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.
A highly efficient power optical coherence tomography configuration is implemented using a Multiwave Photonics broadband source centred at 1060 nm wavelength, FWHM = 50 nm and a Mach Zehnder interferometer. The interferometer contains a fibre acousto-optic modulator in each arm. One is driven at a fixed frequency of 40 MHz while the other via an RF Function Generation. In this way, the en-face OCT signal is modulated on a carrier frequency adjustable in the range 0 kHz to 1.5 MHz. A circulator is placed in the sample arm. Light retroreflected from the sample is sent via the circulator to a balanced coupler where it interferes with the reference beam. A translation stage is used in the reference arm to adjust the optical path difference in the interferometer. The result is photodetected using two InGaAs photodetectors followed by a differential amplifier in a balance detection configuration The system has been used to acquire en-face images as well as cross section optical coherence tomography images from skin and embryos based on T-scans (transversal reflectivity profiles).
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.
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 present an experimental study of the depolarization of circularly polarized (CP) light backscattered from random
media. We employ a polarization sensitive OCT, capable of producing intensity profiles for two orthogonal polarization
channels simultaneously. For CP light backscattered from polystyrene solutions containing spherical particles of sizes
larger than the radiation wavelength, the phenomenon of polarization memory is observed. The degree of circular
polarization (DOCP) as a function of the path the light travels in the medium depends on the scatterers' size. In the case
of scatterers larger than the wavelength, the DOCP exhibits a minimum indicating a helicity cross-over. The copolarized
light then exceeds the intensity of cross-polarized light of backscattered radiation, a phenomenon predicted
theoretically but not observed experimentally so far. The helicity cross-over is observed in the DOCP curves for large
scatterers at small and large concentrations.
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.
Scientific examinations of paintings are routinely carried out in major galleries and museums to assist in conservation treatment and as part of technical or art historical examinations. Care is taken to examine the paintings non-destructively as far as possible. However, in order to study the paint and varnish layers, it is still currently necessary to take tiny samples of a painting to examine the cross section of a small area of the painting under a microscope. In an attempt to solve this problem we evaluate the potential of optical coherence tomography (OCT) in providing high resolution information about paint layers. Two OCT systems have been assembled, operating at 850 nm and 1300 nm, each using two single mode in-fiber couplers. Both systems can produce A (reflectivity profile in depth), T (lateral reflectivity profile), B (cross section image) and C-scans (constant depth image). Using superluminiscent diodes, a depth resolution better than 9 microns is achieved. We present results of applying OCT to sample panels and paintings. We show that infrared OCT is capable of non-destructive examination of paintings in 3D, which shows not only the structure of the varnish layer but also the paint layers. The OCT images present better microscopic tomography of the surface of the varnish and paint layers than any system currently employed in the examination of paintings. OCT could also be used for accurate measurement of the thickness of the varnish layer on a painting.
It is current practice to take tiny samples from a painting to mount and examine in cross-section under a microscope. However, since conservation practice and ethics limit sampling to a minimum and to areas along cracks and edges of paintings, which are often unrepresentative of the whole painting, results from such analyses cannot be taken as representative of a painting as a whole. Recently in a preliminary study, we have demonstrated that near-infrared Optical Coherence Tomography (OCT) can be used directly on paintings to examine the cross-section of paint and varnish layers without contact and the need to take samples. OCT is an optical interferometric technique developed for in vivo imaging of the eye and biological tissues; it is essentially a scanning Michelson's interferometer with a "broad-band" source that has the spatial coherence of a laser. The low temporal coherence and high spatial concentration of the source are the keys to high depth resolution and high sensitivity 3D imaging. The technique is non-invasive and non-contact with a typical working distance of 2 cm. This non-invasive technique enables cross-sections to be examined anywhere on a painting. In this paper, we will report new results on applying near-infrared en-face OCT to paintings conservation and extend the application to the examination of underdrawings, drying processes, and quantitative measurements of optical properties of paint and varnish layers.
By dividing both the object and reference beam in an OCT interferometer, two independent OCT imaging channels are assembled. The depth scanning proceeds simultaneously in the two OCT channels and from the same range, however a differential optical path difference can be introduced between the two channels. In this way, two simultaneous images are generated where the depth differs in each pixel by the differential optical path difference. A dual OCT system working at 850 nm was devised and we demonstrate the capability of the method by simultaneously acquiring images from the optic nerve and fovea of a volunteer. The configuration devised insures a strict pixel to pixel correspondence between the two images irrespective of the axial eye movements while the depth difference between the corresponding pixels is exactly the differential optical path difference. The images are collected by fast en-face scanning (T-scan) which allows both B-scan and C-scan acquisition.
We report a novel imaging procedure and a system implementing it, capable of acquiring two quasi-simultaneous en-face images from a multi-layer target using a single mode fiber optic interferometer. Two optical sources are employed, which are toggled on and off simultaneously with the ramp signal applied to the scanner whose movement determines the line in the raster image. In this way, half of the line in the final frame is generated by a particular source only. Imaging with two different depth resolutions or with two different wavelengths is made possible in this way. Because the switching is much faster than the observer eye can follow, the two images can be interpreted and compared simultaneously. The paper discusses the different functionality of the system depending whether the OCT image is made out of A or T-scans.
KEYWORDS: Polarization, Optical coherence tomography, Birefringence, Single mode fibers, Tissue optics, Reflectivity, In vivo imaging, Signal detection, Mirrors, Tissues
Polarization sensitive optical coherence tomography (PS-OCT) takes into account the vector nature of light waves (state of polarization). The most complete information about the polarization properties of a biological target is given by the depth resolved Mueller matrix elements, however, it is difficult to construct such a system for in-vivo examination. We designed and assembled a simpler system, with two incoherent channels to provide limited information, but essential on the polarization properties of the tissue. The interferometer is a hybrid configuration of bulk optic and single mode optical fiber components. No polarization maintaining fiber is used. The reference and sample beams interfere in single mode optical couplers. The low coherence light source is a superluminescent diode of 850 nm center wavelength and 25 nm spectrum FWHM (which corresponds to a depth resolution of 12 microns in tissue). The system can display either a pair of two polarisation sensitive OCT images, corresponding to linear orthogonal polarisation directions or a pair of images, a polarisation insensitive (pure reflectivity) image and a birefringence retardation map. The 12 bit grayscale images are collected by fast en-face scanning (T-scan) at 2 frames/s. We demonstrate in vivo en face images of the retinal nerve fiber layer, lamina cribrosa, cornea and teeth. A rotation angle of 0.3 degrees per micron was evaluated from the retinal nerve fiber layer and lamina cribrosa.
We discuss the merits of low coherence interferometry to investigate the Faraday effect in a diluted magnetic semiconductor (DMS) crystal placed in one of the interferometer arm. We describe the combination of modulation effects due to the Farady effect in the DMS probe with the modulation due to a vibrating mirror in the interferometer. Essential for the measurement of the Verdet constant is the control of polarization in the two arms of the interferometer. We show that by watching the resulting spectra of the photodetected signal, clear distinction is possible between the circular and linear polarization of the input light.
Optical coherence tomography is a diagnostic imaging technique allowing two dimensional tomographic imaging of tissue architecture. This is a review article on the use of optical coherence tomography in Otolaryngology including original images from human laryngeal tissue and temporal bones (cochlea) in our laboratory. Tissue specimens from normal larynges were imaged with an 850 nm OCT system. Our results showed good correlation between OCT image s and the corresponding haematoxylin-eosin stained histology sections in the normal larynx. Human temporal bones were also imaged using an 1300 nm OCT system. Limited morphological details were obtained due to the high scattering properties of the bony labyrinth.
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.
We have constructed a compact set-up based on incoherent detection in two optical channels to provide real time polarisation sensitive OCT imaging. The system can display either a pair of two polarisation sensitive OCT images, corresponding to linear orthogonal polarisation directions or a pair of images, a polarisation insensitive (pure reflectivity) image and a birefringence retardation map. The images are collected by fast en-face scanning (T-scan) followed by slower scanning along a rectangular transverse direction and depth scanning. B-scan and C-scan images from in-vivo retina and optic nerve are presented.
We present results of applying low coherence interferometry to gallery paintings. Infrared low coherence interferometry is capable of non-destructive examination of paintings in 3D, which shows not only the structure of the varnish layer but also the paint layers.
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.
En-face OCT relies on scanning fast along a direction perpendicular to the optical axis. C-scan images (en-face slices at constant depth) can be obtained at different depths. The quality of the image and what part of a particular layer is visible depends on the properties of the sampling function. In this paper we examine how the scanning configuration, the interface optics, the coherence length of the source used, and the object itself influence the shape of the sampling
function.
We report the first (to the best of out knowledge) en face polarization sensitive optical coherence tomography (PS-OCT) system. The transverse raster scanning of the target is achieved using a pair of galvo-scanner mirrors. The set-up is based on incoherent detection in two optical and electronic channels and employs balanced detection to reduce the excess photon noise generated by the low coherence source (superluminescent diode). The outputs of the two channels are processed using software to provide a polarisation insensitive (pure reflectivity) image and a birefringence retardation map. Images from ex vivo (human tooth) and in vivo targets (human retina) have been acquired. Particulars of en face optical coherence tomography imaging of birefringent tissue are discussed.
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.
Laryngeal cancer is the most common primary head and neck malignancy and the need for early identification is very important for early treatment. Outpatient fiberoptic examination of the larynx is often unreliable in differentiating between benign, pre-malignant and malignant lesions, and therefore surgeons often have to rely on biopsies for a definite diagnosis. This is an invasive procedure requiring general anaesthesia and may have a detrimental effect on patient’s voice. The aim of our study was to investigate the feasibility of optical coherence tomography in imaging of the larynx, which will lay the foundations for investigating its ability to differentiate between benign and malignant disease. Tissue specimens from normal larynges were imaged with a polarisation sensitive OCT system at 850 nm and a second OCT system at 1300 nm. Both OCT systems were capable of providing both B-scan (longitudinal OCT) images as well as C-scan (en-face OCT or at constant depth) images. Imaged specimens were processed with standard histopathological techniques and sectioned in the plane of the B-scan OCT images. Haematoxylin-Eosin stained specimens were compared to the OCT images. Preliminary results showed good correlation between OCT images and histology sections in normal tissues.
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.
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.
An OCT system is built around a confocal optical receiver by adding a reference beam to the beam returned from the target tissue to the photodetector. The amount of light collected by the OCT receiver (both signal and background signal) depends on the parameters of the confocal receiver. We are interested in a configuration that allows the simultaneous display of the confocal signal and the OCT signal, which requires a separate confocal channel in the system. In this case, the S/N performance is different in the two channels and depends on the optical configuration used. The paper discusses the noise sources in the two channels. S/N ratios are numerically evaluated for cases of experimental interest. As far as the penetration depth in OCT is concerned, a value of 20 optical depths was considered achievable in previous reports if only the shot noise was considered. We correct this value by taking into account the excess photon noise and the limitations imposed by the safety power limits. Multiple scattering determines an increase in noise via the excess photon noise term.
We present two different Ac magnetic field sensing configurations exploiting the Faraday effect in the semiconductor compound Cd0.57Mn0.43Te. The first configuration is a typical polarimetric set-up; the periodic magneto-optic polarization rotation of a linearly polarized input in the transducer crystal is converted into intensity modulation by means of a polarization analyzer. In the second configuration, the Faraday modulation appears in the electronic spectrum of the interference signal at the output of a hybrid Mach-Zehnder interferometer. Both set-ups include noise rejection schemes to improve the signal to noise ratio.
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