We present a new design for optical synthetic aperture imaging based on developments in optical coherence tomography. We detail the expected performance and feasibility of using this imaging system from low earth orbit for high resolution imaging of the ground.
We present a novel time-domain polarization sensitive optical coherence tomography configuration operating at 830 nm,
equipped with multichannel acousto-optic deflectors (AOD)s. The system can be used to simultaneously acquire
interference information from multiple polarization-sensitive channels and to enable measurement and imaging of
backscattered intensity, birefringence, and fast optic axis orientation. The system is employed here to demonstrate
polarization sensitive imaging of a thermally damaged muscle tissue.
It is demonstrated that modulation of active gain in each arm may be used to control the optical power that
interrogates multiple paths at the same time. Using this concept, simultaneous interrogation of ten depths in a sample at
75 microns apart is demonstrated, with less attenuation from one recirculation to the next than we previously reported.
In this manuscript, a novel low coherence interferometer configuration is presented, equipped in each arm with an
adjustable optical path length ring. By compensating the losses in the two rings using semiconductor optical amplifiers,
interference of low coherence light after traversing the two rings 18 times is obtained. This configuration can
successfully be employed to produce simultaneous en-face OCT images from different depths.
Results are presented in using low coherence interferometry in quantifying the reflectivity and imaging of different
objects, such as tissue, paintings and fruits. All images have been obtained using en-face flying spot technology. This
allows simultaneous generation of optical coherence tomography and confocal scanning images.
A comparative analysis on the performance of different scanning regimes in time domain optical coherence tomography
is presented in terms of image size. Safety thresholds due to the different continuous irradiation time per transverse pixel
in different scanning regimes are also considered. We present the maximum exposure level for a variety of scanning
procedures, employing either A scanning (depth priority) or T scanning (transverse priority) when generating cross
section images, en-face images or collecting 3D volumes. We present a comparison between such B-scan images, and
different criteria to allow the user to choose the right mode of operation. Mainly, two criteria are detailed, a scanning
criterion and a safety criterion. The scanning criterion depends on the number of pixels along the lateral and axial
directions. The analysis shows that en-face scanning allows wider images while the longitudinal scanning is more
suitable to deep cross sections. The safety criterion refers to safety levels to be observed in each scanning mode. We
show that the flying spot OCT imaging has different safety limits for T- and A- based imaging modes. The analysis leads
to maximum permissible optical power levels that favors T-scan imaging of wide objects. We then apply the analysis
considering as object the eye.
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.
We develop a dual-channel optical coherence tomography/indocyanine green (OCT/ICG) fluorescence system based on our previously reported ophthalmic OCT/confocal imaging system. The confocal channel is tuned to the fluorescence wavelength range of the ICG dye and light from the same optical source is used to generate the OCT image and to excite the ICG fluorescence. The system enables the clinician to visualize simultaneously en face OCT slices and corresponding ICG angiograms of the ocular fundus, displayed side by side. C-scan (constant depth) and B-scan (cross section) images are collected by fast en face scanning (T-scan). The pixel-to-pixel correspondence between the OCT and angiography images enables the user to precisely capture OCT B-scans at selected points on the ICG confocal images.
We report a versatile imaging system combining scanning laser ophthalmoscopy (SLO) and T-scan based en face ultrahigh resolution optical coherence tomography (OCT). The image carrier is generated using the optical path difference modulation introduced by the X-Y galvo-scanner mirrors specific to en face OCT (without optical modulators in the reference arm). The light source is a compact superluminescent diode based source with 150 nm FWHM spectrum, centered at 890 nm. We demonstrate en face B-scan and C-scan ultrahigh resolution OCT imaging of the human retina in vivo, with an axial resolution of 3.2 μm in tissue. The system is capable of acquiring large lateral size ultrahigh resolution OCT scans of a maximum field size of 20°. The acquisition speed is up to 2 frames/s for both OCT B-scans and C-scans. The measured system sensitivity is more than 98 dB, for a power level to the target of 1 mW and maximum lateral scan size. The C-scans are, to the best of our knowledge, the first and the largest size reported ultrahigh resolution C-scans of the human retina in vivo. The instrument is assembled on a chin rest and ready to be used for clinical imaging. SLO and ultrahigh resolution OCT C-scans are acquired simultaneously and displayed side by side. This allows users in a clinical environment to correlate details of the same feature in the area of interest in both images and also choose precisely in the SLO image the location where to perform the ultrahigh resolution en face B-scan.
A versatile time-domain OCT system is presented which, for the first time, can generate cross-section images (B-scans) by using either transverse priority (T-scans) or depth priority (A-scans). Images from the optic nerve are obtained with either regime, with the same system. In different scanning regimes, different values are allowed for the maximum power to be launched to the eye. We present the maximum exposure level for a variety of scanning procedures, such as generation of cross section images and 3D volumes employing either A or T scanning.
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.
A new transmissive grating-based scanning delay-line for optical coherence tomography is proposed, with dispersion compensation capability. Compared to other spectral delay-lines, our implementation has less loss due to a halved number of diffraction grating reflections, and implements a walk off compensation scheme. The performance of the delay line is evaluated. The delay line transmissive geometry targets balance detection configurations.
Two and three SLD diodes are grouped together in order to obtain a compounded source of wide band for OCT investigations. One SLD has a two-lobe spectrum around 840 nm and a compounded spectrum is achieved by using SLDs of smaller wavelengths. This has two advantages: (1) the smaller the wavelength the lower the loss of power through the vitreous due to water absorption; (2) medium band, standard SLDs if used bring a more significant reduction to the coherence length than their counterparts, of medium band but centered at longer wavelength. We show that it is possible to obtain high resolution OCT images with an inexpensive, compact, and easy to operate source.
En-face OCT acquired simultaneously with paired confocal ophthalmoscopic (CO) images provides unprecedented point-to-point correlation between surface and subsurface anatomy of the retina. An advanced prototype of a dual channel OCT/CO instrument was developed in terms of signal to noise ratio and image size. The system can operate in A, B and C-scan regimes. The design is such that there is a strict pixel to pixel correspondence between the OCT and confocal images. An extensive array of clinic pathologies were studied including macular degeneration, central serous retinopathy (CSR), macular hole, macular pucker, cystoid macular edema (CME), diabetic maculopathy, and macular trauma. We report observation of reoccurring patterns in the en-face OCT images which could be identified with different diseases. The system can also simultaneously produce en-face OCT and indocyanine green (ICG) fluorescence images where the same source is used to produce the OCT image and excite the ICG. 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 side by side. The images are collected by fast en-face scanning (T-scan) followed by slower scanning along a transverse direction and depth scanning. The system is capable of providing chosen OCT B-scans at selected points from the ICG image.
Using an advanced prototype of en-face OCT/cSLO instrument, an extensive array of clinic pathologies were studied including macular degeneration, central serous retinopathy (CSR), macular hole, macular pucker, cystoid macular edema (CME), diabetic maculopathy, and macular trauma. We report observation of reoccurring patterns in the en-face OCT images which could be identified with different diseases. Uniquely specific and reoccurring patterns could be characterized for macular hole ("Chrysanthemum flower"), CME ("Swiss cheese wheel"), Macular Pucker ("Star"), CSR ("Target") and RPE Detachment ("Ring of Light"). Other entities such as polypoidal choroidopathy and diabetic edema residues had easily recognizable features but were variable enough to defy specific descriptive comparison. To facilitate the interpretation of the en-face OCT images, a three dimensional interactive simulation was designed which allows the demonstration of characteristic features and artifacts encountered in the acquisition of transverse images.
A review is presented of the developments in Kent in the field of optical coherence tomography (OCT) based instrumentation. Original versatile imaging systems have been devised which allow operation in different regimes under software control. Using such systems, B-scan and C-scan images are demonstrated from retina, anterior chamber, skin and teeth. The systems developed in Kent employ the flying spot concept, i.e. they use en-face scanning of the beam across the target. This has opened the possibility of providing simultaneous en-face OCT and confocal images (C-scans). Application of a standalone OCT/confocal system for investigating the retina in eyes with pathology, the anterior chamber, skin and teeth is demonstrated.
This paper demonstrates the clinical application of a multiplanar imaging system, which simultaneously acquires en-face (C-scan) OCT and corresponding confocal ophthalmoscopic images along with cross-sectional (B-scan) OCT at cursor designated locations on the confocal image. Advantages of the simultaneous OCT/confocal acquisition as well as the challenges of interpreting the C-scan OCT images are discussed. Variations in tissue inclination with respect to th coherence wave surface alters the sampling of structures within the depth in the retina, producing novel slice orientations which are often challenging to interpret. We evaluate for the first time the utility of C-scan OCT for a variety of pathologies including exudative ARMD, macular hole, central serous retinopathy, diabetic retinopathy, polypoidal choroidal vasculopathy and macular pucker. Several remarkable observations of new aspects of clinical anatomy were noted. The versatility of selective capture of C-scan OCT images and B-scan OCT images at precise points on the confocal image affords the clinician a more complete and interactive tool for 3D imaging of retinal pathology.
En-face optical coherence tomography (OCT) technology is employed to produce high resolution images from the retina, cornea, skin and teeth. Longitudinal (B-scan) and transversal (C-scan) images are demonstrated using en-face scanning method. The main advantage of the en-face imaging is that the C-scan images permit a straightforward comparison with the images produced by confocal microscopy. Other developments are also presented as the generation of 3D imaging of different tissue using stacks of en- face OCT images collected at different depths.
We report a system capable of collecting pairs of en-face OCT and confocal images from the anterior chamber. Pairs of such images are collected from up to 7 mm deep in the anterior chamber measured from the top of the cornea. The wavelength is 85micrometers and the power 0.3 mW. The system offers: (i) versatility, being capable of displaying both C-scan OCT images (constant depth, oriented perpendicularly on the optic axis) as well as B-scan OCT images (containing the optic axis or longitudinal); (ii) eye alignment using the Purkinje reflections in the confocal channel; (iii) overall eye guidance, on the confocal image; (iv) correction for the en-face movement in the B-scan images generated by en-face imaging using the confocal image. Animations of such pairs of images demonstrate the utility of the system for in vivo imaging of the anterior segment of the eye.
The utility of a versatile multifunctional standalone Optical Coherence Tomography (OCT)/confocal system for imaging dental tissue was investigated. The system can collect A-scan (reflectivity versus depth graph), longitudinal (B-scan) and en-face (C-scan) OCT images, simultaneously with a confocal image. The power to the sample was 250(mu) W, wavelength (lambda) =850 nm and the depth resolution in air was 16 micrometers . The OCT images showed caries lesions as volumes of reduced reflectivity. Transversal images (C-scan) showed the en-face slices of the tooth tissue like in confocal microscopy. Longitudinal images showed the depth of the lesion into the tooth tissue as well as the different structural layers of sound tooth in the same way as seen in ultrasound images. A-scans performed in locations selected in the en-face images provided quantitative data about the reflectivity versus depth. The confocal channel was extremely useful for guidance and it has also shown the integral of the intensity over depth at transversal locations. We concluded that OCT proved capable to detect an early caries lesion, to show the depth of the lesion into the tissue, and quantitatively assess the degree of demineralization.
We investigate the effect of light source coherence length on longitudinal and en-face OCT images of the retina. The sources used in this comparative study are a superluminescent diode (SLD), a superliminescent diode fitted with an interference filter at its output and a tunable coherence length three-electrode laser device (3EL). We show that the use of sources of shortest coherence length is ideal for longitudinal OCT imaging. However, there are reasons for using adjustable coherence length sources for en-face OCT imaging. The effect of adjustable coherence length (and implicitly spectrum FWHM) on the achievable signal to noise ratio in the Oct is also presented. An increase in the coherence length enhances the excess photon noise but, at the same time, increases the signal collected from scattering tissue due to a larger thickness of the coherence gated backscattering layer in the target tissue. This suggests that the signal to noise ratio should not change with the light source coherence length. Nevertheless, the effect of light source coherence length change on the signal to noise ratio is more complex due to other noise sources in the system.
Two instruments are now available for high depth resolution imaging of the retina. A scanning laser ophthalmoscope is a confocal instruments which can achieve no more than 0.3 mm depth resolution. A longitudinal OCT instrument uses a superluminescent diode which determines a depth resolution better than 20 microns. There is a gap in depth resolution between the two technologies. Therefore, different OCT configurations and low coherence sources are investigated to produce a choice of depth resolutions, and to cover the gap between the old confocal technology and the new OCT imaging method. We show that an instrument with adjustable depth resolution is especially useful for the en-face OCT technology. Such an instrument can bring additional benefits to the investigation process, where different requirements must be met. For instance, a poor depth resolution is required in the process of positioning the patient's eye prior to investigation. A good depth resolution is however necessary when imaging small details inside the eye. The utility of the OCT en-face imaging with adjustable coherence length for diagnostic is illustrated by images taken from the eye of a volunteer. Images with a similar aspect to those produced by a scanning laser ophthalmoscope can now be obtained in real time using the OCT principle.
Utility and modalities of producing en-face optical coherence tomography (OCT) images of the tissue are discussed. Using a versatile system, transversal and longitudinal OCT images from the retina and skin are presented. Using stacks of transversal OCT images, 3D profiles of the tissue are constructed. The stack of en-face OCT images could also be used to infer longitudinal OCT images. These are compared with the longitudinal OCT images produced with the OCT operating in the hardware longitudinal regime. Once en-face OCT images are produced, they could easier be compared with confocal images as their orientation is similar. Images from the optic nerve are shown generated with the two technologies implemented in a stand-alone system.
Using a versatile system, transversal and longitudinal OCT images from the retina and skin are presented. Using stacks of
transversal OCT images, 3D profiles ofretina and skin in vivo are constructed.
Presented here are methods of visualization of the retina, specifically the optic nerve, based on transversal OCT imaging and on the operation of a standalone dual channel OCT transversal/confocal system purpose built for the eye. We also demonstrate that enhanced measurement capabilities of parameters in the fundus are possible using these two channels. This is made possible owing to the unique guidance capability of the confocal channel along with the information it provides about the transversal eye movements. A large number of transversal OCT images are collected from the eye. Then, by software, longitudinal (B-scan) or transversal cuts (C-scan) may be made, post-examination, on the stack of transversal images. The software inferred B-scans are shown to have a similar level of resolution to the hardware generated images after movement errors are corrected. We hope that the system equipped with this 3D software processing could reduce the irradiation time required to collect the necessary data from the eye. We show that 3D visualization of transversal OCT images along with the simultaneous confocal imaging allows easier alignment of the patient eye than is possible with longitudinal OCT imaging alone. We also demonstrate that by this method of transversal imaging, direct comparison may be made between quantitative SLO acquired measurements made on the eye fundus and OCT measurements. For the first time, measurements of volumes and areas in the optic nerve area are demonstrated using transversal OCT imaging, similar to procedures utilized by scanning laser ophthalmoscope users.
Different configurations are presented which allow simultaneous acquisition and presentation of OCT en-face images from different depths. If a sufficient number of slices are collected, 3D rendering of the object in real time is made possible. Results using two configurations to produce simultaneously two such images are presented, as a hardware version towards 3D imaging. Rendering of a 3D profile of the retina is also shown, as a software version, using slices obtained at different depths collected by a single channel en-face OCT instrument.
Different compatibility criteria are discussed for the simultaneous presentation of OCT transversal and confocal images from the retina, in a stand-alone OCT/confocal imaging system.
A new instrument for simultaneous display of an en-face optical coherence tomography (OCT) image and of a confocal image [similar with that produced by the scanning laser ophthalmoscope (SLO)] for the retina in vivo is demonstrated. The system is based on a fiberized OCT configuration, where a separate bulk optical confocal receiver is introduced between the transversal galvanometer scanner and the OCT fiber arm. Criteria for presenting the two images are discussed. These address the resulting impact on the quality of each image as result of differences in the two imaging techniques, low coherence interferometry and confocal imaging. The transversal and depth pixel size are governed by different principles and consequently are generally different. Additionally, the OCT image is disturbed by Doppler shifts introduced by the transversal scanning and speckle.
We have developed a fiberized OCT system which can deliver transversal and longitudinal images. The features and performance of the system are presented in terms of mode of operation, modulation method and image processing method.
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