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Combination of adaptive optics and confocal imaging in a high-resolution retinal imaging instrument
Simon Tuohy, in conjunction with Image Eyes, Orsay, France.
Optical Coherence Tomography (OCT) is a technique that arose out of the telecommunications. It was first devised in 1991 by Huang et al[1]. It is based on Low coherence interferometry. Light from the source is split into two arms. One arm of the interferometer goes into the object while the other arm of the interferometer goes into a reference arm. This reference arm consists of a mirror that can be either scanned or stationary depending with OCT configuration is being built. When light from both arms return from the eye and from the reference arm they interfere when the Optical Path Difference (OPD) between the arms is within the coherence length of the source. This means that features can be imaged in depth with a resolution equal to that of the coherence length of the source.
SLO is in essence a confocal microscope which was first patented by Marvin Minsky in 1961. In a confocal microscope instead of the entire specimen being illuminated a signal point is illuminated. Also in the imaging plane a pinhole is placed. The purpose of the pinhole is to in effect turn the detector into a point detector. This means that light coming from out of focus positions in the specimen is excluded by the pinhole. This improves the resolution.
When imaging the eye one of the main limiting factors of the resolution is aberrations in the eye. Thus the full benefits of an OCT/SLO imaging system can not be realised. However by introducing into the setup wavefront correction it is possible to correct for these aberrations and gain a better resolution in both channels.
Adaptive optics has its origins in Astronomy. It were first theorised by Horace Babcock in 1953 at the Mount Wilson Observatory in California. In his paper entitled "The possibility of compensating astronomical seeing". His idea was that a mirror would be coated with oil, whose shape could be distorted by the application of a charge. However at that time the technology to use adaptive optics was not available. It was not until the end of the 60s when the technology advanced to a sufficient level to allow adaptive optics to be implemented. The corrector usually consists of a mirror whose surface can be deformed so that abberated light incident on the mirror is corrected.
1. Huang D, Swanson EA, Lin CP, Schuman JS, Stinson WG, Chang W, Hee MR, Flotte T, Gregory K, Puliafito CA, et al. Optical coherence tomography. Science. 1991 Nov 22;254(5035):1178-81
Research funded by Marie Curie Actions
Marie Curie Contract Number: MEST-CT-2005-020353