Adaptive Optics for Vision Science

Chapter 10.4.8 - Field Size

10.4.8   Field Size

The constraints on field size for AO-OCT are no different than those for AO
conventional flood illumination and SLO systems. Constraints include the
need for high illumination levels during short image acquisition periods (tens
of milliseconds) as well as a dense sampling of the retinal image (<1 μm/
pixel). The Indiana AO flood illumination OCT and Indiana line illumination
spectral-domain OCT ophthalmoscopes acquire single images of the retina
well within 10 ms, the maximum duration over which the retina is largely stationary
at the cellular level. For single-image acquisition, the field size of these
ophthalmoscopes was limited by the output power of their SLD sources (10
to 20 mW) rather than detector speed. Both the flood and line illumination
ophthalmoscopes had field sizes of about 1/3° even though the detectors they
employed had a sufficient number of pixels to acquire images of the retina up
to 1.7°. For comparison, the Indiana AO conventional flood illumination
ophthalmoscope typically images up to 1.7° patches with exposure durations
around 1 to 4 ms. For the AO en face scanning spectral-domain OCT oph-
thalmoscopes that are being developed, their field of view is limited by the
image acquisition rate of their line scan CCD detectors, as opposed to the
illumination power of their light sources. The fastest reported line scan detector
for spectral-domain OCT operates at 29,300 A-scans/s with exposure
durations of 34 μs [43]. This is 680 times slower than the 50-ns integration
time for the AOSLO reported in Section 10.3.7. Obviously this seriously
constrains the field of view that can be realized in the same period of time as
with the SLO. At Indiana, we have recently evaluated a higher speed line
scan detector that acquires up to 73,000 A-scans/s (2.5 times faster) with
8.5 μs exposure durations. While image signal-to-noise is reduced at the
higher speed, images still carry significant information about the retinal
layers. In principle, volumetric imaging at this higher A-scan rate for a 30-Hz
frame rate can be accomplished for a 49 × 49 × 256 pixel volume of retinal
tissue. While the 49 × 49 en face pixel area is significantly less than that routinely
achieved with AO-SLOs (512 × 512 pixels), the OCT approach has the
advantage of en face information at all depths of the retina.

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