Two factors are of importance to optical coherence tomography (OCT), resolution and sensitivity. Adaptive optics
improves the resolution of a system by correcting for aberrations causing distortions in the wave-front. Balanced
detection has been used in time domain OCT systems by removing excess photon noise, however it has not been used in
Fourier domain systems, as the cameras used in the spectrometers saturated before excess photon noise becomes a
problem. Advances in camera technology mean that this is no longer the case and balanced detection can now be used to
improve the signal to noise ratio in a Fourier domain (FD) OCT system. An FD-OCT system, enhanced with adaptive
optics, is presented and is used to show the improvement that balanced detection can provide. The signal to noise ratios
of single camera detection and balanced detection are assessed and in-vivo retinal images are acquired to demonstrate
better image quality when using balance detection.
We reported recently an active tracking device based on white light coherence ranging using a spectrally interrogated
Michelson interferometer, which was used to monitor and correct for the axial displacement of the eye and head of the
imaged subject in a confocal scanning ophthalmoscope/ en face OCT system (SLO/OCT) by tracking the axial position
of the eye fundus. Both the tracking and imaging interferometers share the eye interface optics and the patient eye and
also an optical path difference (OPD) changing device in the reference (fast voice coil mounted retroreflector), that keeps
them locked at constant OPD values. As a consequence, the sensitivity of the tracking interferometer is not affected by
the spectrometer sensitivity roll-off with increased OPD and mirror term ambiguity tracking errors close to OPD = 0 are
eliminated. Moreover, the axial tracking range is only limited by the voice coil stage travel range and the tracking system
has an update time better than 5 ms. We investigate the potential of the new configuration for acquiring volumetric data
free of axial eye motion artifacts for two different lateral field sizes. Sets of SLO and en face OCT images at
progressively deeper locations in the retina are simultaneously acquired for two lateral sizes, 15°x15° and 3.5°x3.5°. The
large lateral field size provides a means of navigating the retina, while the high magnification small lateral size imaging
reveals interesting microscopic details of the retinal morphology.
We report an active tracking device based on white light coherence ranging using a spectrally interrogated Michelson
interferometer, which is used to monitor and correct for the axial displacement of the eye and head of the subject in a
confocal scanning ophthalmoscope/ en face OCT system (SLO/OCT). The Nyquist limit range of the spectrometer in the
tracking interferometer is ~5.4 mm, which is adequate for monitoring the axial position of axially extended layered
objects like the human eye fundus. Both the tracking and imaging interferometers share the eye interface optics and the
sample and also an optical path (OPD) changing device in the reference (fast voice coil mounted retroreflector), that
keeps them locked at constant OPD values. As a consequence, the sensitivity of the tracking interferometer is not
affected by the spectrometer sensitivity roll-off with increased OPD and mirror term ambiguity tracking errors close to
OPD = 0 are eliminated. Moreover, the axial tracking range is only limited by the voice coil stage travel range. A real
time data acquisition processor board is used to digitize the spectrometer signal and calculate the correction signal
applied to the voice coil with an update time better than 5 ms. We demonstrate axial motion corrected combined
confocal/ en face OCT imaging of the human eye fundus in vivo.
Comparative evaluation of signal-to-noise ratio (SNR) is presented using a Full-field (FF)-OCT configuration, which is
adapted to work in either Swept-Source (SS)-Full-field OCT or Time Domain (TD)-Full-field OCT regime. We
implement the two regimes in the same set-up, using the same CCD camera and the same samples. We describe the
experimental set-up and the procedure implemented to verify the theory which says that Spectral Domain (SD)-OCT is
superior to TD-OCT. A simple theoretical analysis of the signal-to-noise ratio is presented to evaluate the improvement
from TD-OCT to SD-OCT in FF configuration. Experimental results demonstrate that the SNR is indeed better in the
SS-OCT regime, however not to the level predicted by theory. More work is required to understand why the experimental set-up does not achieve the improvement predicted by theory. We also show how to perform the measurements and imaging in the two regimes of operation. The system can deliver B-Scan OCT images in the SS-OCT regime and C-scan OCT images in the TD-OCT regime.
A review is presented of the research on high resolution imaging of the eye based on en-face OCT. This can provide a
dual display of images with different depth resolutions, where the two images are OCT and the other confocal. Two
applications are presented: (i) OCT/ICG systems where the confocal channel is tuned to the fluorescence of indocyanine
green and (ii) aberration corrections in both OCT and confocal channels using closed loop adaptive optics for enhanced
contrast and transversal resolution.
The 1020-1080 nm spectral region appears as a viable alternative to the 700-900 nm spectral band for in vivo eye fundus
OCT imaging due to a local absorption minimum of water (main constituent of the eye aqueous and vitreous). Light at
these wavelengths also experiences less attenuation due to lower scattering and absorption by melanin in the retinal
pigment epithelium and choroid, which results in deeper penetration of the probe beam in the choroid. T-scan based en
face OCT is a modification of the OCT technique that has the unique capability of acquiring both longitudinal (B-scans)
and tranversal (C-scans) OCT images of the eye fundus in real time and allows the addition of a confocal scanning
ophthalmoscope channel to the OCT instrument. We report for the first time a combined T-scan based en face OCT and
confocal scanning opthalmoscopy system for imaging the human eye fundus in vivo in the 1050 nm region. The
instrument allows the visualization of choroidal blood vessels in both the confocal and OCT channels without the use of
contrast agents such as indocyanine green (ICG) dye and could prove an alternative tool for diagnosing eye conditions
like age related macular degeneration that are preceded by choroidal neovascularisation.
The authors report preliminary clinical results using an unique instrument which acquires and displays simultaneously an OCT image, a confocal image similar to that of a scanning laser ophthalmoscope and an indocyanine green fluorescence image. The three images are produced by three channels, an OCT and a confocal channel operating at 793 nm and a confocal channel tuned on the ICG fluorescence spectrum, which peaks at 835 nm. The system is based on our previously described ophthalmic Optical Coherence Tomography (OCT)/confocal imaging system, where the same source is used to produce the OCT image and excite fluorescence in the ICG dye. The system is compact and assembled on a chin rest and it enables the clinician to visualise the same area of the eye fundus in terms of both en-face OCT slices and ICG angiograms, displayed at the same time. The images are collected by fast T-scanning (en-face) which are then used to build B-scan or C-scan images.
An absolute displacement sensing system based on 'Channelled Spectrum' processing capable of simultaneous interrogation of FBGs and FOIS is presented where the role of the optical spectrometer is performed by a scanning Fabry-Perot Filter.
We demonstrate that short in-fibre Bragg gratings may be used to measure MHz acoustic fields and that temperature can be measured simultaneously using a cost-effective monochromator/CCD arrangement.
We describe work on a system able to measure the temperature at a number of points along a single optical fiber probe, designed for in-vivo temperature profile monitoring. The sensing elements are in-fiber Bragg grating sensor and three forms of signal processing have been investigated. System 1 uses interferometric wavelength shift detection with a monochromator providing the WDM. System 2 used a scanning in-line Fabry-Perot filter and system 3 uses a monochromator with a CCD based readout scheme. The performances of the three approaches are compared.
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