Portrait of Professor Adrian Podoleanu

Professor Adrian Podoleanu

Professor of Biomedical Optics
Professor of Biomedical Optics and Committee Member
Head of the Applied Optics Group

About

Adrian Podoleanu received his PhD in Electronics from the Electronics and Telecommunications Faculty, Technical University of Bucharest, Romania in 1984. He began his career teaching at the Technical University Bucharest, Romania. As an associate professor, he taught physics, optics and optoelectronics and developed research on lasers and fast optoelectronics.

Since 2004 he has been Professor of Biomedical Optics in the School of Physical Sciences at the University of Kent, where he  heads the Applied Optics Group. His research interests focus on optical coherence tomography (OCT), imaging the eye, distance measurements using low coherence interferometry, sub-nanosecond dwell time multichannel digital correlation, sensing and secure optical communications. He contributed towards development of the en-face OCT imaging as a novel technology to complement the more conventional longitudinal OCT imaging, towards the dual imaging instrument for the eye, OCT/SLO and towards Fourier domain OCT with no mirror terms (using Talbot bands).

Professor Podoleanu is a Faculty Investigator, National Institute for Health Research, Biomedical Research Centre at Moorfields Eye Hospital NHS Foundation Trust and UCL Institute of Ophthalmology. He is an Honorary Professor of University College London and Visiting Professor of Ophthalmology, New York Medical College, School of Medicine, Touro College. He is also one of eight elected Vice-Presidents of the International Commission of Optics. In this capacity he is the Chair of the IUPAP Young Scientist Prize in Optics. 

Professor Podoleanu's publishing record includes 15 book chapters, over 200 articles in peer-reviewed journals, and over 700 communications at conferences. He is a member of the editorial boards of six journals, has chaired eight international conferences and acted as a member in the organising committees of over 50 international conferences. He has been awarded 21 patents. He is also director of Optopod Ltd, a spin-out of the University of Kent involved in spectral OCT.

Research interests

Professor Podoleanu's research interests include:

  • non-invasive imaging of the tissue, especially optical coherence tomography and confocal microscopy
  • optical sensing
  • fast optoelectronics.

His published works may be accessed at:

He has an h-index ISI excluding self citations of 31; Google Scholar: 43; i-10 index (number of papers with more than 10 citations): 149.

Research grants

Professor Podoleanu currently has research grants as follows:

Previous grants and activities include:

Teaching

Professor Podoleanu is involved in the teaching of non-invasive optical imaging methods with emphasis on OCT, electromagnetism and optics, and medical physics.

Professional

Awards

Fellowships and honours

Publications

Showing 50 of 510 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.
  • Jensen, M. et al. (2019). Noise of supercontinuum sources in spectral domain optical coherence tomography. Journal of the Optical Society of America B [Online] 36:A154-A160. Available at: https://doi.org/10.1364/JOSAB.36.00A154.
    In this paper, we investigate the effect of pulse-to-pulse fluctuations of super continuum sources on the noise in spectral domain optical coherence tomography (OCT) images. The commonly quoted theoretical expression for the OCT noise is derived for a thermal light source, which is not suitable if a super continuum light source is used. We therefore propose a new, measurement-based OCT noise model that predicts the noise without any assumptions on the type of light source. We show that the predicted noise values are in excellent agreement with the measured values. The spectral correlation evaluated for the photo detected signal when using a supercontinuum determines the shape of the OCT noise floor, which must be taken into account when characterising the sensitivity roll-off of a supercontinuum-based OCT system. The spectral correlations using both conventional supercontinuum sources and low-noise all-normal dispersion super continuum sources are investigated, and the fundamental physical effects that cause these correlations are discussed.
  • Manwar, R. et al. (2019). An Application of Simulated Annealing in Compensation of Nonlinearity of Scanners. Applied Sciences [Online] 9:1655. Available at: https://doi.org/10.3390/app9081655.
    Galvo scanners are popular devices for fast transversal scanning. A triangular signal is usually employed to drive galvo scanners at scanning rates close to the inverse of their response
    time where scanning deflection becomes a nonlinear function of applied voltage. To address this, the triangular signal is synthesized from several short ramps with different slopes. An optimization algorithm similar to a simulated annealing algorithm is used for finding the optimal signal shape to drive the galvo scanners. As a result, a significant reduction in the nonlinearity of the galvo scanning is obtained.
  • 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
  • Israelsen, N. et al. (2019). Real-time high-resolution mid-infrared optical coherence tomography. Light: Science & Applications [Online] 8. Available at: https://doi.org/10.1038/s41377-019-0122-5.
    The potential for improving the penetration depth of optical coherence tomography systems by using light sources with longer wavelengths has been known since the inception of the technique in the early 1990s. Nevertheless, the development of mid-infrared optical coherence tomography has long been challenged by the maturity and fidelity of optical components in this spectral region, resulting in slow acquisition, low sensitivity, and poor axial resolution. In this work, a mid-infrared spectral-domain optical coherence tomography system operating at a central wavelength of 4?µm and an axial resolution of 8.6?µm is demonstrated. The system produces two-dimensional cross-sectional images in real time enabled by a high-brightness 0.9- to 4.7-µm mid-infrared supercontinuum source with a pulse repetition rate of 1?MHz for illumination and broadband upconversion of more than 1-µm bandwidth from 3.58–4.63?µm to 820–865?nm, where a standard 800-nm spectrometer can be used for fast detection. The images produced by the mid-infrared system are compared with those delivered by a state-of-the-art ultra-high-resolution near-infrared optical coherence tomography system operating at 1.3??m, and the potential applications and samples suited for this technology are discussed. In doing so, the first practical mid-infrared optical coherence tomography system is demonstrated, with immediate applications in real-time non-destructive testing for the inspection of defects and thickness measurements in samples that exhibit strong scattering at shorter wavelengths.
  • Podoleanu, A. et al. (2019). Progress in Multimodal En Face Imaging: feature introduction. Biomedical Optics Express [Online] 10:2135. Available at: https://doi.org/10.1364/BOE.10.002135.
    This feature issue contains papers that report on the most recent advances in the
    field of en face optical coherence tomography (OCT) and of combinations of modalities
    facilitated by the en face view. Hardware configurations for delivery of en face OCT images
    are described as well as specific signal and image processing techniques tailored to deliver
    relevant clinical diagnoses. The value of the en face perspective for enabling multimodality is
    illustrated by several combination modalities.
  • Mogensen, M. et al. (2018). Two optical coherence tomography systems detect topical gold nanoshells in hair follicles, sweat ducts and measure epidermis. Journal of Biophotonics [Online] 11:e201700348. Available at: https://doi.org/10.1002/jbio.201700348.
    Optical coherence tomography (OCT) is an established imaging technology for in vivo skin investigation. Topical application of gold nanoshells (GNS) provides contrast enhancement in OCT by generating a strong hyperreflective signal from hair follicles and sweat glands, which are the natural skin openings. This study explores the utility of 150 nm diameter GNS as contrast agent for OCT imaging. GNS was massaged into skin and examined in four skin areas of 11 healthy volunteers. A commercial OCT system and a prototype with 3 ?m resolution (UHR?OCT) were employed to detect potential benefits of increased resolution and variability in intensity generated by the GNS. In both OCT?systems GNS enhanced contrast from hair follicles and sweat ducts. Highest average penetration depth of GNS was in armpit 0.64 mm ± SD 0.17, maximum penetration depth was 1.20 mm in hair follicles and 15 to 40 ?m in sweat ducts. Pixel intensity generated from GNS in hair follicles was significantly higher in UHR?OCT images (P = .002) and epidermal thickness significantly lower 0.14 vs 0.16 mm (P =?.027). This study suggests that GNSs are interesting candidates for increasing sensitivity in OCT diagnosis of hair and sweat gland disorders and demonstrates that choice of OCT systems influences results.
  • Israelsen, N. et al. (2018). The value of ultrahigh resolution OCT in dermatology - delineating the dermo-epidermal junction, capillaries in the dermal papillae and vellus hairs. Biomedical Optics Express [Online] 9:2240. Available at: https://doi.org/10.1364/BOE.9.002240.
    Optical coherence tomography (OCT) imaging of the skin is gaining recognition
    and is increasingly applied to dermatological research. A key dermatological parameter inferred
    from an OCT image is the epidermal (Ep) thickness as a thickened Ep can be an indicator of a
    skin disease. Agreement in the literature on the signal characters of Ep and the subjacent skin
    layer, the dermis (D), is evident. Ambiguities of the OCT signal interpretation in the literature is
    however seen for the transition region between the Ep and D, which from histology is known
    as the dermo-epidermal junction (DEJ); a distinct junction comprised of the lower surface of a
    single cell layer in epidermis (the stratum basale) connected to an even thinner membrane (the
    basement membrane). The basement membrane is attached to the underlying dermis.
    In this work, we investigate the impact of an improved axial and lateral resolution on the
    applicability of OCT for imaging of the skin. To this goal, OCT images are compared produced
    by a commercial OCT system (Vivosight from Michaelson Diagnostics) and by an in-house built
    ultrahigh resolution (UHR-) OCT system for dermatology.
    In 11 healthy volunteers, we investigate the DEJ signal characteristics. We perform a detailed
    analysis of the dark (low) signal band clearly seen for UHR-OCT in the DEJ region where we,
    by using a transition function, find the signal transition of axial sub-resolution character, which
    can be directly attributed to the exact location of DEJ, both in normal (thin/hairy) and glabrous
    (thick) skin. To our knowledge no detailed delineating of the DEJ in the UHR-OCT image has
    previously been reported, despite many publications within this field.
    For selected healthy volunteers, we investigate the dermal papillae and the vellus hairs and
    identify distinct features that only UHR-OCT can resolve. Differences are seen in tracing hairs of
    diameter below 20 ?m, and in imaging the dermal papillae where, when utilising the UHR-OCT,
    capillary structures are identified in the hand palm, not previously reported in OCT studies and
    specifically for glabrous skin not reported in any other in vivo optical imaging studies.
  • 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.
  • 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.
  • Jensen, M. et al. (2018). All-depth dispersion cancellation in spectral domain optical coherence tomography using numerical intensity correlations. Scientific Reports [Online] 8. Available at: https://doi.org/10.1038/s41598-018-27388-z.
    In ultra-high resolution (UHR-) optical coherence tomography (OCT) group velocity dispersion (GVD) must be corrected for in order to approach the theoretical resolution limit. One approach promises not only compensation, but complete annihilation of even order dispersion effects, and that at all sample depths. This approach has hitherto been demonstrated with an experimentally demanding ‘balanced detection’ configuration based on using two detectors. We demonstrate intensity correlation (IC) OCT using a conventional spectral domain (SD) UHR-OCT system with a single detector. IC-SD-OCT configurations exhibit cross term ghost images and a reduced axial range, half of that of conventional SD-OCT. We demonstrate that both shortcomings can be removed by applying a generic artefact reduction algorithm and using analytic interferograms. We show the superiority of IC-SD-OCT compared to conventional SD-OCT by showing how IC-SD-OCT is able to image spatial structures behind a strongly dispersive silicon wafer. Finally, we question the resolution enhancement of 2–? that IC-SD-OCT is often believed to have compared to SD-OCT. We show that this is simply the effect of squaring the reflectivity profile as a natural result of processing the product of two intensity spectra instead of a single spectrum.
  • 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.
  • 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.
  • Toadere, F. et al. (2017). 1 MHz Akinetic Dispersive Ring Cavity Swept Source at 850 nm. IEEE Photonics Technology Letters [Online] 29:933-936. Available at: https://doi.org/10.1109/LPT.2017.2695083.
    A fast dual mode-locked akinetic optical swept source at 850 nm central wavelength is presented using a dispersive cavity. We demonstrate that single mode fiber can be successfully used as dispersive element to induce mode locking. A first locking condition is imposed by driving the optical gain at a high frequency, to induce mode locking, while a second locking condition involves sweeping at a rate close to resonance value. In this regime, using the same fiber length in the loop, sweeping rates of 0.5 MHz and 1 MHz are demonstrated with proportional reduction in the tuning bandwidth. The axial range of the swept source was evaluated by scanning through the channeled spectrum of a Michelson interferometer.
  • Maria, M. et al. (2017). Q-switch-pumped supercontinuum for ultra-high resolution optical coherence tomography. Optics Letters [Online] 42:4744. Available at: https://doi.org/10.1364/OL.42.004744?
    In this Letter, we investigate the possibility of using a commercially available
  • 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.
  • Avanaki, M. and Podoleanu, A. (2017). En-face time-domain optical coherence tomography with dynamic focus for high-resolution imaging. Journal of Biomedical Optics [Online] 22:56009. Available at: https://doi.org/10.1117/1.JBO.22.5.056009.
    Optical coherence tomography (OCT) is capable of imaging microstructures within translucid samples. A time-domain version of the OCT technology is employed here due to its compatibility with the dynamic focus (DF) procedure. DF means moving the confocal gate in synchronism with the depth scanning via the coherence gate. A DF-OCT setup was implemented for imaging samples at 1300 nm. Its confocal gate of 180 ?m allows the achievement of good and similar transversal resolution along its much larger axial range. Images of a phantom, human skin, teeth, and larynx with and without DF are demonstrated.
  • 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.
  • 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.
  • Adabi, S. et al. (2017). An overview of methods to mitigate artifacts in optical coherence tomography imaging of the skin. Skin Research and Technology [Online]. Available at: https://doi.org/10.1111/srt.12423.
    Background: Optical coherence tomography (OCT) of skin delivers three-dimensional images of tissue microstructures. Although OCT imaging offers a promising high-resolution modality, OCT images suffer from some artifacts that lead to misinterpretation of tissue structures. Therefore, an overview of methods to mitigate artifacts in OCT imaging of the skin is of paramount importance. Speckle, intensity decay, and blurring are three major artifacts in OCT images. Speckle is due to the low coherent light source used in the configuration of OCT. Intensity decay is a deterioration of light with respect to depth, and blurring is the consequence of deficiencies of optical components.
    Method: Two speckle reduction methods (one based on artificial neural network and one based on spatial compounding), an attenuation compensation algorithm (based on Beer-Lambert law) and a deblurring procedure (using deconvolution), are described. Moreover, optical properties extraction algorithm based on extended Huygens-Fresnel (EHF) principle to obtain some additional information from OCT images are discussed.
    Results: In this short overview, we summarize some of the image enhancement algorithms for OCT images which address the abovementioned artifacts. The results showed a significant improvement in the visibility of the clinically relevant features in the images. The quality improvement was evaluated using several numerical assessment measures.
    Conclusion: Clinical dermatologists benefit from using these image enhancement algorithms to improve OCT diagnosis and essentially function as a noninvasive optical biopsy.

Book section

  • Adabi, S. et al. (2018). Mitigation of Speckle Noise in Optical Coherence Tomograms. in: Optics, Photonics and Laser Technology. Springer, pp. 115-135. Available at: https://doi.org/10.1007/978-3-319-98548-0_6.
    Optical Coherence Tomography (OCT) is a promising high-resolution imaging technique that works based on low coherent interferometry. However, like other low coherent imaging modalities, OCT suffers from an artifact called, speckle. Speckle reduces the detectability of diagnostically relevant features in the tissue. Retinal optical coherence tomograms are of a great importance in detecting and diagnosing eye diseases. Different hardware or software based techniques are devised in literatures to mitigate speckle noise. The ultimate aim of any software-based despeckling technique is to suppress the noise part of speckle while preserves the information carrying portion of that. In this chapter, we reviewed the most prominent speckle reduction methods for OCT images to date and then present a novel and intelligent speckle reduction algorithm to reduce speckle in OCT images of retina, based on an ensemble framework of Multi-Layer Perceptron (MLP) neural networks.
  • 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

  • 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. (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. SPIE. 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. (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.
  • 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.
  • Jensen, M. et al. (2018). Phase estimation for global defocus correction in optical coherence tomography. 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/10.1117/12.2292547.
    In this work we investigate three techniques for estimation of the non-linear phase present due to defocus in optical
    coherence tomography, and apply them with the angular spectrum method. The techniques are: Least squares
    fitting the of unwrapped phase of the angular spectrum, iterative optimization, and sub-aperture correlations.
    The estimated phase of a single en-face image is used to extrapolate the non-linear phase at all depths, which
    in the end can be used to correct the entire 3-D tomogram, and any other tomogram from the same system.
  • Bai, F. et al. (2018). Superpixel guided active contour segmentation of retinal layers in OCT volumes. in: Podoleanu, A. G. H. and Bang, O. eds. Second Canterbury Conference on Optical Coherence Tomography, 2017, Canterbury, United Kingdom. SPIE. Available at: http://dx.doi.org/10.1117/12.2282326.
    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.
  • Abuabboud, L. et al. (2018). Modern evaluation of the quality of the techniques of root canal dental obturation. 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/10.1117/12.2286452.
    1) Background: One of the key factors for a successful endodontic therapy is to adequately fill the root canals. The aim
    of this in vitro study was to compare the quality of three different techniques of root canal obturation: tapered singlecone,
    cold lateral condensation, warm vertical condensation and injection system, using non-invasive Optical Coherence
    Tomography (OCT).
    2) Materials and methods: A total of 30 extracted single-rooted teeth, prepared with Reciproc
    System (VDW, silver Reciproc ) were divided into three groups, based on the filling method: the first group obturated
    with “single-cone” (n=10) Reciproc guttapercha single-cone tapered according to the diameter, length and conicity of the
    preparation, the second group obturated with cold lateral condensation (n=10) and the third group, “combined-system”
    (E&Q META BIOMED) (n=10). OCT technology can generate high-resolution cross-sectional imaging, capable to
    evaluate the micro-leakages of dental restorations and endodontic fillings.
    3) Results: OCT showed that none of the root canal filled teeth were gape-free; the highest percentage of filling material was observed in the combined system group
    (warm vertical and injection system). 4) Conclusions: OCT represents a valuable method for investigation with high
    depth resolution which can be used for evaluation of endodontic fillings.
  • Fleischhauer, F. et al. (2018). Impact of absorption in the top layer of a two layer sample on spectroscopic spectral domain interferometry of the bottom layer. 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/10.1117/12.2281928.
    Spectroscopic spectral domain interferometry and spectroscopic optical coherence tomography combine depth information with spectrally-resolved localised absorption data. These additional data can improve diagnostics by giving access to functional information of the investigated sample. One possible application is measuring oxygenation levels at the retina for earlier detection of several eye diseases. Here measurements with different hollow glass tube phantoms are shown to measure the impact of a superficial absorbing layer on the precision of reconstructed attenuation spectra of a deeper layer. Measurements show that a superficial absorber has no impact on the reconstructed absorption spectrum of the deeper absorber. Even when diluting the concentration of the deeper absorber so far that an incorrect absorption maximum is obtained, still no influence of the superficially placed absorber is identified.
  • 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.
  • Podoleanu, A. et al. (2018). New ways of combating demineralization: link between classic and allopathic medicine. 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/10.1117/12.2281725.
    Prevention and allopathic medicine gained attention, since it is possible for dentists to prevent demineralization, using plants and natural substances with well-known efficacy. The purpose of this study was to present new methods for teeth remineralization. It was made a selection of 10 extracted teeth, maintained in physiological serum, with no color fading, decay or demineralization. It was induced demineralization, with ortho-phosphoric acid (concentration 45 %), for one minute. The probes were visually and with optical coherence tomography (OCT) inspected. The natural product and the bonding with additional nanoparticles of argent were created and applied on the demineralization zone of the both groups of teeth. Each tooth in the first group had one plain surface demineralized. The second group of teeth had a cavity prepared on one of each tooth’s side. The pastes were applied on the demineralized surfaces and in the demineralized cavities for two minutes. After time expired, the pasta applied on the first group of teeth was washed away; the bonding above the second group of teeth was light cured. The probes were again visually and with OCT inspected. It was observed an improvement in remineralizing the white marks on plain surfaces and in the created cavities, the OCT being able to detect different levels of remineralization. The efficacy of natural pasta depends on the time it is applied and the concentration of the different main substances. Also, the type of surface, plain or occlusal facets, may influence the substances’ penetration ability. The non-invasive specific feature of these products, low costs and safety are strong positive aspects of this method of remineralization. However, the natural process of remineralization is a long-lasting one; perfecting the main substances in order to accelerate the process, in addition to several in vivo studies would be necessary to be fulfilled.
  • Bravo Gonzalo, I. et al. (2018). Ultra-low noise supercontinuum source for ultra-high resolution optical coherence tomography at 1300 nm. in: Raghavachari, R. and Liang, R. eds. SPIE BiOS, 2018. SPIE, p. . Available at: https://doi.org/10.1117/12.2282412.
    Supercontinuum (SC) sources are of great interest for many applications due to their ultra-broad optical bandwidth, good beam quality and high power spectral density [1]. In particular, the high average power over large bandwidths makes SC light sources excellent candidates for ultra-high resolution optical coherence tomography (UHR-OCT) [2-5]. However, conventional SC sources suffer from high pulse-to-pulse intensity fluctuations as a result of the noise-sensitive nonlinear effects involved in the SC generation process [6-9]. This intensity noise from the SC source can limit the performance of OCT, resulting in a reduced signal-to-noise ratio (SNR) [10-12]. Much work has been done to reduce the noise of the SC sources for instance with fiber tapers [7,8] or increasing the repetition rate of the pump laser for averaging in the spectrometer [10,12]. An alternative approach is to use all-normal dispersion (ANDi) fibers [13,14] to generate SC light from well-known coherent nonlinear processes [15-17]. In fact, reduction of SC noise using ANDi fibers compared to anomalous dispersion SC pumped by sub-picosecond pulses has been recently demonstrated [18], but a cladding mode was used to stabilize the ANDi SC. In this work, we characterize the noise performance of a femtosecond pumped ANDi based SC and a commercial SC source in an UHR-OCT system at 1300 nm. We show that the ANDi based SC presents exceptional noise properties compared to a commercial source. An improvement of ~5 dB in SNR is measured in the UHR-OCT system, and the noise behavior resembles that of a superluminiscent diode. This preliminary study is a step forward towards development of an ultra-low noise SC source at 1300 nm for ultra-high resolution OCT.
  • 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.
  • 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.
  • Podoleanu, A. (2018). Advances in Optical Coherence Tomography. in: 20th Anniversary International Conference on Transparent Optical Networks (ICTON) 2018. USA: IEEE, pp. 1-4. Available at: https://doi.org/10.1109/ICTON.2018.8473778.
    Traditionally applied to imaging the eye, optical coherence tomography (OCT) is now being extended to fields outside ophthalmology and optometry. The tremendous increase in acquisition speed of the spectral domain OCT technology in the last decade has enabled the OCT community to contemplate real time volume display, has opened the field of no-dye angiography and that of fast interrogation of deformation patterns in elastography. The presentation will review the OCT applications in ophthalmology and endoscopy as well as the dynamic field of broadband and fast tunable optical sources for OCT. Current research in Kent combined spectral domain and time domain OCT principles into a new method, Master/Slave OCT, that delivers fast display of any number of en-face OCT images. The Master/Slave method simplifies the OCT technology, the signal processing as well as gives parallel, direct access to information from multiple depths in the tissue. A review is presented on the advances of OCT that make the technology useful for numerous directions in medical imaging and for non-destructive testing.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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. (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.
  • Bondu, M. et al. (2017). Using a single supercontinuum source for visible multispectral photoacoustic microscopy and 1300 nm optical coherence tomography. in: European Conferences on Biomedical Optics. SPIE. Available at: http://dx.doi.org/10.1117/12.2286119.
    We present a bimodal system driven by a supercontinuum source to perform photoacoustic-based spectral selective absorption measurements from 500 nm to 800 nm and structural optical coherence tomography imaging at 1300 nm. An energy of 5 to 40 nJ is achieved on sample within a 50 nm bandwidth in the visible range in the photoacoustic channel. Also, a few mW power is also achieved on the sample in the optical coherence tomography channel.
  • 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.
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