattered photons (light also propagates twice through the same object region) (Fercher, Drexler, Hitzenberger, & Lasser, 2003). The main parameters of an OCT are optical power, wavelength, penetration, resolution, sensitivity and acquisition rate. These parameters either are interdependent or determined by the properties of the object of interest for example, the choice of wavelength of the light source is depend on the major makeup molecules of the object of interest to minimize the absorption coefficient. Unlike most of the tomographic technique, OCT’s resolution is split into two parts. The first part is called axial resolution and the second part is called transverse resolution and they depends on different factors (Jaffe & Caprioli, 2004). The axial resolution depends on the wavelength and bandwidth of the incident light. The wavelength determines the penetration depth of the chosen light to the object of interest. A correctly chosen wavelength light source can greatly reduce the absorption of light by the object and improve the penetration depth. A boarder spectral bandwidth also enhances axial resolution by producing a shorter coherence light beam (Jaffe & Caprioli, 2004).

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