Optics III: Advanced imaging techniques

Lateral resolution (∆x)

• Ability to resolve structure across the retina

• Limited by PSF size or airy disk size

  • Pupil size → Diffraction + Aberrations

Axial resolution (∆z)

• Resolve structure along the Z-axis _{(In and out)} of the retina

  • Depth

• Limited by the imaging method used

What size lateral and axial resolution is best?

• Low (∆z) and low (∆x)

How to determine lateral resolution in diffraction-limited system

Rayleigh criterion

• f → Focal length (mm)

• D → Aperture diameter

Rayleigh criterion states…

• Two point sources can be identified if they are separated by the radius of their airy disks

For 550 nm light, what is the optimal pupil diameter for rayleigh criterion?

Assume: 22mm focal length

∆x = 1.22 * f * λ / D AND Foveal cones are 2.5 microns

• The optimal pupil size is 6mm _(Image)

  • Without aberrations

Imaging cones

• Refers to individual foveal cones being used as a benchmark for imaging

• Foveal cone size = 2.5 microns or 0.5 arc mins

• Requirements

  • 6mm pupil

  • Diffraction-limited system (Rayleigh criterion)

Requirements to resolve one “cycle”

One black-white pair needs to fall on two adjacent cones

• ∆x of…

  • 5 microns

  • 1 min arc

  • 60 CPD

Is it easier imaging cones in the periphery or fovea?

Peripheral cones!

Why?

• They get bigger as you move away from the fovea

  • Time to reclass bro

Absorption of macular pigment

• 400nm to 550nm light

  • Peak near 458nm

Absorption of melanin in the RPE and choroid

• Absorption ↓ as wavelength ↑

Absorption patterns of retinal vasculature

• Blood absorbs between 400 and 450nm light

• Light between 520nm and 590nm is dependent on oxygenation of hemoglobin

Is foveal or parafoveal reflectance greater?

Parafoveal reflectance

• Foveal cones have more short wavelength-absorbing pigment

What wavelengths are used for retinal imaging? Why?

700 to 900 nm (Near infrared)

• Not absorbed by retinal structures

• Less visible to subject → Less photophobia

Things that will fuck up retinal imaging

• Accommodation

• Tear film between blinks

• Fixational eye movements

• Patient becomes uncomfortable → Moves around

Adaptive optics (AO) systems

Compensates for fluctuations in accommodation and tear film changes during retinal imaging

• Continuously monitors quality of retinal image

• Adjusts according to changes in accommodation/tear film

Fixational eye movements

Factor degrading retinal imaging quality

• Displace image up to 10 microns

• Solution

  • Short imaging exposure

    • <10 msec

  • Eye tracking recordings

Confocal scanning laser ophthalmoscope (cSLO)

Problem: Each retinal layer scatters light differently → Image quality degraded

cSLO solution

• Excludes scattered light from every layer except the one of interest

  • Via a pin-hole that only allows reflected light from the desired layer

cSLO is used in the diagnosis of…

• Glaucoma

• Macular degeneration

• Retinal disorders

How exactly does a cSLO work?

• Horizontal and vertical scanning mirrors move laser over the retina _{(Raster scan)}

  • Uses reflected intensity from each spot to form the image

• Pinhole used to only focus the reflected light from the plane you want to image at _(Image)

  • Confocal = The desired layer is in line with the pinhole

Optical sectioning

• Changing the layer of the retina that us confocal with the cSLO pinhole allows high resolution image of different sections of the optical media

  • Can form a 3D scan based on individual images

Axial resolution of cSLO is dependent on…

• Dependent on f-number

  • focal length / D

What is numerical aperture?

Math concept relating f-number to cSLO imaging

NA = 1 / (2 * f #)

Equation to determine axial resolution (∆z) of cSLO

Factors lowering axial resolution

𝛥z= 1.4𝛌/NA²

𝛥z= 5.6𝛌 f/#²

f/# = f / D

• Smaller pupil

  • ↓ D = ↑ f/# = ↑ ∆z = Lower resolution

• Longer wavelength

• Uncorrected aberrations

Optical coherence tomography (OCT) offers very high…

Axial resolution (∆z)

OCT is analogous to what type of imaging?

Ultrasound

Ultrasound

• Measures time for sound to bounce back

• Axial resolution ~100um

OCT

• Measures time for light to bounce back

• Axial resolution ~5-10um

Two main types of OCT’s

• Time domain OCT

• Spectral (Fourier) domain OCT

Low-coherence interferometry

Refers to interferometer emitting a beam but…

• Light source is only “coherent” over a short distance before dissociating

How do OCT’s induce short coherence lengths?

Light source of super-luminescent diode (SLD)

How does an OCT work?

• OCT shoots short coherence light onto the retina

  • One beam is reflected from the tissue sample

  • One beam reflected off mirror in the OCT

Both beams interfere with each other over a short distance

• Limitation in distance allows you to pinpoint the depth at which the interference signal is coming from

Therefore axial resolution of the OCT = coherence length of the source

Time-domain OCT (TD-OCT)

Measures interference as a function of tissue depth

• Reference and tissue beam shoot out (Normal OCT)

• Mirror inside OCT moves around measuring

  • Depth of signal

  • Intensity of reflected ‘echoes’

• Generates an A-scan

Spectral (Fourier) OCT

• Stationary reference mirror _{(Contrast to TD-OCT)}

• Spectrometer separates reflected light by wavelength

• Resulting interpretation measures depth of scan to form OCT image

  • Faster than TD-OCT

A,B,C scans

• A scan → Single axial scan

• B scan → Series of A scans across the retina, retinal slice

• C scan → Vertical stack of B-scans in coronal plane

Equation for axial resolution of OCTs

Bandwidth

Refers to the range of wavelengths within a light source

• Lasers → Narrow bandwidth

• SLDs → Broader bandwidth

Best combined SD-OCT and cSLO system

Heidelberg engineering cSLO and OCT

Adaptive optics systems can be applied to…

• Fundus cameras

• SLO

• OCT

3 main components of adaptive optics systems

• Wavefront sensor (WFS)

  • Measures wavefront aberrations

• Wavefront corrector

  • Corrects aberrations with deformable mirror

• Control computer

  • Controls the WFS and DM