Adaptive Optics for Vision Science

Chapter 10.3 - Scanning Laser Imaging

10.3   SCANNING LASER IMAGING

Scanning laser imaging systems were invented in 1955 by Marvin Minsky [19].
Scanning laser ophthalmoscopes (SLOs) were invented by Robert Webb in
1980 [20]. The SLO is the same as a scanning laser microscope except that
the eye is used as the objective lens and the retina is always the sample being
imaged. A SLO image is acquired over time as the scattered light is recorded,
pixel by pixel, from the focused spot as it scans in a raster pattern across the
retina. Effective implementation of this imaging technique relies on the property
of scanning and descanning of the beam. By rule of reversibility, the
scattered light from the retina returns along the incoming light’s path. Once
the scattered light reflects back off the scanning mirror (or mirrors), the beam
is no longer scanning but follows directly along the stationary part of the
incoming light path. This allows one to place a single small detector in the
returned light path to detect the light that forms the image. The image is
formed digitally in a frame grabber, which combines horizontal and vertical
position information from the scanning mirrors (termed hsync and vsync,
respectively) with the digitized values of the analog intensity stream from the
detector, to form an extended field image. A feature of the SLO is that scattered
light from the focused point on the retina can be reimaged prior to
detection. At the location of the aerial image of the spot, one can place a
“confocal” pinhole for which only the light that passes through gets detected.
The confocal pinhole serves to limit the light reaching the detector to that
originating from the focal plane only. The operation of the confocal pinhole
is illustrated in Figure 10.3. The result is that images have a higher contrast
than what would be obtained with a conventional flood-illuminated imaging
system. Another advantage is that one can move the focal plane through the
retina to see different features in depth.

SLOs are suited for real-time imaging and have been used routinely for
real-time as well as single-shot applications. Video imaging applications have
been used for fluorescein angiography, retinal eye tracking, and blood flow
measurements based on laser Doppler principles (Heidelberg Retina Flowmeter,
Heidelberg Engineering GmbH). Snapshot-based imaging has been
used for routine screening and diagnosis (Optomap, Optos plc). Auto-
flourescence imaging has been used to look at the accumulation of lipofuscein in
normal and diseased eyes [21]. The addition of polarization sensitivity has
been used to measure properties of the nerve fiber layer (GDx, Laser Diagnostic
Technologies) and different wavelength and detection strategies have
been used to image subretinal features [22]. Finally, optical sectioning has
been used to measure the topography of the nerve fiber layer, particularly in
the area of the optic disk (Heidelberg Retina Topographer, Heidelberg
Engineering GmbH).

SLOs are not only used for imaging. In some systems, the laser that is being
raster scanned on the retina is modulated to produce complicated retinal


FIGURE 10.3 Schematic to illustrate optical sectioning in a confocal scanning laser ophthalmoscope. In the illumination path, (a) the light is focused to a specific plane in the retina. Although the light is focused to a plane, the incident light will scatter from all layers of the retina. Light that scatters from the plane of focus (b) passes through the optical system and is focused to an aerial image, where the confocal pinhole is placed. Only light from the focal plane passes through the confocal pinhole and contributes to forming the image. Light that scatters from deeper (or more anterior) layers (c) is reimaged in a plane other than the confocal pinhole plane and is blocked from reaching the detector. The confocal pinhole limits the light in the image to that originating from the scattered light near the plane of focus.


patterns that not only the patient can see but can also be seen in the image.
This makes it possible to image the retina while it is fixating, tracking a
moving object, or reading text or while presenting and recording electroretinograph
stimuli [23, 24].

But since an SLO uses light-based imaging, it is limited in resolution by
aberrations in the optics of the eye. The incorporation of AO into the SLO
improves lateral resolution, as well as axial resolution.

 

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