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PROGRESS ON DEVELOPING ADAPTIVE OPTICS-OPTICAL

COHERENCE TOMOGRAPHY FOR IN VIVO RETINAL IMAGING:

MONITORING AND CORRECTION OF EYE MOTION ARTIFACTS

PRESENTED BY SWETHA.P

S7 EC1 ROLL NO.67

Retinal Imaging Modalities

Fundus camera

Scanning Laser Ophthalmoscope (SLO)

Optical Coherence Tomography (OCT)

FUNDUS CAMERA

Provide magnified view of fundus

Low power microscope with an attached camera

Reflected light pass through eyepiece

Button is pressed to take a picture

Normal fundus photographs of left

eye (right image)

SCANNING LASER OPTHALMOSCOPE (SLO)

Technique of confocal laser scanning microscopy for

diagnostic

Combined with adaptive optics technology to provide sharper

images

Uses horizontal and vertical scanning mirror

SLO Retinal image of a left eye

OPTICAL COHERENCE TOMOGRAPHY

Optical signal acquisition and processing method

Three-dimensional images from within optical scattering media (e.g., biological tissue)

The principle of OCT is white light or low coherence interferometry

Captures micrometer-resolution, three-dimensional images

Ultrashort pulsed lasers and supercontinum lasers are used

A- SCAN AND B-SCAN

A-SCAN

Determine eye length for calculation of intraocular lens

Performed using ultrasonic wave

It gives the information in the form of one dimensional

B SCAN

Brightness scan

Produce a two-dimensional, cross-sectional view of the eye

Adaptive optics

Makes necessary optical correction

Permit faster modulation (higher frame rates)

FEATURES

Reduced speckle size

Increased lateral resolution

Components of Adaptive optics

Wavefront sensor

Wavefront corrector

Control system

The wavefront sensor and corrector measure and correct the

eye's wave aberrations

The AOcontroller controls the interaction between the

wavefront sensor and the corrector element

Interprets the wavefront sensor data and computes the

appropriate wavefront corrector drive signals.

AO systems operating in closed-loop

Measured wavefront is the error signal that gets fed back to the

controller

AO-OCT OCT design configurations have been combined with AO

Assessed using -standard AO and OCT metrics

More accurate and clear image

Point-spread functions achieved by

different retinal imaging systems

Comparison in resolution •

OCT axial resolution Δz

depends only on the

coherence properties of the

light source

SLO system depends on the focusing

geometry of the beam used for

imaging

Transverse resolution (Δx) in OCT and SLO depends on the light source wavelength and

NA of the imaging optics.

DRAWBACKS OF TRANSVERSE RESOLUTION

Limited depth range over which the retinal structures remain in

focus

Structures in focus show high contrast

Out-of-focus areas appear similar to those in the low lateral

resolution OCT scan

MOTION ARTIFACTS

Micro saccadic jerks - constant move of eye from one feature

to other

Slow drift of the eye

High frequency micro tremors - involuntary movement due

muscle contraction

determination of motion artifacts

Co-register the b-scans

Co-register the b-scans to maximize the cross-correlation

between each adjacent pair

Real-time hardware-based correction

Additional light beam is used to determine distance to cornea

advanced SYSTEM

Yellow rays-AO-OCT path

Blue rays-AO-SLO path

Red rays-common path for both systems.

Hslo-slo horizontal scanner

HOCT-OCT horizontal scanner

V-vertical scanner

D-dichroic mirror

Dm-deformable mirror for adaptive optics

Pbs-pellicle beam splitter

Lf-low-pass optical filter,

SHWFS-shack-Hartman wavefront sensor.

AO-OCT Data Acquisition

OCT use 836 nm light with a bandwidth of 112 nm

For SLO use 683.4 nm light with a bandwidth of 8.2 nm

SLO and OCT beams share the same path

Hslo,Hoct-separate SLO light from OCT light for the

horizontal scanning mirrors

Both AO-OCT volume and AO-SLO image cover the same

lateral area of the retina

B-scan acquisition is repeated for different positions of the

OCT horizontal scanner

AO-SLO image cover the same lateral area of the retina

Timing diagrams of vertical scanner (VS) and

horizontal OCT/SLO scanners HSOCT, HSSLO for

volumetric data acquisition.

Schematic of main acquisition planes of AO-OCT

(B-scan) and AO-SLO (C-scan) in our instrument

Experimental setup

Detection of Eye Motion

Artifacts Uses AO-SLO image series to track eye movement

Construct a reference image

Several frames distributed throughout the AO-SLO image

Avoid frames that have significantly lower overall brightness

Register the reference frames to maximize the cross

correlation between each selected frame

Average the registered frames

Divide the AO-SLO images into equally sized strips

Displacement of a strip from its expected location on the

reference image is the average displacement of the target

Correction of eye motion

artifacts

Advantages OCT can build up clear 3D images of thick samples by

rejecting background signal

Images can be obtained by ‘non-contact’

OCT as an echo technique is similar to ultrasound imaging

The motion corrected AO-OCT volumes reveal the actual

sampling pattern that are affected by motion artifacts

Disadvantages

If the AO-SLO images do not exhibit a distinct texture, the

algorithm will not be able to construct a good composite

reference image

The conversion factors to change eye motion detected in

AO-OCT pixels must be accurate to ensure the correct

replacement of A-scans to their actual sampling locations

Future scope

Improved diagnostic and monitoring of many eye diseases

(eg. Glaucoma)

Averaging of multiple volumes might allow visualization of

cellular structures that are not visible on a single volume due

to insufficient signal intensity or presence of coherence noise

Successful visualization of retinal ganglion cells(the cells that

send the signals from the retina to other parts of the brain)

CONCLUSION AO-OCT is a relatively novel retinal imaging technology that

still continues to develop

Transverse chromatic aberrations shift the relative lateral position of OCT and SLO beams on the retina

Motion correction of in vivo AO-OCT volumes of the human retina has potentially significant benefits for vision and physiology

Reduce the number of scans need to be captured, ultimately reducing imaging time and increasing patient comfort

REFERENCE Optical coherence tomography

Source: http://en.wikipedia.org/w/index.php

Adaptive Optics in Retinal Imaging

Source: http:/ / www.adaptiveoptics. org/ News_1006_3. html

Guyton AC, Hall JE. Human physiology and mechanisms of

disease. 6th ed. Philadelphia (PA): W.B. Saunders

Company;1997. p. 400–15