Optical Instruments II Instruments for Imaging the Retina

1. Fundus Camera Fundus camera optics are very similar to those of the indirect ophthalmoscope.

principle of indirect ophthalmoscope GTT 04 same principle for fundus camera

GTT 04

Practical Retinal Illumination System GTT 04

GTT 05

hole in 45 o mirror

camera or CCD Fundus camera

2. Scanning Laser Ophthalmoscope (SLO) Uses much lower light levels than fundus camera – continuous viewing. Many wavelengths including IR—no mydriasis

Confocal Principle Red cell in thick sample imaged by lens Blue cell, nearer to surface, imaged at different point Pinhole in image plane passes all light from blue cell Pinhole blocks most of light from red cell Based on Webb, RH, Rep Prog Phys 59:427

Fast (35  S) horizontal line scan Slower vertical scan (17 mS) Video rate raster pattern SLO Raster Scan

laser AOM rotating faceted mirror—40,000 rpm vertical scan—60 Hz photo- detector video monitor laser-beam raster on retina pinhole

laser laser-beam raster on retina Video source (computer, camera) Acousto- Optic Modulator video monitor

SLO more light efficient than fundus camera iris pupilillumination exit pupil illumination exit pupil FUNDUS CAMERA SLO

SLO with Adaptive Optics (AO) Corrects laser beam aberrations caused by eye’s optics. Results in very high resolution images of retina.

AO SLO laser micromirror array X – Y scan beamsplitting mirror Hartmann- Shack wavefront sensor aberration signals

Hartmann-Shack Principle

AO turned on Human retina AO SLO A. Roorda UC Berkley

AO SLO optical sectioning (images in depth)

3. Optical Coherence Tomography (OCT)

Coherence of Light Waves

Laser Beam Coherence Laser coherence length

Michelson Interferometer reference arm sample arm

Interference Fringes in Michelson Interferometer

low coherence lengthlong coherence length

Michelson Interferometer Optical Coherence Tomography electronics video monitor

sample video monitor electronics reference arm sample arm

Fringes form when reference mirror path length matches path length of a reflective piece in the tissue in the sample arm. Fringes only form when the path difference is within the coherence length of the light source. IN MICHELSON INTERFEROMETER

video monitor electronics A SCAN B SCAN

OCT using fiber optics electronics photodetector SLD sample reference

Time Domain OCT’s Axial (‘A’) scan comes from mirror movement in time. Resolution in both directions about 10  m. About 750 A scans/sec 1 – 2 sec for one complete image Eye movements a problem

Fourier Domain OCT (FDOCT) Reference mirror stationary Reflectance of tissue at each depth recorded simultaneously Two types: Swept Source (SSOCT) & Spectral Domain SDOCT) Called this because raw output of the OCT is the Fourier transform of the depth reflectance signal.

Swept-Source FDOCT swept laser fixed ref mirror inverse Fourier transform electronics 1/  (wavenumber) I Distance (  m)

FDOCT provides improved resolution and reduced image formation times compared to TDOCT TDOCT FDOCT  10  m < 3  m ,000 1– 2sec 0.03 sec Axial & lateral resolution A-scans/sec Image formation time (512 A scans)

Drexler W et al. Nature 2001

J. Izatt

Bioptigen Inc. 1,000 A scans. 17 images/sec Fundus image from 3D data Volumetric 3D image (5.7 sec)