EYE-OCT.(OPTICAL COHERANCE TOMOGRAPHY)Submitted by:-

SANJIB PASHI

APPLIED ELECTRONICS AND INSTRUMENTATION ENG

HALDIA INSTITUTE OF TECHNOLOGY

Acknowledgment

I am sincerely thankful to Mr.Priyonko Das and also thankful to Mr.Monodipan Shaoo& Mr.Soumya Roy- Faculty of AEIE Dept. for this report. I thank them for their total support & UNENDING help to me during the entire report. I am also thankful to our friends who have helped me very much during the report for any kind of information, data, format, etc. Last but not the least; i am thankful to our college & its library for providing me the needful and supporting material for my report..

SANJIB PASHI

TABLE OF CONTENTS:

INTRODUCTION WHAT IS OCT? PRINCIPLE OF OCT DIFFERENT KIND OF OCT WHY WE NEED OCT? SIX WAYS MEASLES CAN AFFECT THE EYES OCT SCANS PROCEDURE IDEA ABOUT RETINA DIFFERENT KIND OF ANOMALOUS SCANNING TIPS CONCLUSION

INTRODUCTION

Eey Optical coherence tomography (EYE-OCT) is an established medical imaging technique.It is a non-invasive optical medical diagnostic imaging modality which enables in vivo cross-sectional tomographic visualization of the internal microstructure in biological systems.Since its invention (pioneered in Vienna by Prof. A.F. Fercher) in the late 1980s and early 1990s, the original concept of OCT was to enable non-invasive optical biopsy.OCT is based on low-coherence interferometry.In conventional interferometry with long coherence length (i.e., laser interferometry), interference of light occurs over a distance of meters. In OCT, this interference is shortened to a distance of micrometers, owing to the use of broad-bandwidth light sources (i.e., sources that emit light over a broad range of frequencies). Light with broad bandwidths can be generated by using superluminescent diodes or lasers with extremely short pulses (femtosecond lasers). White light is an example of a broadband source with lower power.

WHAT IS OCT?

Optical Coherence Tomography, or OCT, is a noncontact, noninvasive imaging technique used to obtain high resolution cross-sectional images of the retina and anterior segment.

Three-dimensional imaging technique with ultrahigh spatial resolution

Measures reflected light from tissue discontinuities

Based on interferometry

involves interference between reflected light and a reference beam.

Optical coherence tomography obtains imaging of subsurface mucosa with a resolution on the order of a low power microscop

PRINCIPLE OF OCT

Interferometry is the technique of superimposing (interfering ) two or more waves, to detect differences between them.

Interferometry works because two waves with the same frequency that have the same phase will add each other while two waves that have opposite phase will subtract.

Light from a source is directed onto a partially reflecting mirror and is split into a reference and a measurement beam.

The measurement beam reflected from the specimen with different time delays according to its internal microstructure.

The light in the reference beam is reflected from a reference mirror at a variable distance which produces a variable time delay.

The light from the specimen, consisting of multiple echoes, and the light from the reference mirror, consisting of a single echo at a known delay are combined and detected.

DIFFERENT KIND OF OCT

1>Time domain OCT : In time domain OCT the pathlength of the

reference arm is translated longitudinally in time. A property of low coherence interferometry is that interference, i.e. the series of dark and bright fringes, is only achieved when the path difference lies within the coherence length of the light source. This interference is called auto correlation in a symmetric interferometer (both arms have the same reflectivity), or cross-correlation in the common case. The envelope of this modulation changes as pathlength difference is varied, where the peak of the envelope corresponds to pathlength matching.

The interference of two partially coherent light beams can be expressed in terms of the source intensity, , as

where represents the interferometer beam splitting ratio, and is called the complex degree of coherence, i.e. the interference envelope and carrier dependent on reference arm scan or time delay , and whose recovery of interest in OCT. Due to the coherence gating effect of OCT the complex degree of coherence is represented as a Gaussian function expressed as[15]

where represents the spectral width of the source in the optical frequency domain, and is the centre optical frequency of the source

2> Frequency domain OCT (FD-OCT ): In frequency domain OCT the broadband interference is acquired with spectrally separated detectors (either by encoding the optical frequency in time with a spectrally scanning source or with a dispersive detector, like a grating and a linear detector array). Due to the Fourier relation (Wiener-Khintchine theorem between the auto correlation and the spectral power density) the depth scan can be immediately calculated by a Fourier-transform from the acquired spectra, without movement of the reference arm

3> Spatially encoded frequency domain OCT (spectral domain or Fourier domain OCT): SEFD-OCT extracts spectral information by distributing different optical frequencies onto a detector stripe (line-array CCD or CMOS) via a dispersive element (see Fig. 4). Thereby the information of the full depth scan can be acquired within a single exposure. However, the large signal to noise advantage of FD-OCT is reduced due to the lower dynamic range of stripe detectors with respect to single photosensitive diodes, resulting in an SNR (signal to noise ratio) advantage of ~10 dB at much higher speeds. This is not much of a problem when working at 1300 nm, however, since dynamic range is not a serious problem at this wavelength range.

4>Time encoded frequency domain OCT (also swept source OCT: TEFD-OCT tries to combine some of the advantages

of standard TD and SEFD-OCT. Here the spectral components are not encoded by

spatial separation, but they are encoded in time. The spectrum either filtered or

generated in single successive frequency steps and reconstructed before Fourier-

transformation. By accommodation of a frequency scanning light source (i.e.

frequency scanning laser) the optical setup becomes simpler than SEFD, but the

problem of scanning is essentially translated from the TD-OCT reference-arm into

the TEFD-OCT light source. Here the advantage lies in the proven high SNR

detection technology, while swept laser sources achieve very small instantaneous

bandwidths (=linewidth) at very high frequencies (20–200 kHz).

WHY WE NEED OCT?

Measles infections can harm the front or back of the eye, possibly causing vision loss or blindness

For better treatment we need OCT

SIX WAYS MEASLES CAN AFFECT THE EYES

Retinopathy: While rare, there are documented cases where the measles virus destroys the retina, a layer of cells in the back of the eye that convert light energy into electrical impulses that go to the brain. Retinitis can cause temporary and, in some cases permanent, vision loss.

Optic neuritis: This inflammation affects the optic nerve that sends signals from the back of the eye to the brain. This complication is relatively rare but can occur in measles patients who also develop encephalitis, or brain swelling, though cases without encephalitis have also surfaced. Acute cases may be treated with corticosteroids. Like retinitis, vision loss from optic neuritis can be temporary or permanent.

Conjunctivitis: Redness and watery eyes from conjunctivitis occur in nearly all measles patients. This type of pink eye usually develops early on in the disease and is a hallmark symptom along with fever, cough and a runny nose, often occurring before the telltale rash. This condition usually abates on its own as the disease runs its course.

Keratitis: Keratitis is an infection of the cornea, the window of the eye. In comparison with conjunctivitis, keratitis has more severe symptoms and far more dangerous

consequences. Symptoms include pain, redness, tearing and light sensitivity. It can feel as though foreign particles are lodged in the eye. Often treated with medicated drops, keratitis will cause temporary blurred vision, but if it leads to scarring the loss of vision can be permanent.

Corneal scarring: Corneal ulcers are open sores on the front of the eye that can occur when infected, either via the measles virus or from a bacterial infection that develops secondary to measles. Ulcers may appear as white dots on the front of the eye and are usually treated with topical antiviral or antibiotic drops. When the ulcers heal, they can scar over and leave opaque scar tissue that may inhibit vision and cause blindness

Blindness: Measles is a leading cause of childhood blindness in developing countries

where immunization programs for this disease are less established or often interrupted by conflict. When compounded by malnutrition, particularly vitamin A deficiency, measles is associated with corneal scarring from ulceration and keratitis, two of the most likely reasons for blindness from measles. Blindness from optic neuritis has also been noted.

OCT SCANNING PROCEDURE

REGIONS OF RETINA

At first eye of the patient is introduce before the sensor of the OCT

After few min. the full scane of the eye thus obtained which is quite

For purposes of analysis, the OCT image of the retina can be subdivided vertically into four regions

I. the pre-retina

II. the epi-retina

III. the intra-retina

IV.the sub-retina

Retinal Anatomy Compared to OCT:

The vitreous is the black space on the top of the image

We can identify the fovea by the normal depression

The nerve fiber layer (NFL) and the retinal pigment epithelium (RPE) are easily identifiable layers as they are more highly reflective than the other layers of the retina

This higher reflectivity is represented by the "hotter" colors (red, yellow, orange, white) in the false color representation of the OCT.

DIFFERENT KIND OF ANOMALOUS

A pre-retinal membrane with traction on the fovea:

a pigment epithelial detachment is causing the convexity:

Macular cyst:

Preretinal macular fibrosis has been variously termed by various investigators as surface wrinkling retinopathy, cellophane membrane or cellophane maculopathy, preretinal traction membrane, macular pucker etc.

The outer segments of the photoreceptors receive oxygen and nutrition from the choroid. If the retina is detached from the choroid, the photoreceptors will fail. The fovea has no retinal blood vessels and depends wholly on the choroid for its oxygen, so detachment of the macula leads to permanent damage to the cones and rods at the posterior pole, and loss of vision.

A cyst caused by vitreous traction in the macula, the tiny oval area made up of millions of nerve cells located at the center of the retina responsible for sharp, central vision.

Macular hole:

Serous retinal pigment epithelial detachment (PED):

The center of the macula is the fovea. Foveal cysts are caused by vitreous traction. Eyes that develop foveal cysts may remain stable, develop full-thickness macular holes

A serous pigment epithelial detachment occurs when the sensory retina detaches from the pigment epithelial layer due to inflammation, injury, or vascular abnormalities. A serous pigment epithelial detachment often occurs in conjunction with central serous chorioretinopathy. This is because of the fluid that accumulates in the subretinal

space.

SCANNING TIPS

Minimize patient fatigue by keeping scan time to a minimum. Never scan an eye for more than 10 minutes (FDA regulation).

Keep the cornea lubricated. Use artificial tears and have the patient blink when you are not saving a scan pass.

Reflectivity may be further enhanced by moving the focus knob on the side of the OCT unit.

CONCLUSIONOptical coherence tomography is a potentially useful technique for high depth resolution, cross-sectional examination of the fundus.To demonstrate optical coherence tomography for high-resolution, noninvasive imaging of the human retina. Optical coherence tomography is a new imaging technique analogous to ultrasound B scan that can provide cross-sectional images of the retina with micrometer-scale resolution.

References:

www.Google.comwww.Wikipedia.orgwww.ncbi.nlm.nih.govwww.classteacher.comwww.geteyesmart.orgwww.macula.org