Air Force Air Force Portable Device for Retinal Imaging Portable Device for Retinal Imaging

Abdelhamid JnaneAbdelhamid JnaneMentors: Zhongping ChenMentors: Zhongping Chen

QiangQiang WangUniversity of California, IrvineUniversity of California, Irvine

Beckman Laser InstituteBeckman Laser Institute

OUTLINEOUTLINE

Background Goals Device setup Components System parameters Device component assembly Sample OCT Images Experimental design Wiener filter implementation

BackgroundBackground

Field-deployable screening device to detect lesions of the retina– To facilitate early diagnostic of ocular laser

induced injuries caused by laser weapons and laser guided aiming devices.

– To reduce the risk of catastrophic failure of vision.

– To ensure that flight personnel will conduct missions with full visual acuity.

BackgroundBackground

Replacement of the current photographic fundus imaging device– Standard procedure for evaluating ocular lesions, it

provides only two-dimensional retinal imaging

The newly developed optical coherence tomography technology with integrated micro electro mechanical system (MEMS)

To provide a three-dimensional of retinal images’ surface

BackgroundBackground

The proposed field deployable imaging device will find many other applications– In ophthalmology clinical practice where

evaluation and diagnosis of retinal health is very important

– Anyone engaging in a potentially dangerous activity requiring superb visual performance

GoalGoal Designing a compact & portable OCT system

– Features Dimensions: 8” x 8” x 4” Sensitivity of 99.3 db High speed (20,000 A-lines/s ) High resolution (12 m ) Coherence length of 9.8mm at 2 kHz sweep rate

Testing our device in vivo animal and human subjects– Evaluating retinal injuries in hamster and rats

Filtering the noise that corrupted the source signal (ripples)

Device SetupDevice Setup

Figure 1: Schematic of the FDOCT system: collimator; Atte., neutral density attenuator; D1 and D2, photodetectors.

ComponentsComponents

– Collimator (OFR Inc., 1060 nm)– Gold mirror (Thorlabs Inc., ½”)– 2 circulators (Agiltron Inc., 1060 nm)– 70/30 2x2 couplers (AC Photonics Inc. 1060 nm)– 50/50 2x2 couplers (AC Photonics Inc. 1060 nm)– X stage (Newport Inc.)– Light source (Santec Inc., 1060 nm, 28 kHz)– Aluminum case (Hammond Manufacturing, 10.03” x

9.63” x 3.84”)

System ParametersSystem Parameters

Imaging speed: 20 k A-lines/secondPower delivered to sample arm: 1.2 mwPower delivered to reference arm: 0.4 mw

Device HardwareDevice Hardware

Compact OCT systemNon-compact OCT system

Device ComponentDevice ComponentAssemblyAssembly

Sample OCT Image Sample OCT Image

Human eye image

Sample OCT Image Sample OCT Image

Sample OCT Image Sample OCT Image

Experimental DesignExperimental Design In vivo animal clinical trial experiment will be conducted to test the

effectiveness of the new device.

Imaging devices– Portable OCT system– Photographic fundus imaging system

Evaluation of images by ophthalmologists – Group 1: photographic fundus images only– Group 2: photographic fundus images and OCT images

Ophthalmologist evaluation compared to histology evaluation

Group A (5 Rats)

• 23.6 mw, 0.25 s

Wiener FilterWiener Filter

The wiener filter is an adaptive filter.

It tailors itself to be the “best possible filter” for a given dataset.

Below is a simple version of the derivation for the wiener formula.

Wiener FilterWiener Filter

Standard equation to model a signal with noise:

y [n] =x [n] +n [n] (1)

We want to pass this y [n] through a filter ‘h’

– To get back something that very closely matches our original signal x, ( x)

Design a filter that minimizes the difference between x and x.

– minimizing the least mean square error between x and x [x − ˜x] 2 (2)

Wiener FilterWiener Filter

Since x is h*y, we have: [ x − h * y ] (3)

Expanding this expression and taking the Fourier transform of the expression to get the power spectra:

– ∑(j) ( (Xj − HjYj))2 (4)– ∑(j) ((Xj − Hj (Xj + Nj))2 (5)

After simplification we get the following formula for H

– H (f) = (|X (f) |)2/ (|X (f) |)2 + (|N (f) |)2 (6)

Spectral ReshapingSpectral Reshaping

Small ripples in the light source spectrum caused by antireflection coating of the semiconductor optical amplifier

Ripples & other types of dispersions in the optical fiber components modulate the fringe contrast of the spectral interference signal

Spectrum of light with ripple and side lobes

Spectra and Gaussian Fit from Spectra and Gaussian Fit from Glass SlideGlass Slide

• Solid curve: Spectrum determined from a single image

•Dashed curve: Gaussian fit to each spectrum

Coherence Envelopes Determined from Glass Slide

• Dotted curve: Uncorrected response

• Solid curve: Corrected response

Design of Wiener FilterDesign of Wiener Filter

Determine the envelope of the wavelength-dependent fringe contrast– Design a spectral shaping filter from the OCT spectral signals

Spectral interference signal can be rewritten as follows:

– S j (k) = Se (k) *cos φ j(k) (1)

contrast envelope* fringe where φ j(k) is the phase of the j-th fringe

Design of Wiener FilterDesign of Wiener Filter The ensemble average of spectral interference in j is expressed as

– (Sj(k)^2)j = Se(k)^2 cos^2φ j(k)j

Simplified equation of the contrast envelope:

– Se(k) =√(2/N*Σ(j)Sj(k)^2).

Substituting Se(k) into the following the Wiener filter:

– W(k) =(Se(k)/(Se(k)2 +nc))*Gauss(k)– nc is a constant depending on the SNR of the detection system– Gauss(k) is a Gaussian window to reshape the spectrum to a Gaussian profile.

Implementation of Wiener Implementation of Wiener Filter Filter

SpecEnv[j] = A + B1*j + B2*j^2 + B3*j^3

+ B4*j^4 + B5*j^5 + B6*j^6 + B7*j^7+ B8*j^8

– Where: A=23.84411; B1=0.95364; B2= -0.0062 ;B3=9.5098E 5; B4= -7.19185E7; B5=2.49286E-9; B6= -4.32256E-12; B7=3.66599E-15;B8=1.21285E-18

GuassionEnv[j] = 100*exp(-(j-425.0)*

(j-425.0)/240.0/240.0); SpecReshape[j]=GuassionEnv[j]/

(2.0*SpecEnv[j] +constant factor);

Axial PSF without Spectral Axial PSF without Spectral ReshapingReshaping

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Axial PSf without spectral reshaping

Single image of the glass mirror

Coherence Envelopes Determined from Glass Slide

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Spectrum envelope of the source

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Specrtrum envelope of the source

Axial PSF with Spectral Axial PSF with Spectral ReshapingReshaping

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Single image of the glass mirror

ConclusionConclusion

We have developed FDOCT device with 1m swept light source which has the following specifications:

Dimensions of 8” x 8” x 4”; Sensitivity of 99.3 db; High speed (20,000 A-lines/s); High resolution (12 m); Coherence length of 9.8mm at 2 kHz sweep rate.

ConclusionConclusion

We have also shown that FDOCT resolution can be improved by reducing the effects of the tails or side lobes by implementing the Wiener Filter algorithm to get a gaussian shape of spectral light source.

The algorithms we have used take advantage of the convolution property of Fourier transformation. Therefore, extensive computation that can slow the speed of the OCT is not necessary.

ProgressProgress1. Prepare medication and supplies for animal

study (anesthesia, eye-drops, heat pads, surgical instruments, etc.)

2. Set up portable OCT system3. Optimize and calibrate portable OCT system4. Order lab rats5. Begin imaging rat, rabit and hamster retina6. Design Wiener filter to reduce noise Conduct the clinical trial experiment

AcknowledgementsAcknowledgements

Dr. Zhongping ChenDr. Zhongping ChenDr. QiangDr. Qiang WangUniversity of California, IrvineUniversity of California, IrvineBeckman Laser InstituteBeckman Laser InstituteIM-SURE ProgramIM-SURE ProgramSaid ShokairSaid ShokairNSFNSF