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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