Portrait of Dr Adrian Bradu

Dr Adrian Bradu

Lecturer in Applied Optics

About

Dr Adrian Bradu studied for his Msc in Optics, Optoelectronics and Microwaves at École Nationale Supérieure d’Électronique et de Radioélectricité de Grenoble (ENSERG), Grenoble, France; his thesis was 'Spectrophotometry of turbid media using optical fibre probes'. He went on to complete his PhD in the group of Professor Jacques Derouard at Joseph Fourier University, Grenoble, France in 2004. His thesis title was 'Optical methods used to investigate biological tissues. Cerebral tissue spectroscopy using small optical fibre probes and optical coherence tomography imaging'.

Publications

Showing 50 of 195 total publications in the Kent Academic Repository. View all publications.

Article

  • Podoleanu, A., Cernat, R. and Bradu, A. (2019). Down-conversion en-face optical coherence tomography. Biomedical Optics Express [Online] 10:772-788. Available at: https://doi.org/10.1364/BOE.10.000772.
    We present an Optical Coherence Tomography (OCT) method that can deliver an en-face OCT image from a sample in real-time, irrespective of the tuning speed of the swept source. The method, based on the Master Slave interferometry technique, implements a coherence gate principle by requiring that the optical path difference (OPD) between the arms of an imaging interferometer is the same with the OPD in an interrogating interferometer. In this way, a real-time en-face OCT image can originate from a depth in the sample placed in the imaging interferometer, selected by actuating on the OPD in the interrogating interferometer, while laterally scanning the incident beam over the sample. The generation of the en-face image resembles time domain OCT, with the difference that here the signal is processed based on spectral domain OCT. The optoelectronic processor operates down-conversion of the chirped radio frequency signal delivered by the photo-detector. The down-conversion factor is equal to the ratio of the maximum frequency of the photo-detected signal due to an OPD value matching the coherence length of the swept source, to the sweeping rate. This factor can exceed 106 for long coherence swept sources.
  • Rivet, S. et al. (2019). Fast spectrally encoded Mueller optical scanning microscopy. Scientific Reports [Online] 9:3972. Available at: https://doi.org/10.1038/s41598-019-40467-z.
    Mueller microscopes enable imaging of the optical anisotropic properties of biological or non-biological samples, in phase and amplitude, at sub-micrometre scale. However, the development of Mueller microscopes poses an instrumental challenge: the production of polarimetric parameters must be sufficiently quick to ensure fast imaging, so that the evolution of these parameters can be visualised in real-time, allowing the operator to adjust the microscope while constantly monitoring them. In this report, a full Mueller scanning microscope based on spectral encoding of polarization is presented. The spectrum, collected every 10 μs for each position of the optical beam on the specimen, incorporates all the information needed to produce the full Mueller matrix, which allows simultaneous display of all the polarimetric parameters, at the unequalled rate of 1.5 Hz (for an image of 256×256 pixels). The design of the optical blocks allows for the real-time display of linear birefringent images which serve as guidance for the operator. In addition, the instrument has the capability to easily switch its functionality from a Mueller to a Second Harmonic Generation (SHG) microscope, providing a pixel-to-pixel matching of the images produced by the two modalities. The device performance is illustrated by imaging various unstained biological specimens.
  • Duma, V. et al. (2019). Optical Coherence Tomography Investigations and Modeling of the Sintering of Ceramic Crowns. Materials [Online] 12:947. Available at: https://dx.doi.org/10.3390/ma12060947.
    Dental prostheses are sintered in ovens that sometimes suffer from a loss of calibration. This can lead to variations of the sintering temperature outside the range recommended by the manufacturer. Stress and even fractures in dental ceramics may occur, and this leads to the necessity to rebuild the dental construct. The aim of this work is to monitor the quality of sintering processes using an established biomedical imaging technique—optical coherence tomography (OCT). Conventional current procedures imply the fabrication of supplemental samples that add to the expenses and are only evaluated visually. To our knowledge, we were the first to propose the use of OCT, a non-destructive method that brings objectivity for such assessments, focusing, in a previous study, on metal ceramic dental prostheses. Here, a different material, pressed ceramics, is considered, while we propose a quantitative assessment of the results—using reflectivity profiles of en-face (i.e., constant-depth) OCT images of sintered samples. The results for both the pressed ceramics and metal ceramics prostheses are discussed by obtaining the analytic functions of their reflectivity profiles. A multi-parametric analysis demonstrates the best parameter to characterize the loss of calibration of dental ovens. Rules-of-thumb are extracted; producing dental prostheses with defects can thus be avoided
  • Bradu, A. et al. (2018). Recovering distance information in spectral domain interferometry. Scientific Reports [Online] 8. Available at: http://dx.doi.org/10.1038/s41598-018-33821-0.
    This work evaluates the performance of the Complex Master Slave (CMS) method, that processes the spectra at the interferometer output of a spectral domain interferometry device without involving Fourier transforms (FT) after data acquisition. Reliability and performance of CMS are compared side by side with the conventional method based on FT, phase calibration with dispersion compensation (PCDC). We demonstrate that both methods provide similar results in terms of resolution and sensitivity drop-off. The mathematical operations required to produce CMS results are highly parallelizable, allowing real-time, simultaneous delivery of data from several points of different optical path differences in the interferometer, not possible via PCDC.
  • Rivet, S. et al. (2018). Group refractive index and group velocity dispersion measurement by complex master slave interferometry. Optics Express [Online] 26:21831-21842. Available at: https://doi.org/10.1364/OE.26.021831.
    This paper demonstrates that the complex master slave interferometry (CMSI)
    method used in spectral domain interferometry (SDI) can efficiently be used for accurate
    refractive index and group velocity dispersion measurements of optically transparent samples.
    For the first time, we demonstrate the relevance of the phase information delivered by CMSI
    for dispersion evaluations with no need to linearize data. The technique proposed here has
    been used to accurately measure the group refractive index and the group velocity dispersion
    of a strong dispersive sample (SF6 glass), and a weak dispersive one (distilled water). The
    robustness of the technique is demonstrated through the manipulation of several sets of
    experimental data.
  • Hutiu, G. et al. (2018). Assessment of Ductile, Brittle, and Fatigue Fractures of Metals Using Optical Coherence Tomography. Metals [Online] 8:3-18. Available at: http://dx.doi.org/10.3390/met8020117.
    Some forensic in situ investigations, such as those needed in transportation (for aviation, maritime, road, or rail accidents) or for parts working under harsh conditions (e.g., pipes or turbines) would benefit from a method/technique that distinguishes ductile from brittle fractures of metals—as material defects are one of the potential causes of incidents. Nowadays, the gold standard in material studies is represented by scanning electron microscopy (SEM). However, SEM instruments are large, expensive, time-consuming, and lab-based; hence, in situ measurements are impossible. To tackle these issues, we propose as an alternative, lower-cost, sufficiently high-resolution technique, Optical Coherence Tomography (OCT) to perform fracture analysis by obtaining the topography of metallic surfaces. Several metals have been considered in this study: low soft carbon steels, lamellar graphite cast iron, an antifriction alloy, high-quality rolled steel, stainless steel, and ductile cast iron. An in-house developed Swept Source (SS) OCT system, Master-Slave (MS) enhanced is used, and height profiles of the samples’ surfaces were generated. Two configurations were used: one where the dimension of the voxel was 1000 ?m3 and a second one of 160 ?m3—with a 10 ?m and a 4 ?m transversal resolution, respectively. These height profiles allowed for concluding that the carbon steel samples were subject to ductile fracture, while the cast iron and antifriction alloy samples were subjected to brittle fracture. The validation of OCT images has been made with SEM images obtained with a 4 nm resolution. Although the OCT images are of a much lower resolution than the SEM ones, we demonstrate that they are sufficiently good to obtain clear images of the grains of the metallic materials and thus to distinguish between ductile and brittle fractures—especially with the higher resolution MS/SS-OCT system. The investigation is finally extended to the most useful case of fatigue fracture of metals, and we demonstrate that OCT is able to replace SEM for such investigations as well.
  • Marques, M. et al. (2018). Complex master-slave for long axial range swept-source optical coherence tomography. OSA Continuum [Online] 1:1251-1259. Available at: https://doi.org/10.1364/OSAC.1.001251.
    Using complex master-slave interferometry, we demonstrate extended axial range optical coherence tomography for two commercially available swept sources, well beyond the limit imposed by their k-clocks. This is achieved without k-domain re-sampling and without engaging any additional Mach-Zehnder interferometer providing a k-clock signal to the digitizer. An axial imaging range exceeding 17 mm with an attenuation of less than 30 dB is reported using two commercially available swept sources operating at 1050 nm and a 100 kHz repetition rate. This procedure has more than trebled the range achievable using the k-clock signal provided by the manufacturers. An analysis is presented on the impact that the digitization has on the axial range and resolution of the system.
  • Sinescu, C. et al. (2017). Effects of Temperature Variations during Sintering of Metal Ceramic Tooth Prostheses Investigated Non-Destructively with Optical Coherence Tomography. Applied Sciences [Online] 7:552. Available at: https://doi.org/10.3390/app7060552.
    Calibration loss of ovens used in sintering metal ceramic prostheses leads to stress and 18 cracks in the material of the prostheses fabricated, and ultimately to failure of the dental treatment. 19 Periodic calibration may not be sufficient to prevent such consequences. Evaluation methods based 20 on firing supplemental control samples are subjective, time consuming, and rely entirely on the 21 technician’s skills. The aim of this study was to propose an alternative procedure for such 22 evaluations. Fifty prostheses were sintered in a ceramic oven at a temperature lower, equal or 23 larger than the temperature prescribed by the manufacturer. A non-destructive imaging method, 24 swept source (SS) optical coherence tomography (OCT) was used to evaluate comparatively the 25 internal structure of prostheses so fabricated. A quantitative assessment procedure is proposed, 26 based on en-face OCT images acquired at similar depths inside the samples. Differences in 27 granulation and reflectivity depending on the oven temperature are used to establish 28 rules-of-thumb on judging the correct calibration of the oven. OCT evaluations, made on a regular 29 basis allow an easy and objective monitoring of correct settings in the sintering process. This 30 method can serve rapid identification
  • Caujolle, S. et al. (2017). Speckle variance OCT for depth resolved assessment of the viability of bovine embryos. Biomedical Optics Express [Online] 8:5139-5150. Available at: http://dx.doi.org/10.1364/BOE.8.005139.
    The morphology of embryos produced by in vitro fertilization (IVF) is commonly used to estimate their viability. However, imaging by standard microscopy is subjective and unable to assess the embryo on a cellular scale after compaction. Optical coherence tomography is an imaging technique that can produce a depth-resolved profile of a sample and can be coupled with speckle variance (SV) to detect motion on a micron scale. In this study, day 7 post-IVF bovine embryos were observed either short-term (10 minutes) or longterm (over 18 hours) and analyzed by swept source OCT and SV to resolve their depth profile and characterize micron-scale movements potentially associated with viability. The percentage of en face images showing movement at any given time was calculated as a method to detect the vital status of the embryo. This method could be used to measure the levels of damage sustained by an embryo, for example after cryopreservation, in a rapid and non-invasive way.
  • Bondu, M. et al. (2017). Multispectral photoacoustic microscopy and optical coherence tomography using a single supercontinuum source. Photoacoustics [Online] 9:21-30. Available at: https://doi.org/10.1016/j.pacs.2017.11.002.
    We report on the use of a single supercontinuum (SC) source for multimodal imaging. The 2-octave bandwidth
    (475-2300 nm) makes the SC source suitable for optical coherence tomography (OCT) as well as for multispectral
    photoacoustic microscopy (MPAM). The IR band centered at 1310 nm is chosen for OCT to penetrate deeper into
    tissue with 8 mW average power on the sample. The 500-840 nm band is used for MPAM. The source has the ability
    to select the central wavelength as well as the spectral bandwidth. An energy of more than 35 nJ within a less than 50
    nm bandwidth is achieved on the sample for wavelengths longer than 500 nm. In the present paper, we demonstrate
    the capabilities of such a multimodality imaging instrument based on a single optical source. In-vitro mouse ear B-scan
    images are presented.
  • Cernat, R. et al. (2017). Gabor fusion master slave optical coherence tomography. Biomedical Optics Express [Online] 8:813-827. Available at: http://dx.doi.org/10.1364/BOE.8.000813.
    This paper describes the application of the Gabor filtering protocol to a Master/Slave (MS) swept source optical coherence tomography (SS)-OCT system at 1300 nm. The MS-OCT system delivers information from selected depths, a property that allows operation similar to that of a time domain OCT system, where dynamic focusing is possible. The Gabor filtering processing following collection of multiple data from different focus positions is different from that utilized by a conventional swept source OCT system using a Fast Fourier transform (FFT) to produce an A-scan. Instead of selecting the bright parts of A-scans for each focus position, to be placed in a final B-scan image (or in a final volume), and discarding the rest, the MS principle can be employed to advantageously deliver signal from the depths within each focus range only. The MS procedure is illustrated on creating volumes of data of constant transversal resolution from a cucumber and from an insect by repeating data acquisition for 4 different focus positions. In addition, advantage is taken from the tolerance to dispersion of the MS principle that allows automatic compensation for dispersion created by layers above the object of interest. By combining the two techniques, Gabor filtering and Master/Slave, a powerful imaging instrument is demonstrated. The master/slave technique allows simultaneous display of three categories of images in one frame: multiple depth en-face OCT images, two cross-sectional OCT images and a confocal like image obtained by averaging the en-face ones. We also demonstrate the superiority of MS-OCT over its FFT based counterpart when used with a Gabor filtering OCT instrument in terms of the speed of assembling the fused volume. For our case, we show that when more than 4 focus positions are required to produce the final volume, MS is faster than the conventional FFT based procedure.
  • Zhang, L. et al. (2017). From Macro to Micro: Autonomous Multiscale Image Fusion for Robotic Surgery. IEEE Robotics & Automation Magazine [Online] PP:1-1. Available at: https://doi.org/10.1109/MRA.2017.2680543.
  • Avanaki, M., Bradu, A. and Podoleanu, A. (2017). Optimization of excitation of fiber Fabry–Perot tunable filters used in swept lasers using a phase-correction method. Applied Optics [Online] 56:3378. Available at: https://doi.org/10.1364/AO.56.003378.
    In this paper, we investigate a phase correction method for compensation of the nonlinearity of conventional wavelength swept laser sources based on a fiber Fabry-Perot tunable filter as a wavelength selective element. A triangular waveform signal is commonly used to drive the filter. We however extract the zero crossings from the interferograms and modify the shape of the triangular signal accordingly. This algorithm was tested for different values of the optical path length difference (OPD) in the interferometer set-up. Significant compensation for the nonlinearity of the filter was obtained.
  • Rivet, S. et al. (2017). Passive optical module for polarization-sensitive optical coherence tomography systems. Optics Express [Online] 25:14533-14544. Available at: https://doi.org/10.1364/OE.25.014533.
    The paper presents a proof-of-concept polarization-sensitive swept source Optical Coherence Tomography (OCT) system that performs measurements of the retardance as well as of the axis orientation of a linear birefringent sample. The system performs single input state polarization-sensitive OCT and employs an optical module based on optically passive elements such as two beam displacers and a Faraday rotator. Our implementation of the PS-OCT system does not need any calibration step to compensate for the polarimetric effect of the fibers, and its operation does not require a balanced polarization-diversity detector. The optical module allows
    measurement of the two polarization properties of the sample via two measurements which are performed simultaneously.
  • Rivet, S. et al. (2016). Complex Master Slave Interferometry. Optics Express [Online] 24:2885-2904. Available at: http://dx.doi.org/10.1364/OE.24.002885.
    A general theoretical model is developed to improve the novel Spectral Domain Interferometry method denoted as Master/Slave (MS) Interferometry. In this model, two functions, g and h are introduced to describe the modulation chirp of the channeled spectrum signal due to nonlinearities in the decoding process from wavenumber to time and due to
    dispersion in the interferometer. The utilization of these two functions brings two major improvements to previous implementations of the MS method. A first improvement consists in reducing the number of channeled spectra necessary to be collected at Master stage. In previous MSI implementation, the number of channeled spectra at the Master stage
    equated the number of depths where information was selected from at the Slave stage. The paper demonstrates that two experimental channeled spectra only acquired at Master stage suffice to produce A-scans from any number of resolved depths at the Slave stage. A second improvement is the utilization of complex signal processing. Previous MSI implementations discarded the phase. Complex processing of the electrical signal determined by the channeled spectrum allows phase processing that opens several novel avenues. A first consequence of such signal processing is reduction in the random component of the phase without affecting the axial resolution. In previous MSI implementations, phase instabilities were reduced by an average over the wavenumber that led to reduction in the axial resolution.
  • Bradu, A., Rivet, S. and Podoleanu, A. (2016). Master/slave interferometry – ideal tool for coherence revival swept source optical coherence tomography. Biomedical Optics Express [Online] 7:2453-2468. Available at: http://doi.org/10.1364/BOE.7.002453.
    In this paper, we demonstrate that the master slave (MS) interferometry method can significantly simplify the practice of coherence revival swept source optical coherence tomography (OCT) technique. Previous implementations of the coherence revival technique required considerable resources on dispersion compensation and data resampling. The total tolerance of the MS method to nonlinear tuning, to dispersion in the interferometer and to dispersion due to the laser cavity, makes the MS ideally suited to the practice of coherence revival. In addition, enhanced versatility is allowed by the MS method in displaying shorter axial range images than that determined by the digital sampling of the data. This brings an immediate improvement in the speed of displaying cross-sectional images at high rates without the need of extra hardware such as graphics processing units or field programmable gate arrays. The long axial range of the coherence revival regime is proven with images of the anterior segment of healthy human volunteers.
  • Chin, C. et al. (2016). Acousto-optic tunable filter for dispersion characterization of time-domain optical coherence tomography systems. Applied Optics [Online] 55:5707-5714. Available at: http://dx.doi.org/10.1364/AO.55.005707.
    A broadband supercontinuum light source with an acousto-optic tunable filter (AOTF) are used to characterize dispersion in two time-domain OCT systems, at 850 and 1300 nm. The filter is designed to sweep across two spectral ranges, which are restricted here from 800 to 900 nm and from 1200 to 1500 nm, respectively. Dispersion compensation for 850 nm was achieved with a spectral delay line. Dispersion compensation for 1300 nm was achieved using BK 7 rod glasses in the reference arm. The AOTF allows evaluation of dispersion in under as well as overcompensated systems. The AOTF method is based on wavelength dependence of the optical path difference corresponding to the maximum strength of the interference signal recorded using a mirror as object. Comparison is made between the AOTF method and the more usual method based on measurement of the full width at half-maximum of the autocorrelation peak. This comparison shows that the AOTF method is more accurate in terms of evaluation of the dispersion left uncompensated after each adjustment. The AOTF method additionally provides information on the direction of dispersion compensation.
  • Rivet, S. et al. (2016). Optical module to extend any Fourier-domain optical coherence tomography system into a polarisation-sensitive system. Journal of Optics [Online] 18:5607. Available at: http://dx.doi.org/10.1088/2040-8978/18/6/065607.
    This article presents a theoretical study on an optical module (OM) that can be inserted between an object under investigation and a Fourier-domain optical coherence tomography system, transforming the latter into a polarisation-sensitive optical coherence tomography optical coherence tomography (OCT) system. The module consists of two electro-optic modulators, a Faraday rotator, a linear polariser and a quarter-wave plate. A detailed description on how the module can be used to extract both the net retardance and the fast axis orientation of a linear birefringent sample is presented. This is achieved by taking two sequential measurements for different values of retardance produced by the electro-optic modulator. The module keeps measurements free from undesired polarimetric effects due to birefringence in the single-mode optical fibre and diattenuation in fibre-based couplers within OCT systems. Simulations have been carried out in order to evaluate the effects of chromatic behaviour of the components within the OM.

Book section

  • Bradu, A. et al. (2017). Future developments - Adaptive Optics Applied to Glaucoma Imaging. in: Iester, M., Garway-Heath, D. and Lemij, H. eds. Glaucoma Imaging. Savona, Italy: European Glaucoma Society, pp. 137-143.
    Scanning laser ophthalmoscopy (SLO) and optical coherence tomography (OCT) have substantially advanced ophthalmic imaging by improving the transversal resolution and axial resolution beyond that which can be achieved by the fundus camera. SLO improved the transversal resolution by using a small aperture in front of the photodetector and OCT improved the axial resolution by employing principles of coherence gating. Combining the two technologies into a single instrument produces an OCT instrument with the transversal resolution of an SLO but the finer axial resolution afforded by OCT principles, typically 5-10 um or less. Transversal resolution of both SLO and OCT instruments is limited by the aberrations of the eye to more than 15 um with a pupil size of less than 3 mm. Transversal resolution in both instruments can, in principle, be improved to less than 3 um resolution by enlarging the beam diameter. In practice, this often fails due to imperfections in the crystalline lens, cornea, intraocular fluid, and tear film that disturb the wavefront, and hence distort the round uniformity of the spot illuminating the retina. Applying adaptive optics (AO) principles, aberrations of the eye can be both measured and compensated

Conference or workshop item

  • Marques, M. et al. (2019). Long axial range swept-source OCT instrument enhanced by Complex Master-Slave processing. in: Optical Coherence Tomography and Coherence Domain Optical Methods in Biomedicine XXIII.. Available at: https://dx.doi.org/10.1117/12.2508638.
    We report on the use of the Complex Master-Slave (CMS) method to obtain a long axial range in a swept-source 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.
  • Fauchart, M. et al. (2019). Evaluation of a commercial-grade camera for line field spectral-domain optical coherence tomography. in: Optical Coherence Tomography and Coherence Domain Optical Methods in Biomedicine XXIII.. Available at: https://dx.doi.org/10.1117/12.2511401.
    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.
  • Podoleanu, A. et al. (2019). Speeding up master slave optical coherence tomography by matrix manipulation. in: Goda, K. and Tsia, K. K. eds. SPIE BiOS. SPIE. Available at: https://doi.org/10.1117/12.2511404.
    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.
  • Rivet, S., Bradu, A. and Podoleanu, A. (2019). Employing the phase in master slave interferometry. in: Optical Coherence Tomography and Coherence Domain Optical Methods in Biomedicine XXIII.. Available at: https:/dx./doi.org/10.1117/12.2511405.
    In this paper, we extend the master slave (MS) method, so far applied to the modulus of the spectra acquired in spectral domain interferometry, to processing complex spectra. We present the algorithm of complex master slave interferometry (CMSI) method and illustrate the importance of phase processing for signal stability and strength. We demonstrate better stability of the signal driving a direct en-face OCT image by processing both real part and imaginary part of the CMS signal. Then we show that by processing the phase, novel avenues can be opened for the master slave method. A first avenue detailed here is that of dispersion measurements.
  • Marques, M. et al. (2018). Snapshot polarization-sensitive plug-in optical module for a Fourier-domain optical coherence tomography system. in: SPIE BiOS 2018. SPIE. Available at: http://dx.doi.org/10.1117/12.2291502.
    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.
  • Caujolle, S. et al. (2018). Assessing embryo development using swept source optical coherence tomography. in: Podoleanu, A. G. H. and Bang, O. eds. Second Canterbury Conference on Optical Coherence Tomography, 2017, Canterbury, United Kingdom. SPIE, p. . Available at: https://doi.org/10.1117/12.2282912.
    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.
  • Maroiu, A. et al. (2018). Optoelectronic evaluation of indirect dental veneers interfaces. in: Todea, C., Podoleanu, A. G. H. and Duma, V. -F. eds. Seventh International Conference on Lasers in Medicine. SPIE. Available at: https://doi.org/10.1117/12.2282648.
    Dental indirect veneers have become the most functional and cost-effective method for providing high-aesthetic results whenever smile design enhancement is required. Yet, clinical failures have been reported due to the detachment of the veneers from the dental hard tissues, as well as to chipping and microleakage. Many experimental studies have been conducted in order to identify clinical and technical solutions for enhancing the adhesive and biomechanical properties of the veneers, by promoting the same classical, linear marginal contour of these particular indirect restorations. Thus, the aim of this study is to develop a novel design of the veneers deemed to augment the interfacial adhesive forces and, furthermore, to investigate the bonded interfaces by using optical coherence tomography.
  • Sinescu, C. et al. (2018). Dental Impression Technique Using Optoelectronic Devices. in: Podoleanu, A. G. H. and Bang, O. eds. Second Canterbury Conference on Optical Coherence Tomography, 2017, Canterbury, United Kingdom. SPIE. Available at: https://doi.org/10.1117/12.2281775.
    The use of Optical Coherence Tomography (OCT) as a non-invasive and high
    precision quantitative information providing tool has been well established by researches within the last decade. The marginal discrepancy values can be scrutinized in optical biopsy made in three dimensional (3D) micro millimetre scale and reveal detailed qualitative and quantitative information of soft and hard tissues. OCT-based high resolution 3D images can provide a significant impact on finding recurrent caries, restorative failure, analysing the precision of crown preparation, and prosthetic elements marginal adaptation error with the gingiva and dental hard tissues. During the CAD/CAM process of prosthodontic restorations, the circumvent of any error is important for the practitioner and the technician to reduce waste of time and material. Additionally, OCT images help to achieve a new or semi-skilled practitioner to analyse their crown preparation works and help to develop their skills faster than in a conventional way. The aim of this study is to highlight the advantages of OCT in high precision prosthodontic restorations.
  • Podoleanu, A. et al. (2018). From Doppler to speckle variance measurements in optical coherence tomography. in: Todea, C., Podoleanu, A. G. H. and Duma, V. -F. eds. Seventh International Conference on Lasers in Medicine. SPIE. Available at: https://doi.org/10.1117/12.2283530.
    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.
  • Bradu, A. et al. (2018). Current capabilities and challenges for optical coherence tomography as a high impact non-destructive imaging modality. in: Todea, C. C., Podoleanu, A. G. and Duma, V. -F. eds. Seventh International Conference on Lasers in Medicine. SPIE. Available at: https://doi.org/10.1117/12.2281502.
    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.
  • Hutiu, G. et al. (2018). Roughness measurements using optical coherence tomography: a preliminary study. in: Todea, C. C., Podoleanu, A. G. H. and Duma, V. -F. eds. Seventh International Conference on Lasers in Medicine. SPIE. Available at: https://doi.org/10.1117/12.2282807.
    To determine the roughness is an important aspect in both industrial and biomedical applications. We propose and utilize for roughness evaluations, a non-destructive evaluation methods, Optical Coherence Tomography (OCT). For the metallic surfaces investigated from this point of view, the Ra and Rz parameters are utilized, according to ISO 4287/1988. Also, according to ISO 4280 and ISO 3274 standards, the measurements have been made on 12.5 mm portions. In order to accommodate such evaluations with the specific OCT field-of-view, four consecutive OCT images have been made for each sample, and an appropriate processing of the data collected from the surface profiles has been made. A validation of the results obtained with OCT has been completed with the gold standard for such evaluations, using a contact mechanical method, with a Mitutuyo profiler.
  • Sinescu, C. et al. (2018). Temperature variations in sintering ovens for metal ceramic dental prostheses: Non-destructive assessment using OCT. in: SPIE BiOS, 2018, San Francisco, California, United States.. Available at: https://doi.org/10.1117/12.2289162.
    We present a recent investigation regarding the use of optical coherence tomography (OCT) in the monitoring of the calibration loss of sintering ovens for the manufacturing of metal ceramic dental prostheses. Differences in the temperatures of such ovens with regard to their specifications lead to stress and even cracks in the prostheses material, therefore to the failure of the dental treatment. Evaluation methods of the ovens calibration consist nowadays of firing supplemental samples; this is subjective, expensive, and time consuming. Using an in-house developed swept source (SS) OCT system, we have demonstrated that a quantitative assessment of the internal structure of the prostheses, therefore of the temperature settings of the ovens can be made. Using en-face OCT images acquired at similar depths inside the samples, the differences in reflectivity allow for the evaluation of the differences in granulation (i.e., in number and size of ceramic grains) of the prostheses material. Fifty samples, divided in five groups, each sintered at different temperatures (lower, higher, or equal to the prescribed one) have been analyzed. The consequences of the temperature variations with regard to the one prescribed were determined. Rules-of-thumb were extracted to monitor objectively, using only OCT images of currently manufactured samples, the settings of the oven. The method proposed allows for avoiding producing prostheses with defects. While such rules-of-thumb achieve a qualitative assessment, an insight in our on-going work on the quantitative assessment of such losses of calibration on dental ovens using OCT is also made.
  • Cernat, R. et al. (2018). Time efficient Gabor fused Master Slave optical coherence tomography. in: SPIE BiOS: Optical Coherence Tomography and Coherence Domain Optical Methods in Biomedicine XXII. San Francisco, California, United States: SPIE. Available at: https://doi.org/10.1117/12.2292058.
    In this paper the benefits in terms of operation time that Master/Slave (MS) implementation of optical coherence tomography can bring in comparison to Gabor fused (GF) employing conventional fast Fourier transform based OCT are presented. The Gabor Fusion/Master Slave Optical Coherence Tomography architecture proposed here does not need any data stitching. Instead, a subset of en-face images is produced for each focus position inside the sample to be imaged, using a reduced number of theoretically inferred Master masks. These en-face images are then assembled into a final volume. When the channelled spectra are digitized into 1024 sampling points, and more than 4 focus positions are required to produce the final volume, the Master Slave implementation of the instrument is faster than the conventional fast Fourier transform based procedure.
  • Niels, I. et al. (2018). Resolution dependence on phase extraction by the Hilbert transform in phase calibrated and dispersion compensated ultrahigh resolution spectrometer based OCT. in: Podoleanu, A. G. H. and Bang, O. eds. Second Canterbury Conference on Optical Coherence Tomography, 2017, Canterbury, United Kingdom. SPIE. Available at: https://doi.org/10.1117/12.2282295.
    Ultrahigh resolution optical coherence tomography (UHR-OCT) is enabled by using a broad band source.
    Simultaneously, this makes the OCT image more sensitive to dispersion mismatch in the interferometer. In
    spectral domain OCT, dispersion left uncompensated in the interferometer and detector non-linearities lead
    together to an unknown chirp of the detected interferogram. One method to compensate for the chirp is to
    perform a pixel-wavenumber calibration versus phase that requires numerical extraction of the phase. Typically a Hilbert transform algorithm is employed to extract the optical phase versus wavenumber for calibration and dispersion compensation. In this work we demonstrate UHR-OCT at 1300 nm using a Super continuum source and highlight the resolution constraints in using the Hilbert transform algorithm when extracting the optical phase for calibration and dispersion compensation. We demonstrate that the constraints cannot be explained purely by the numerical errors in the data processing module utilizing the Hilbert transform but must be dictated by broadening mechanisms originating from the experimentally obtained interferograms.
  • Todor, R. et al. (2018). Investigation of firing temperature variation in ovens for ceramic-fused-to-metal dental prostheses using swept source optical coherence tomography. in: 2nd Canterbury Conference on OCT with Emphasis on Broadband Optical Sources. SPIE. Available at: https://doi.org/10.1117/12.2281892.
    One of the most common fabrication techniques for dental ceramics is sintering, a process of heating of the ceramic to ensure densification. This occurs by viscous flow when the firing temperature is reached. Acceptable restorations require the alloy and ceramic to be chemically, thermally, mechanically, and aesthetically compatible. Thermal and mechanical compatibility include a fusing temperature of ceramic that does not cause distortion of the metal substructure. Decalibration of ovens used for firing of the ceramic layers for metal ceramic dental prostheses leads to stress and cracks in the veneering material, and ultimately to the failure of the restoration. 25 metal ceramic prostheses were made for this study. They were divided in five groups, each sintered at a different temperature: a group at the temperature prescribed by the producer, two groups at lower and two groups at higher temperatures set in the ceramic oven. An established noninvasive biomedical imaging method, swept source (SS) optical coherence tomography (OCT) was employed, in order to evaluate the modifications induced when using temperatures different from those prescribed for firing the samples. A quantitative assessment of the probes is performed by en-face OCT images, taken at constant depths inside the samples. The differences in granulation, thus in reflectivity allow for extracting rules-of-thumb to evaluate fast, by using only the prostheses currently produced the current calibration of the ceramic oven. OCT imaging can allow quick identification of the oven decalibration, to avoid producing dental prostheses with defects.
  • SZUHANEK, C. et al. (2018). Optical coherence tomography study regarding the enamel structure before and after debonding. in: Todea, C. C., Podoleanu, A. G. and Duma, V. -F. eds. Seventh International Conference on Lasers in Medicine. SPIE. Available at: https://doi.org/10.1117/12.2282650.
    Orthodontic treatments imply the use of different types of adhesives and brackets. However, concerns have been raised regarding the effect of these treatments on the structure of the involved teeth. The debonding process is especially regarded as a concern: due to the use of different pliers tensile and pulling forces the develop on the surface of the tooth. The finishing bur is also a concern. Optical Coherence Tomography (OCT), an emerging technology that performs transverse sections of biological systems has been used in order to obtain a more accurate assessment of enamel quality due to its wide applicability and to its non-invasive properties.OCT, analogous to ultrasound imaging(with the difference that it uses light instead of sound), provides cross-sectional images of the tissue structure on the micron scale, in vivo and in real time. Regarding fixed orthodontic treatments, patients are often subjected in the process to a high risk of enamel decalcification and carious processes. Demineralization usually occurs in the area adjacent to the orthodontic bracket location, where bacterial plaque control is difficult. Therefore, in this study we evaluate using OCT the degree of demineralisation produced in the enamel structure, following the removal of the orthodontic bracket. Also, the amount of adhesive remnants after the removal of the adhesive and the finishing of the dental surface with specific instrumentation is evaluated.
  • Rivet, S. et al. (2018). From master slave interferometry to complex master slave interferometry: theoretical work. in: Podoleanu, A. G. H. and Bang, O. eds. Second Canterbury Conference on Optical Coherence Tomography, 2017, Canterbury, United Kingdom. SPIE. Available at: https://doi.org/10.1117/12.2303761.
    A general theoretical framework is described to obtain the advantages and the drawbacks of two novel Fourier Domain Optical Coherence Tomography (OCT) methods denoted as Master/Slave Interferometry (MSI) and its extension denoted as Complex Master/Slave Interferometry (CMSI). Instead of linearizing the digital data representing the channeled spectrum before a Fourier transform can be applied to it (as in OCT standard methods), channeled spectrum is decomposed on the basis of local oscillations. This replaces the need for linearization, generally time consuming, before any calculation of the depth profile in the range of interest. In this model two functions, g and h, are introduced. The function g describes the modulation chirp of the channeled spectrum signal due to nonlinearities in the decoding process from wavenumber to time. The function h describes the dispersion in the interferometer. The utilization of these two functions brings two major improvements to previous implementations of the MSI method. The paper details the steps to obtain the functions g and h, and represents the CMSI in a matrix formulation that enables to implement easily this method in LabVIEW by using parallel programming with multi-cores.
  • Adrian, B., David, J. and Adrian, P. (2018). Long axial imaging range using conventional swept source lasers in optical coherence tomography via re-circulation loops. in: Podoleanu, A. and Bang, O. eds. Second Canterbury Conference on Optical Coherence Tomography, 2017, Canterbury, United Kingdom. SPIE. Available at: https://doi.org/10.1117/12.2282116.
    Typically, swept source optical coherence tomography (SS-OCT) imaging instruments are capable of a longer axial range than their camera based (CB) counterpart. However, there are still various applications that would take advantage for an extended axial range. In this paper, we propose an interferometer configuration that can be used to extend the axial range of the OCT instruments equipped with conventional swept-source lasers up to a few cm. In this configuration, the two arms of the interferometer are equipped with adjustable optical path length rings. The use of semiconductor optical amplifiers in the two rings allows for compensating optical losses hence, multiple paths depth reflectivity profiles (Ascans) can be combined axially. In this way, extremely long overall axial ranges are possible. The use of the recirculation loops produces an effect equivalent to that of extending the coherence length of the swept source laser. Using this approach, the achievable axial imaging range in SS-OCT can reach values well beyond the limit imposed by the coherence length of the laser, to exceed in principle many centimeters. In the present work, we demonstrate axial ranges exceeding 4 cm using a commercial swept source laser and reaching 6 cm using an “in-house” swept source laser. When used in a conventional set-up alone, both these lasers can provide less than a few mm axial range.
  • Podoleanu, A. et al. (2018). Supercontinuum applications in high resolution non invasive optical imaging. in: CLEO: Applications and Technology 2018. OSA. Available at: https://doi.org/10.1364/CLEO_AT.2018.AW3S.1?
    Progress will be presented in adapting supercontinuum sources to a variety of applications with emphasis on signal processing procedures. These are customised to alleviate noise and take full advantage of the large bandwidth and large power spectral density of modern supercontinuum sources.
  • Adrian, B., Sylvain, R. and Adrian, P. (2017). Master/slave: the ideal tool for coherence revival based optical coherence tomography imaging instruments. in: Fujimoto, J. G., Izatt, J. A. and Tuchin, V. V. eds. Optical Coherence Tomography and Coherence Domain Optical Methods in Biomedicine XXI. SPIE Society of Photo-Optical Instrumentation Engineers. Available at: http://dx.doi.org/10.1117/12.2253638.
    In this communication, we present the utility of the Master/Slave (MS) method in combination with the coherence revival technique to perform full axial range Optical Coherence Tomography (OCT). The MS method eliminates two major drawbacks of the conventional Fourier Transformed (FT) based OCT technology when applied to the coherence revival technique: the need of data re-sampling as well as the need to compensate for unbalanced dispersion in the interferometer.
  • Bradu, A. et al. (2017). Master/slave: a better tool for Gabor filtering optical coherence tomography imaging instruments. in: European Conference of Biomedical Optics. Society of Photo-optical Instrumentation Engineers, p. 104160N. Available at: http://dx.doi.org/10.1117/12.2286057.
    In this report, the benefits that the Master/Slave (MS) implementation of optical coherence tomography (OCT) can bring to a Gabor filtering (GF) imaging instrument are illustrated. The MS allows simultaneous display of three categories of images in one frame: multiple depth en-face OCT images, two B-scan OCT and a confocal like image. The power of MS is illustrated here by showing 3D images of constant transversal resolution from different objects, obtained by merging sub-volumes collected for four different focus positions. By combining the two techniques, GF and MS, a powerful imaging instrument is demonstrated. We show that when more than four focus positions are required, MS can produce fused volumes faster than the conventional FT based procedure. © (2017) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
  • Adrian, B., Sylvain, R. and Adrian, P. (2017). Master/slave based optical coherence tomography for in-vivo, real-time, long axial imaging range of the anterior segment. in: Manns, F., Söderberg, P. G. and Ho, A. eds. Opthalmic Technologies XXVII. SPIE Society of Photo-Optical Instrumentation Engineers. Available at: http://dx.doi.org/10.1117/12.2251873.
    In this report, we demonstrate that in a coherence revival (CR) based swept source optical coherence tomography (SS-OCT) set-up, real-time cross-sectional long-range images can be produced via the Master Slave (MS) method. The total tolerance of the MS method to nonlinear tuning, to dispersion in the interferometer and to dispersion due to the laser cavity, makes the MS ideally suited to the practice of coherence revival. In addition, enhanced versatility is allowed by the MS method in displaying shorter axial range images than that determined by the digital sampling of the data. This brings an immediate improvement in the speed of displaying cross-sectional images at high rates without the need of extra hardware such as graphics processing units or field programmable gate arrays. The long axial range of the coherence revival regime is proven with images of the anterior segment of healthy human eye.
  • Marques, M. et al. (2017). Polarization-sensitive plug-in optical module for a Fourier-domain optical coherence tomography system. in: Fujimoto, J. G., Izatt, J. A. and Tuchin, V. V. eds. Optical Coherence Tomography and Coherence Domain Optical Methods in Biomedicine XXI. SPIE Society of Photo-Optical Instrumentation Engineers. Available at: http://dx.doi.org/10.1117/12.2254832.
    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 communication 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.
  • Adrian, B., Sylvain, R. and Adrian, P. (2017). Replacing the Fourier transformation in optical coherence tomography with multiple comparison operations. in: Photonics West, San Francisco.
    The conventional spectral domain (SD) and Fourier domain (FD) OCT method deliver a 1D reflectivity profile in the sample investigated by applying a Fourier transform (FT) to the channeled spectrum, CS, at the interferometer output. We discuss here the advantages of a novel OCT technology, Master Slave (MS). The MS method radically changes the main building blocks of a SD (FD)-OCT set-up. The serially provided electrical signal in conventional technology is replaced by multiple signals, a signal for each OPD point along an electrical output for each depth in the object investigated. In this way, it is possible to: (1) direct access to information from selected depths; (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, reduction in the time to display an image and lower cost OCT assembly; (iii) OCT interferometer tolerant to dispersion left unbalanced.
  • Marques, M. et al. (2017). Novel software package to facilitate operation of any spectral (Fourier) OCT system. in: Wojtkowski, M., Boppart, S. A. and Oh, W. -Y. eds. European Conferences on Biomedical Optics. SPIE. Available at: http://dx.doi.org/10.1117/12.2284184.
    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.
  • Hutiu, G. et al. (2017). Analysis of the fractures of metallic materials using optical coherence tomography. in: SPIE Optical Metrology, 2017. SPIE. Available at: http://dx.doi.org/10.1117/12.2270117.
    Forensic in situ investigations, for example for aviation, maritime, road, or rail accidents would benefit from a method that may allow to distinguish ductile from brittle fractures of metals - as material defects are one of the potential causes of such accidents. Currently, the gold standard in material studies is represented by scanning electron microscopy (SEM). However, SEM are large, lab-based systems, therefore in situ measurements are excluded. In addition, they are expensive and time-consuming. We have approached this problem and propose the use of Optical Coherence Tomography (OCT) in such investigations in order to overcome these disadvantages of SEM. In this respect, we demonstrate the capability to perform such fracture analysis by obtaining the topography of metallic surfaces using OCT. Different materials have been analyzed; in this presentation a sample of low soft carbon steel with the chemical composition of C 0.2%, Mn 1.15%, S 0.04%, P 0.05 % and Fe for the rest has been considered. An in-house developed Swept Source (SS) OCT system has been used, and height profiles have been generated for the sample surface. This profile allowed for concluding that the carbon steel sample was subjected to a ductile fracture. A validation of the OCT images obtained with a 10 microns resolution has been made with SEM images obtained with a 4 nm resolution. Although the OCT resolution is much lower than the one of SEM, we thus demonstrate that it is sufficient in order to obtain clear images of the grains of the metallic materials and thus to distinguish between ductile and brittle fractures. This study analysis opens avenues for a range of applications, including: (i) to determine the causes that have generated pipe ruptures, or structural failures of metallic bridges and buildings, as well as damages of machinery parts; (ii) to optimize the design of various machinery; (iii) to obtain data regarding the structure of metallic alloys); (iv) to improve the manufacturing technologies of metallic parts.
  • Gabor, A. et al. (2016). OCT evaluation of single ceramic crowns: comparison between conventional and chair-side CAD/CAM technologies. in: Todea, C. C., Podoleanu, A. G. H. and Duma, V. -F. eds. Fifth Congress of the World Federation for Laser Dentistry and Sixth International Conference on Lasers in Medicine. Bellingham, Washington, United States: SPIE Society of Photo-Optical Instrumentation Engineers, p. 96700Z. Available at: http://doi.org/10.1117/12.2191907.
    Digital impressions were introduced to overcome some of the obstacles due to traditional impression materials and techniques. The aim of this in vitro study is to compare the accuracy of all ceramic crowns obtained with digital impression and CAD-CAM technology with the accuracy of those obtained with conventional impression techniques. Two groups of 10 crowns each have been considered. The digital data obtained from Group 1 have been processed and the all-ceramic crowns were milled with a CAD/CAM technology (CEREC MCX, Sirona). The all ceramic crowns in Group 2 were obtained with the classical technique of pressing (emax, Ivoclar, Vivadent). The evaluation of the marginal adaptation was performed with Time Domain Optical Coherence Tomography (TD OCT), working at a wavelength of 1300 nm. Tri-dimensional (3D) reconstructions of the selected areas were obtained. Based on the findings in this study, one may conclude that the marginal accuracy of all ceramic crowns fabricated with digital impression and the CAD/CAM technique is superior to the conventional impression technique. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
  • Bradu, A. et al. (2016). En-face optical coherence tomography revival. in: Todea, D. C., Podoleanu, A. G. H. and Duma, V. -F. eds. Fifth Congress of the World Federation for Laser Dentistry and Sixth International Conference on Lasers in Medicine. Bellingham, Washington, United States: SPIE Society of Photo-Optical Instrumentation Engineers, p. 9670 OT. Available at: http://dx.doi.org/10.1117/12.2191541.
    Quite recently, we introduced a novel Optical Coherence Tomography (OCT) method, termed as Master Slave OCT (MS-OCT), especially to deliver en-face images. MS-OCT operates like a time domain OCT, selecting signal from a selected depth only while scanning the laser beam across the sample. Time domain OCT allows real time production of an en-face image, although relatively slowly. As a major advance, the Master Slave method allows collection of signals from any number of depths, as required by the user. MS-OCT is an OCT method that does not require resampling of data and can be used to deliver en-face images from several depths simultaneously. However, as the MS-OCT method requires important computational resources, the number of multiple depth en-face images produced in real-time is limited. Here, we demonstrate that taking advantage of the parallel processing feature of the MS-OCT technology by harnessing the capabilities of graphics processing units (GPU)s, information from 384 depth positions is acquired in one raster with real time display of 40 en-face OCT images. These exhibit comparable resolution and sensitivity to the images produced using the traditional Fourier domain based method. The GPU facilitates versatile real time selection of parameters, such as the depth positions of the 40 images out of a set of 384 depth locations, as well as their axial resolution. Here, we present in parallel with the 40 en-face OCT images of a human tooth, a confocal microscopy lookalike image, together with two B-scan OCT images along rectangular directions. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
  • Marques, M. et al. (2016). Spectral-domain, polarization-sensitive optical coherence tomography system insensitive to fiber disturbances. in: Izatt, J. A., Fujimoto, J. G. and Tuchin, V. V. eds. SPIE BiOS. International Society for Optics and Photonics. Available at: http://www.dx.doi.org/10.1117/12.2213479.
    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. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE).
  • Duma, V. et al. (2016). Handheld scanning probes for Optical Coherence Tomography: Developments, applications and perspectives. in: Todea, C. C., Podoleanu, A. G. H. and Duma, V. -F. eds. Fifth Congress of the World Federation for Laser Dentistry and Sixth International Conference on Lasers in Medicine. Bellingham, Washington, United States: SPIE Society of Photo-Optical Instrumentation Engineers. Available at: http://dx.doi.org/10.1117/12.2178590.
    We present the handheld scanning probes that we have recently developed in our current project for biomedical imaging in general and for Optical Coherence Tomography (OCT) in particular. OCT is an established, but dynamic imagistic technique based on laser interferometry, which offers micrometer resolutions and millimeters penetration depths. With regard to existing devices, the newly developed handheld probes are simple, light and relatively low cost. Their design is described in detail to allow for the reproduction in any lab, including for educational purposes. Two probes are constructed almost entirely from off-the-shelf components, while a third, final variant is constructed with dedicated components, in an ergonomic design. The handheld probes have uni-dimensional (1D) galvanometer scanners therefore they achieve transversal sections through the biological sample investigated - in contrast to handheld probes equipped with bi-dimensional (2D) scanners that can also achieve volumetric (3D) reconstructions of the samples. These latter handheld probes are therefore also discussed, as well as the possibility to equip them with galvanometer 2D scanners or with Risley prisms. For galvanometer scanners the optimal scanning functions studied in a series of previous works are pointed out; these functions offer a higher temporal efficiency/duty cycle of the scanning process, as well as artifact-free OCT images. The testing of the handheld scanning probes in dental applications is presented, for metal ceramic prosthesis and for teeth. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
  • Marcauteanu, C. et al. (2016). Evaluation of Tizian overlays by means of a swept source optical coherence tomography system. in: Todea, C. C., Podoleanu, A. G. H. and Duma, V. -F. eds. Fifth Congress of the World Federation for Laser Dentistry and Sixth International Conference on Lasers in Medicine. Bellingham, Washington, United States: SPIE Society of Photo-Optical Instrumentation Engineers, p. 96700X. Available at: http://doi.org/10.1117/12.2191905.
    The teeth affected by pathologic attrition can be restored by a minimally invasive approach, using Tizian overlays. In this study we prove the advantages of a fast swept source (SS) OCT system in the evaluation of Tizian overlays placed in an environment characterized by high occlusal forces. 12 maxillary first premolars were extracted and prepared for overlays. The Tizian overlays were subjected to 3000 alternating cycles of thermo-cycling (from -10°C to +50°C) and to mechanical occlusal overloads (at 800 N). A fast SS OCT system was used to evaluate the Tizian overlays before and after the mechanical and thermal straining. The SS (Axsun Technologies, Billerica, MA) has a central wavelength of 1060 nm, sweeping range of 106 nm (quoted at 10 dB) and a 100 kHz line rate. The depth resolution of the system, measured experimentally in air was 10 ?m. The imaging system used for this study offers high spatial resolutions in both directions, transversal and longitudinal of around 10 ?m, a high sensitivity, and it is also able to acquire entire tridimensional (3D)/volume reconstructions as fast as 2.5 s. Once the full dataset was acquired, rendered high resolutions en-face projections could be produced. Using them, the overlay (i.e., cement) abutment tooth interfaces were remarked both on B-scans/two-dimensional (2D) sections and in the 3D reconstructions. Using the system several open interfaces were possible to detect. The fast SS OCT system thus proves useful in the evaluation of zirconia reinforced composite overlays, placed in an environment characterized by high occlusal forces. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
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