With the aim of applying complex master/slave interferometry (CMSI) to polarizationsensitive optical coherence tomography (PS-OCT) for birefringent tomographic measurements of biological tissue, we present the impact of temperature instability on an all-fiber-based depth-encoded PS-OCT system and the practicality of temperature control for birefringence measurements. In our PS-OCT system, two orthogonally polarized interrogating beams were separated to a depth of approximately 1 mm using a 5-meter long polarization-maintaining fiber (PMF) as a passive delay unit, which is susceptible to temperature instability. The variation in resolution and delay due to temperature change of PMF were investigated. Furthermore, it is shown that tomographic birefringence can be measured under temperature-controlled operation utilizing the advantages of CMSI. We found that changes to the location of generated masks caused by an emerging temperature drift between the channels can be corrected with our presented characterization.
We present a multiscale imaging system in which a conventional microscope stand is inserted in the sample arm of a Spectral-Domain Optical Coherence Microscopy (SD-OCM) system. The instrument was designed to facilitate localisation and maturity assessment of oocytes and early-stage embryos to be used for in vitro fertilisation (IVF), by a versatile and easy switch of microscope objectives. The dispersion variation due to change of the microscope objectives is dealt with by employing the Complex Master Slave procedure that enables correct operation without the use of matching glass in the reference arm for dispersion compensation.
The most successful methods for speckle reduction in Optical Coherence Tomography (OCT) are based on deformation of the wavefront used in scanning. Here, a simple method is presented where the wavefront is distorted by lateral translation of the lens between the 2D galvo-scanner and sample. The report demonstrates that the method can be implemented with a small piezoelectric transducer. Up to 33% improvement in Speckle Contrast Ratio (SCR) of B-scan OCT images is demonstrated.
This paper presents a review of the activities on optical coherence tomography (OCT) in the Applied Optics Group (AOG), University of Kent, encompassing optical devices, sources for OCT as well as OCT applications. Out of the directions of applications approached, two fields are selected, applications in medical imaging with emphasis on ophthalmology and endoscopy and in non destructive testing. An important advantage of OCT is that high axial resolution is achievable at comfortable working distances, which is an important requirement for safe scanning of patients as well as of valuable materials or objects of art.
We present a optical coherence tomography system for minimally-disturbance imaging of porcine embryos, placed inside an incubator which ensures adequate environmental parameters including temperature, humidity, and gas ratios.
We analyze identification documents using optical coherence tomography imaging. Owing to its sub-surface imaging capabilities, we established its usefulness for quantitative visualization of embedded security features in these documents, increasing the accuracy in forgery detection.
We have introduced the Master Slave (MS) interferometry method to address the limitations due to the use of conventional FTs or its derivatives in OCT data processing. The novel MS technology replaces the FT operator with a parallel batch of correlators. An electrical signal proportional to the channeled spectrum at the interferometer output is correlated with P masks producing P signals, a signal for each point out of P in the A-scan.
In this way, it is possible to: (i) directly access the information from selected depths in the sample placed in the slave interferometer; (ii) eliminate the process of resampling, required by the FT based conventional technology, with immediate consequences in improving the decay of sensitivity with depth, achieving the expected axial resolution limit and reducing the time to display an en-face OCT image, while slightly lowering the cost of OCT assembly and (iii) tolerate the dispersion left unbalanced in the slave interferometer.
The lecture will present several developments based on the MS-OCT technology, such as: (a) an equivalent OCT/SLO (scanning laser ophthalmoscopy), where no extra optical channel for the SLO is needed; (b) coherence revival swept source OCT employing the MS tolerance to dispersion: (c) Gabor filtering, where large number of repetitions with different focus adjustments can be performed more time efficiently than when employing FT based OCT; (d) MS phase processing, which opens novel avenues in phase- and polarization-sensitive modalities; (e) achieving the theoretical axial resolution when using a ultra wide broadband source such as a supercontinuum laser; (f) down-conversion OCT that can deliver an en-face OCT image from a sample in real-time, irrespective of the tuning speed of the swept source where the mask signals are generated in real time (by a physical master interferometer) while sweeping the frequency of the swept source.
In this communication, we evaluate the suitability of Master-Slave (MS) optical coherence tomography (OCT) for processing of interferograms generated by an interferometer driven by an akinetic, electrically-tunable swept source from Insight with an ultra-large instantaneous coherence length. The akinetic source is programmed to sweep linearly, but within the sweep, at predictable times, the laser tuning introduces invalid regions in the interferogram, which are normally removed post-acquisition using a pre-calibration file. This makes sure that any optical frequency component is used once only and enables correct operation of a Fourier transform (FT). A FT applied to an unprocessed emitted spectrum leads to wide and numerous peaks in the A-scan. MS processing was introduced to avoid the necessary corrections demanded by conventional FT signal processing or its derivatives. The MS procedure consists of comparing photo-detected signals at the output of two interferometers, a Slave and a Master interferometer. The MS method was advanced along two avenues, either by using (i) electricallygenerated master signals (making use of the same interferometer twice) or (ii) optically-generated master signal via a recently introduced MS down-conversion procedure. We apply both avenues to the Insight source. Approach (i) tests the MS principle as an alternative to the Insight file correction while (ii) demonstrates near coherencelimited operation at a large axial range (>80 mm) for which a too-high sampling rate digitizer would have been needed. In this communication, we evaluate for the first time the suitability of the MS procedure to OCT measurements performed with the akinetic swept source commercialized by Insight. Two modalities are evaluated to implement the MS processing, based on: (i) digital generation of the master signals using the OCT interferometer and (ii) down conversion using a second interferometer driven by the swept source.
In this communication, we present a method to measure the thickness of transparent media employing a low coherence interferometer and a multi-element array as a photo-detector. The multi-element array employed is part of a consumer-grade digital camera (< 0.5 k$). A two-beam interferometer is created by inserting the slab of a transparent material half-way through into the measurement beam. The method is evaluated on a thin microscope cover slip and on a thicker microscope glass slide.
We present here advances on the Master Slave (MS) concept, applicable to spectral/Fourier/frequency-domain optical coherence tomography (OCT) technology. Instead of obtaining an A-scan from the sample investigated via a Fourier Transform (FT) or equivalent, the amplitude of the A-scan for each resolvable point along the depth is obtained along a separate output. A multiplier produces the product of the photo-detected signal from the OCT system with that generated by an Electrical or an Optical Master. This allows acquisition at a frequency comparable to that of the sweeping, much inferior to the frequency bandwidth of the channeled spectrum. 3 advantages of the down-conversion method are demonstrated here: (a) real time delivery of an en-face image; (b) axial optical path difference (OPD) range at the level of the source’s dynamic coherence length and (c): tolerance to fluctuations in the sweep of the swept source. The most important advantage of the down-conversion method is that it reduces the signal bandwidth considerably, to the level of the sweeping rate. This facilitates real-time operation. Conventional A-scan production can only be performed real-time if the FT processing is carried out in a time comparable to or less than the sweep time, which depending on the number of sampled points and dynamic range determines a limit of ∼ MHz sweep rate. Before even calculating a FT, acquisition may also be limited by the sampling rate of the digitiser. In conventional SS-OCT, the number of depth points can exceed 1,000, which for a sweeping time of 1 μs would determine signals in the GHz range. Using long coherence length swept sources, this number of depths could be even larger, hence the conventional FT-based method faces a bottleneck due to the time needed to calculate the FT, combined with the need to acquire data at many GS/s.
Tattoos can be used in forensic human identification as a secondary means of identification (other means being, but not limited to, personal descriptions and artefacts) allowing the identification procedure to be strengthened in this way. Despite this, the decomposition of tattoos is a topic not extensively studied in taphonomic research (study of how organisms decay). In this communication, we assess optical coherence tomography (OCT) as a method to reliably identify tattoos before and after decomposition, by imaging tattooed porcine samples. OCT was able to penetrate up to 3mm below the surface and visualize parts of tattoos after 16 days of decomposition, which were no longer visible and recognizable using conventional photography-based methods. We believe this imaging modality has the potential to increase the reliability of tattoos in forensic human identification.
We report a compact rigid instrument capable of delivering en-face optical coherence tomography (OCT) images alongside (epi)-fluorescence endomicroscopy (FEM) images by means of a robotic scanning device. Two working imaging channels are included: one for a one-dimensional scanning, forward-viewing OCT probe and another for a fiber bundle used for the FEM system. The robotic scanning system provides the second axis of scanning for the OCT channel while allowing the field of view (FoV) of the FEM channel to be increased by mosaicking. The OCT channel has resolutions of 25 / 60 μm (axial/lateral) and can provide en-face images with an FoV of 1.6 × 2.7 mm2. The FEM channel has a lateral resolution of better than 8 μm and can generate an FoV of 0.53 × 3.25 mm2 through mosaicking. The reproducibility of the scanning was determined using phantoms to be better than the lateral resolution of the OCT channel. Combined OCT and FEM imaging were validated with ex-vivo ovine and porcine tissues, with the instrument mounted on an arm to ensure constant contact of the probe with the tissue. The OCT imaging system alone was validated for in-vivo human dermal imaging with the handheld instrument. In both cases, the instrument was capable of resolving fine features such as the sweat glands in human dermal tissue and the alveoli in porcine lung tissue.
This paper presents the last leg of the evolution of the Master Slave (MS) optical coherence tomography (OCT) technology, towards complex master slave (CMS), where phase information is also delivered. We will show how matrix manipulation of signals can lead to real time display. We have demonstrated that this can be executed on central processing units (CPU)s with no need for graphic processing units (GPU)s, yielding simultaneous display of multiple en-face OCT images (C-scans), two cross-section OCT images (B-scans) and an aggregated image, equivalent to a scanning laser ophthalmoscopy (SLO) image when imaging the retina, which is similar to a confocal microscopy image. The same protocol can obviously be applied employing GPUs when using faster acquisition engines, such as multi MHz swept optical sources.
We investigate the utilization of a high frame rate, 2-D commercial-grade camera in a spectral domain (SD) OCT system driven by a super-luminescent (SLD) light source, using parallel illumination on the sample with a line focus (line-field SD-OCT, LF-SD-OCT). To this goal, several regimes of operation of the camera are evaluated, for different values of the exposure time, ISO and image size, assessing their suitability for depth resolved imaging. A-scans and B-scans of specular and scattering samples are produced, albeit of lesser quality than those we obtained in the past with a relatively expensive, high bit-depth, scientific camera. A comparative study involving several of the camera parameters and their impact on the system’s imaging range and resolution is presented.
We report on the use of the Complex Master-Slave (CMS) method to obtain a long axial range in a sweptsource OCT system, well above the axial range limit imposed by the k-clock of the optical source. This is achieved without the need for software-based k-domain re-sampling or employing an additional Mach-Zehnder interferometer providing a stable k-clock signal to the digitizer board. An imaging range of over 17 mm is reported in each case using a commercially available swept source from either Axsun and Santec operating in the 1 μm region, with a 100 kHz repetition rate, which is about three times the range achievable using either source’s built-in k-clock. We have also analyzed the impact the digitization has on the axial range and resolution of the system.
A short-review of optical coherence tomography (OCT) technologies employed to evaluate and image flow and tiny movements is presented. Over the time, the progress of OCT from time domain to spectral (Fourier) domain SD-OCT has led to new approaches in measuring flow and tissue (object) vibration. In the present document, several procedures are presented of what is known today as OCTA, used to visualize tiny vessels in the human retina and replacing the need for injection in angiography. These methods are now extended to measurements of minuscule spatial variations due to action potential, cell division or tissue deformation in elastography.
Non-destructive sensing and imaging within the body of materials is essential for quality control and very importantly for the development of new materials, equally for industrial and medical applications. Conventional non-destructive testing (NDT) methods, such us ultrasound, exhibit low imaging resolutions, of hundreds of microns and typically require a direct contact between the probe and the sample to be investigated. The speed at which the standard NDT methods performs is also quite restricted. The development of optical coherence tomography (OCT) applications in the field of NDT have grown immensely over the past years, offering faster, higher resolution images in a completely contactless environment with the sample. Optical Coherence Tomography brings a plethora of benefits to the current non-destructive methods. However, a multitude of challenges still need to be overcome to truly make OCT the technique of choice for NDT applications. In this paper, a short overview of the main challenging of producing cross-sectional, transversal and volumetric OCT images are presented with an emphasize on OCT’s capabilities and limitations in producing images in real-time. Real-time OCT images of various samples produced using the Master/Slave technique developed within the Applied Optics Group at the University of Kent are demonstrated.
We present a proof-of-concept for side-viewing endoscopic optical coherence tomography probes designed for real-time imaging. The design employs a commercial rotating micro-engine (Kinetron), with a 1 mm outer diameter and maximum speed of 10,000 rpm, to steer the beam from a GRIN lens-terminated optical fiber (Agiltron), 360 degrees around the probe body. The engine is encapsulated inside a PET tube with an outer diameter of 1.6 mm, and coupled to a swept-source based optical coherence tomography (SS-OCT) system operating at 1300 nm with an A-scan rate of 100 kHz.
Large material dispersion mismatch between the reference and sample arms, which would otherwise degrade the axial resolution, is compensated for by using the Master-Slave OCT technique to process the interferograms. This allows for more convenient interchange of probes without exactly matching the fibre length. The probe can be configured using GRIN lenses at the end of the fibers with different focal lengths, and the fiber lengths can differ by several cm. When the probe is changed, the reference path is adjusted, but no dispersion compensation is needed, due to use of the Master Slave method developed by our group. We will demonstrate the performance of such systems with full 360 degree tomographic images of scattering phantoms taken at different probe driving speeds, with different configurations of the probe heads.
A detailed assessment of embryo development would assist biologists with selecting the most suitable embryos for transfer leading to higher pregnancy rates. Currently, only low resolution microscopy is employed to perform this assessment. Although this method delivers some information on the embryo surface morphology, no specific details are shown related to its inner structure. Using a Master-Slave Swept-Source Optical Coherence Tomography (SS-OCT), images of bovine embryos from day 7 after fertilization were collected from different depths. The dynamic changes inside the embryos were examined, in detail and in real-time from several depths. To prove our ability to characterize the morphology, a single embryo was imaged over 26 hours. The embryo was deprived of its life support environment, leading to its death. Over this period, clear morphological changes were observed.
Retinal OCT image segmentation is a precursor to subsequent medical diagnosis by a clinician or machine learning algorithm. In the last decade, many algorithms have been proposed to detect retinal layer boundaries and simplify the image representation. Inspired by the recent success of superpixel methods for pre-processing natural images, we present a novel framework for segmentation of retinal layers in OCT volume data. In our framework, the region of interest (e.g. the fovea) is located using an adaptive-curve method. The cell layer boundaries are then robustly detected firstly using 1D superpixels, applied to A-scans, and then fitting active contours in B-scan images. Thereafter the 3D cell layer surfaces are efficiently segmented from the volume data. The framework was tested on healthy eye data and we show that it is capable of segmenting up to 12 layers. The experimental results imply the effectiveness of proposed method and indicate its robustness to low image resolution and intrinsic speckle noise.
In this communication, we present a proof-of-concept polarization-sensitive Optical Coherence Tomography (PS-OCT) which can be used to characterize the retardance and the axis orientation of a linear birefringent sample. This module configuration is an improvement from our previous work1, 2 since it encodes the two polarization channels on the optical path difference, effectively carrying out the polarization measurements simultaneously (snapshot measurement), whilst retaining all the advantages (namely the insensitivity to environmental parameters when using SM fibers) of these two previous configurations. Further progress consists in employing Master Slave OCT technology,3 which is used to automatically compensate for the dispersion mismatch introduced by the elements in the module. This is essential given the encoding of the polarization states on two different optical path lengths, each of them having dissimilar dispersive properties. By utilizing this method instead of the commonly used re-linearization and numerical dispersion compensation methods an improvement in terms of the calculation time required can be achieved.
We present a novel software method (master-slave) to facilitate operation of any SDOCT system. This method relaxes constraints on dispersion compensation and k-domain re-sampling in SDOCT methods without requiring any changes in the hardware used.
In this manuscript we communicate a theoretical study on a plug-in optical module to be used within a Fourier-domain optical coherence tomography system (FD-OCT). The module can be inserted between the object under investigation and any single-mode fiber based FD-OCT imaging instrument, enabling the latter to carry out polarization measurements on the former. Similarly to our previous communication1 this is an active module which requires two sequential steps to perform a polarization measurement. Alternating between the two steps is achieved by changing the value of the retardance produced by two electro-optic polarization modulators, which together behave as a polarization state rotator. By combining the rotation of the polarization state with a projection against a linear polarizer it is possible to ensure that the polarization measurements are free from any undesirable polarization effects caused by the birefringence in the collecting fiber and diattenuation in the fiber-based couplers employed in the system. Unlike our previous work, though, this module adopts an in-line configuration, employing a Faraday rotator to ensure a non-reciprocal behavior between the forward and backward propagation paths. The module design also allows higher imaging rates due to the use of fast electro-optic modulators. Simulations have been carried out accounting for the chromatic effects of the polarization components, in order to evaluate the theoretical performance of the module.
In this report we applied the principle of Master-Slave Interferometry (MSI) to an Optical Coherence Tomography (OCT)
employing a Super-Continuum (SC) light source. A-scans and B-scan images of biological and non-biological sample are
presented in order to demonstrate similar performance with the images obtained with the resampled Fourier Transform
(FT) based OCT technique. Dispersion tolerance of MSI method is demonstrated as a constant axial resolution over the
depth range even though dispersion is left uncompenstaed in the system.
This communication presents a spectral-domain, polarization-sensitive optical coherence tomography (PS-OCT)
system based on a fiber interferometer using single-mode fibers and couplers. The two orthogonal polarization
components which define the polarization state are sequentially detected by a single line camera. Retardance
measurements can be affected by polarimetric effects in fibers and couplers. This configuration bypasses such
issues by performing the polarization selection before the collection fiber, employing a combination of a polarization
rotator and a linear polarizer. Numerical simulations are carried out to verify the tolerance of the proposed
configuration to fiber-based disturbances; this was further experimentally verified with similar net retardance
maps of a birefringent phantom being obtained for two different settings of induced fiber birefringence.
In this study, we present a new algorithm based on an artificial neural network (ANN) for reducing
speckle noise from optical coherence tomography (OCT) images. The noise is modeled for different parts
of the image using Rayleigh distribution with a noise parameter, sigma, estimated by the ANN. This is
then used along with a numerical method to solve the inverse Rayleigh function to reduce the noise in the
image. The algorithm is tested successfully on OCT images of retina, demonstrating a significant increase
in the signal-to-noise ratio (SNR) and the contrast of the processed images.
Spectrometer-based Optical Coherence Tomography (Sp-OCT) introduced a number of improvements in terms of imaging speed and overall sensitivity over the existing OCT methods. This variant uses a spectrometer to acquire channelled spectra resulting from white light interferometry, which are then processed to yield the depth profiles for each transverse position. A spectrometer can be constructed using a dispersive medium such as a diffraction grating, focusing optics and a linear camera, which allows for a certain degree of control over the spectrometer parameters (exposure time, wavelength range and resolution). The recent advances in line camera technology also allow for fast camera readings, effectively increasing the frame rate of OCT B-scan images acquired and minimizing motion artifacts, which is especially important when imaging moving samples. In the presented work the usage of a commercial line camera (Basler Sprint spl-140km) employed in a Talbot bands based OCT system is reported. An analysis of how the configured parameters (exposure time, pixel binning, number of pixels read) affect the spectrometer’s resolution and the OCT sensitivity profile is also described.
The purpose of this study was to show how to favorably mix two e_ects to improve the sensitivity with depth
in Fourier domain optical coherence tomography (OCT): Talbot bands (TB) and Gabor-based fusion (GF)
technique. TB operation is achieved by directing the two beams, from the object arm and from the reference
arm in the OCT interferometer, along parallel separate paths towards the spectrometer. By changing the lateral
gap between the two beams in their path towards the spectrometer, the position for the maximum sensitivity
versus the optical path difference in the interferometer is adjusted. For five values of the focus position, the gap
between the two beams is readjusted to reach maximum sensitivity. Then, similar to the procedure employed in
the GF technique, a composite image is formed by edging together the parts of the five images that exhibited
maximum brightness. The combined procedure, TB/GF is examined for four different values of the beam
diameters of the two beams. Also we demonstrate volumetric FD-OCT images with mirror term attenuation
and sensitivity profile shifted towards higher OPD values by applying a Talbot bands configuration.
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