The technique of low coherence reflectometry has been extended to tomographic imaging in the systems of the biological. The light from the coherence property which is reflected from the retina provides the required information from the reflective boundaries of the delay on the time-of-flight. This delayed information helps in determining the longitudinal site location of the reflection. This is achieved by the use of OCT system’s scans providing the location of the reflected site map in a two-dimension. It uses a mode that operates in the analogous which is then converted to ultrasonic pulse-echo imaging .
The resolution of the confocal microscopy of the longitudinal depends on the numerical aperture available. The resolution can be maintained by the OCT to high depth when the aperture is small.
The SLD output is tied into a solo mode fiber and divided at PZT the 50/50 coupler into sample and SLD reference arms. These reflections from 50/50 the two arms are put together at the Sample coupler and sensed by the photo-diode. The reference mirror which is used to do longitudinal scanning is then translated by the stepper motor Detector stage at 1.6 creating a reference 3.8 khz Doppler shift. Then the piezo-electric transducer (PZT) provides a further 21.2- kHz phase modulation to the interferometric signal.
The photodetector detects the output intensity of the interferometric modulation when the sample arm delay and the reference are almost matched. This is followed by the demodulation of the detector output of the 25 kHz frequency producing the interferometric signal. The signal that is modulated is digitized (AD) and further stored on the computer. The procedure is repeated producing a series of longitudinal scans. In addition to that, after each longitudinal scan, the lateral beam position is translated.
The high sensitivity of 10 fW is then accomplished by the optical heterodyne detection. The piezoelectric and Droppler shift modulation modulates the interferometric signal at a high frequency. This interferometer output filter the frequency in the demodulation separating the required interferometric from the noise outside the signal bandwidth.
The OCT is used for diagnosing the retinal structure at a high-resolution imaging for various diseases like macular degeneration, glaucoma and macular edema. Currently there is a big problem when managing and diagnosing the glaucoma since it is difficult to use the intraocular pressure to predict the progress of the glaucoma. When diagnosing the glaucoma, the optic nerves of the retinal nerve are to be taken into consideration and the techniques to be employed is the retinal nerve fiber layer (RNFL)
Schematic of the OCT Scanner
The site of reflective boundaries in the sample is measured with a high accuracy. Since the light travels slower in tissues than in air, the sample of the reflection is repeatedly to locate the spatial resolution of less than 2p,m delaying the optical which is then divided by the tissue’s group velocity to obtain the exact depth of the tissue.
The imaging optics and the beam steering devices are incorporated onto a standard ophthalmic slit-lamp biomicroscope in order to perform the accurate two-dimensional imaging of the retina. Then scanning is done at a high speed which is achieved by a fast slewing reference mirror. The collimated super luminescent diode light is focussed onto two orthogonally mounted computer controlled galvanometric beam-steering devices. A 60-D Volk lens which is combined with the patient’s eye optic, transmit the slit lamp onto the retina .
The Volk lens transmits the pupil of the super-luminescent diode path to the pupil’s entrance reducing the vignetting. The data is then control by the computer allowing the scanned patterns of the retina giving the actual image on the computer monitor. Always the scanning must be done in a fast image acquisition time in order to have a high resolution in vivo measurement.
The image acquisition time is given by
ϵscan depthscan velocityno.transverse pixels
ϵ is the efficiency factor which is 1.125. The acquisition times can be obtained by minimizing the number of transverse pixels
The RNFL is the main technique used since it has a moderately highly scattering layer likened to the subjacent retinal and vitreous structures. Since the nerve fibers are cylindrical in nature, the strength of the backscattered signal that is produced from the RNFL is very strong regardless of the incident angle of the light. The dependence of the light angle accounts for the reduction of the RNFL signal that is perceived at the margin of the optic disk where the nerve fibers incline into the optic nerve. By the use of the clinical ultrasound system, the tomograph images the optic disk and the retina with higher depth resolution. This scanning provides the detailed information about the thickness and the contour of the retinal structures forming a basis in clinical diagnostic procedure. For example, measureable valuation of the RNFL thickness is a main advance in the management of glaucoma.The cross-sectional images obtained by the RNFL are used for glaucoma diagnosis which gives a clear difference between healthy eyes and infected one . In addition, C-mode method is used to visualize the inner layers of an OCT scan manually. It permits the image to be segmented in a plane perpendicular to the scanning axis for easier visualization of the inner layers of an OCT cube scan. The method allows the user to evaluate the pathologies such as age-segmented and macular edema .
A- and B-scan ultrasonography is regularly used in ophthalmologic biometry and diagnosis to image and distinguish orbital disease and intraocular anatomy. These ultrasound capacities need physical contact with the eye and typically provide 150 mm of longitudinal resolution by using a 10-MHz transducer. The ultrasound of a high-frequency of about 50 MHz can be used to achieve an axial resolution of 30 mm, but its infiltration depth is limited to 4 mm because of acoustic attenuation in ocular media. Hence, the anterior segment of the eye is imaged with high resolution .
- There is no much computation required when using OCT for image reconstruction.
- Has the high detection sensitivity from the reflected signal.
- Structure of the eye can be captured in 3D.
- The device has database that can be used to highlight the local defects.
- Fine structures are captured since the device used a high-speed imaging giving more important information required.
- Allows earlier detection of the ocular diseases
- The specimen is not removed since the OCT provides real time visualization of tissues structure .
- There is a polarization mismatch between the interferometer arms when using a resolution in the ultrahigh resolution bringing changes in the shapes.
- There are unequal beam splitting in power and wavelength resulting in resolution reduction.
- There are interferences of the signal since there are electronics systems used resulting in the degradation of the axial OCT resolution.
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