Portrait of Dr Adrian Bradu

Dr Adrian Bradu

Lecturer in Applied Optics

About

Dr Adrian Bradu is a Lecturer in Applied Optics in the School of Physical Sciences and a member of the Applied Optics Group, where he develops imaging techniques for applications in biosciences and medicine. In particular his research is focused on imaging techniques based on optical coherence tomography, elastography and photo-acoustics.

After graduating in 1997 with an BSc in Physics, Adrian obtained a MSc in Optics, Optoelectronics and Microwaves at École Nationale Supérieure d'Électronique et de Radioélectricité de Grenoble (ENSERG), France, with a viva in 1998 on “Spectrophotometry of turbid media using optical fibre probes”. Here, he had the opportunity to work in the “Functional and Metabolic Neuroimaging” Center Inserm/UJF 594 (Grenoble University Hospital), as a MSc and then as a PhD student. He completed his PhD in the group of Prof. Jacques Derouard at Joseph Fourier University, Grenoble, France in 2004 with a thesis entitled, Optical methods to investigate biological tissues. Cerebral tissue spectroscopy using small optical fibre probes and optical coherence tomography imaging.

After 2004, Adrian took up various positions as Postgraduate Research Associate in the Applied Optics Group, at the University of Kent. In 2017, in became a lecturer in the School of Physical Sciences where he is developing research programmes on optical coherence elastography and photo-acoustics tomography.

Research interests

The study of interaction of light with various materials (biological media in particular) has captured Adrian’s interest during his undergraduate studies. His Master studies, convinced him to pursue a career focused on Applied Optics whereas the PhD in Grenoble strengthened his beliefs that understanding light behaviour while propagating through various materials can lead to novel optical methods and instruments of vital utility in the modern society. After finishing his PhD thesis, Adrian’s research activity has been focused on high-resolution non-invasive optical imaging of superficial tissue based on Optical Coherence Tomography (OCT). Currently he is interested:

  1. To develop medical imaging tools able to produce an images of what tissue ‘feels’ like by evaluating its elastic properties using optical coherence elastography
  2. To develop techniques that can offer not only structural but also functional information of biological tissues with excellent resolution and high contrast, by listening to the sound produced when a laser pulse interacts with the absorbing tissue (photo-acoustics tomography).

and also in:

  • Developing software platforms data acquisition, display and analysis of the images using cutting-edge techniques and methods for OCT systems and not only.
  • Non-invasive imaging of biological tissues: optical coherence tomography and confocal microscopy techniques for biological tissue imaging and adaptive optics techniques for retinal imaging.

Teaching

Adrian is involved with the teaching of several subjects: Electric Circuits, Magnetostatics, Modern Optics and Special Relativity, Image Processing, Physics Research Projects and Biomedical Optics.  

Supervision

Potential MSc and PhD candidates (for degrees in Physics) are encouraged to contact Adrian to discuss about self-funded positions or apply for external funding. Funded positions are advertised when available. Adrian can host in his lab, over the summer, several undergraduate students. Please contact him in advance for arrangements.

Example projects that may be undertaken by self-funded students (PhD or MSc) under Adrian’s supervision are listed below, but a range of other projects can be designed to match your skills and interests:

Publications

Showing 50 of 212 total publications in the Kent Academic Repository. View all publications.

Article

  • Říha, R., Marques, M., Hughes, M., Bradu, A. and Podoleanu, A. (2020). Direct en-face, speckle-reduced images using angular-compounded Master-Slave optical coherence tomography. Journal of Optics [Online] 22. Available at: https://doi.org/10.1088/2040-8986%2Fab8285.
    In this paper, an angular compounding method to achieve speckle contrast reduction in optical coherence tomography (OCT) imaging is explored in detail. The angular compounding approach involves collecting multiple images at different angles of incidence, registering the images to correct for induced distortions, and then incoherently summing the images to reduce speckle. The method was experimentally demonstrated with a spectrometer-based Master-Slave enhanced OCT instrument capable of directly generating en-face images. We have investigated the impact of the angular range and number of averaged frames on the degree of speckle artefact reduction, as well as the effect on image resolution and sharpness. The minimum angular step necessary to secure a sufficiently small speckle pattern correlation between the images has also been determined, and the method has subsequently been validated on a biological sample (potato cells).
  • Dasa, M., Nteroli, G., Bowen, P., Messa, G., Feng, Y., Petersen, C., Koutsikou, S., Bondu, M., Moselund, P., Podoleanu, A., Bradu, A., Markos, C. and Bang, O. (2020). All-fibre supercontinuum laser for in vivo multispectral photoacoustic microscopy of lipids in the extended near-infrared region. Photoacoustics [Online] 18. Available at: https://doi.org/10.1016/j.pacs.2020.100163.
    Among the numerous endogenous biological molecules, information on lipids is highly coveted for understanding both aspects of developmental biology and research in fatal chronic diseases. Due to the pronounced absorption features of lipids in the extended near-infrared region (1650−1850 nm), visualisation and identification of lipids become possible using multi-spectral photoacoustic (optoacoustic) microscopy. However, the spectroscopic studies in this spectral region require lasers that can produce high pulse energies over a broad spectral bandwidth to efficiently excite strong photoacoustic signals. The most well-known laser sources capable of satisfying the multi-spectral photoacoustic microscopy requirements (tunability and pulse energy) are tunable nanosecond optical parametric oscillators. However, these lasers have an inherently large footprint, thus preventing their use in compact microscopy systems. Besides, they exhibit low-repetition rates. Here, we demonstrate a compact all-fibre, high pulse energy supercontinuum laser that covers a spectral range from 1440 to 1870 nm with a 7 ns pulse duration and total energy of 18.3 μJ at a repetition rate of 100 kHz. Using the developed high-pulse energy source, we perform multi-spectral photoacoustic microscopy imaging of lipids, both ex vivo on adipose tissue and in vivo to study the development of Xenopus laevis tadpoles, using six different excitation bands over the first overtone transition of C–H vibration bonds (1650−1850 nm).
  • Marques, M., Hughes, M., Vyas, K., Thrapp, A., Zhang, H., Bradu, A., Gelikonov, G., Giataganas, P., Payne, C., Yang, G. and Podoleanu, A. (2019). En-face optical coherence tomography/fluorescence endomicroscopy for minimally invasive imaging using a robotic scanner. Journal of Biomedical Optics [Online] 24. Available at: https://doi.org/10.1117/1.JBO.24.6.066006.
    We report a compact rigid instrument capable of delivering en-face optical coherence tomography (OCT) images alongside (epi)-fluorescence endomicroscopy (FEM) images by means of a robotic scanning device. Two working imaging channels are included: one for a one-dimensional scanning, forward-viewing OCT probe and another for a fiber bundle used for the FEM system. The robotic scanning system provides the second axis of scanning for the OCT channel while allowing the field of view (FoV) of the FEM channel to be increased by mosaicking. The OCT channel has resolutions of 25  /  60  μm (axial/lateral) and can provide en-face images with an FoV of 1.6  ×  2.7  mm2. The FEM channel has a lateral resolution of better than 8  μm and can generate an FoV of 0.53  ×  3.25  mm2 through mosaicking. The reproducibility of the scanning was determined using phantoms to be better than the lateral resolution of the OCT channel. Combined OCT and FEM imaging were validated with ex-vivo ovine and porcine tissues, with the instrument mounted on an arm to ensure constant contact of the probe with the tissue. The OCT imaging system alone was validated for in-vivo human dermal imaging with the handheld instrument. In both cases, the instrument was capable of resolving fine features such as the sweat glands in human dermal tissue and the alveoli in porcine lung tissue.
  • Luca, R., Todea, C., Duma, V., Bradu, A. and Podoleanu, A. (2019). Quantitative assessment of rat bone regeneration using complex master-slave optical coherence tomography. Quantitative Imaging in medicine and surgery [Online]. Available at: http://qims.amegroups.com/article/view/25894.
    Background: The need for hard and soft tissues in oral implantology determined the development of methods and techniques to increase bone volume and their quality with different alternative materials used as substituents of patient’s natural bone. In addition, laser radiation can be used to accelerate the repair of fractures and to produce an increased volume of formed callus, as well as an increased bone mineral density.
    Methods: The aim of this work is to evaluate the capability of an in-house developed multimodal complex master slave (CMS) enhanced swept source (SS) optical coherence tomography (OCT) imaging instrument to analyze the increase in the quantity and the improvement of the quality of newly-formed bone using low level laser therapy (LLLT). Bone formation is quantitatively assessed in 5 mm cylindrical defects made in the calvaria part of the skull of living rats. Samples are divided in three study groups: A, a negative control group, for which the natural healing process of the defect is investigated; B, a positive control group, for which bovine graft is used to stimulate bone formation, and C, a study group, in which bovine graft is added to the created defects and LLLT is applied throughout the entire healing period. The animals are sacrificed after 14, 21, and 30 days, and the samples are imaged using the multimodal CMS/SS-OCT instrument.
    Results: The method allows for the simultaneous monitoring of the bone tissue via two perpendicular cross-sections and nine en-face images taken at adjustable depths into the sample. A global image with course axial resolution allows for the positioning of the field-of-view of the system on the area of interest on the tissue. The quantitative assessment of the process of bone formation is completed using the differences in brightness between the native bone, the artificial bone graft, and the newly-formed bone.
    Conclusions: Group C is demonstrated to have a higher volume of newly-formed bone than Group B, which is better from this point of view than Group A. By analyzing the evolution of this volume of new bone in time, the most significant difference was after 21 days, therefore approximately after two thirds of the total time interval analyzed. After 30 days, the volumes of bone tend to move closer, as they begin to fill the available gap. The study demonstrates that OCT can assess quantitatively the positive impact of LLLT on bone regeneration.
  • Duma, V., Sinescu, C., Bradu, A. and Podoleanu, A. (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
  • Rivet, S., Dubreuil, M., Bradu, A. and Le Grand, Y. (2019). Fast spectrally encoded Mueller optical scanning microscopy. Scientific Reports [Online] 9. 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.
  • 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.
  • Marques, M., Rivet, S., Bradu, A. and Podoleanu, A. (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., Israelsen, N., Maria, M., Marques, M., Rivet, S., Feuchter, T., Bang, O. and Podoleanu, A. (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., Bradu, A., Bairstow, F., Forriere, H. and Podoleanu, A. (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., Duma, V., Demian, D., Bradu, A. and Podoleanu, A. (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.
  • Bondu, M., Marques, M., Moselund, P., Lall, G., Bradu, A. and Podoleanu, A. (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.
  • Caujolle, S., Cernat, R., Silvestri, G., Marques, M., Bradu, A., Feuchter, T., Robinson, G., Griffin, D. and Podoleanu, A. (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.
  • Rivet, S., Marques, M., Bradu, A. and Podoleanu, A. (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.

Book section

  • Harvey, A., Carles, G., Bradu, A. and Podoleanu, A. (2019). The physics, instruments and modalities of retinal imaging. In: Trucco, E., MacGillivray, T. and Xu, Y. eds. Computational Retinal Image Analysis. Elsevier, pp. 19-57. Available at: http://dx.doi.org/10.1016/B978-0-08-102816-2.00003-4.
    The characteristics of retinal images are determined by the physics and principles of the retinal imaging process and hence the appearance of images varies in both profound and subtle ways between imaging modalities. We describe the principles of operation of the main modalities used for clinical retinal imaging, such as the fundus camera, scanning laser ophthalmoscope and optical coherence tomography and discuss the image characteristics that may inform the optimization of algorithms for retinal-image analysis.
  • Szuhanek, C., Sinescu, C., Todea, C., Pop, O., Duma, V., Topala, F., Rominu, M., Negrutiu, M., Bradu, A. and Podoleanu, A. (2018). Optical coherence tomography study regarding the enamel structure before and after debonding. In: Todea, C. C., Podoleanu, A. 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., Dimb, A., Duma, V., Demian, D., Bradu, A. and Podoleanu, A. (2018). Roughness measurements using optical coherence tomography: a preliminary study. 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/10.1117/12.2282807.
    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.
  • Bradu, A., Marques, M., Rivet, S. and Podoleanu, A. (2018). Current capabilities and challenges for optical coherence tomography as a high impact non-destructive imaging modality. In: Todea, C. C., Podoleanu, A. 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.
  • Maroiu, A., Rominu, M., Sinescu, C., Negrutiu, M., Topala, F., Duma, V., Bradu, A. and Podoleanu, A. (2018). Optoelectronic evaluation of indirect dental veneers interfaces. In: Todea, C., Podoleanu, A. G. and Duma, V.-F. eds. Seventh International Conference on Lasers in Medicine. SPIE. Available at: https://doi.org/10.1117/12.2282648.
    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.
  • Podoleanu, A., Caujolle, S., Marques, M., Cernat, R., Rajendram, R. and Bradu, A. (2018). From Doppler to speckle variance measurements in optical coherence tomography. In: Todea, C., Podoleanu, A. G. and Duma, V.-F. eds. Seventh International Conference on Lasers in Medicine. SPIE. Available at: https://doi.org/10.1117/12.2283530.
    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.
  • 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. 2nd Canterbury Conference on OCT With Emphasis on Broadband Optical Sources. Bellingham, Washington: 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.
  • Israelsen, N., Maria, M., Feuchter, T., Bradu, A., Podoleanu, A. and Bang, O. (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. and Bang, O. eds. 2nd Canterbury Conference on OCT With Emphasis on Broadband Optical Sources. 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.
  • Rivet, S., Bradu, A., Maria, M., Feuchter, T., Leick, L. and Podoleanu, A. (2018). From master slave interferometry to complex master slave interferometry: theoretical work. In: Podoleanu, A. G. and Bang, O. eds. 2nd Canterbury Conference on OCT With Emphasis on Broadband Optical Sources. 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.
  • Sinescu, C., Barua, S., Topala, F., Negruţiu, M., Duma, V., Gabor, A., Zaharia, C., Bradu, A. and Podoleanu, A. (2018). Dental Impression Technique Using Optoelectronic Devices. In: Podoleanu, A. G. and Bang, O. eds. 2nd Canterbury Conference on OCT With Emphasis on Broadband Optical Sources. 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.
  • Caujolle, S., Cernat, R., Silvestri, G., Marques, M., Bradu, A., Feuchter, T., Robinson, G., Griffin, D. and Podoleanu, A. (2018). Assessing embryo development using swept source optical coherence tomography. In: Podoleanu, A. G. and Bang, O. eds. 2nd Canterbury Conference on OCT With Emphasis on Broadband Optical Sources. SPIE. 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.
  • Todor, R., Negrutiu, M., Sinescu, C., Topala, F., Bradu, A., Duma, V., Rominu, M. and Podoleanu, A. (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.
  • Ďuriš, M., Bradu, A., Podoleanu, A. and Hughes, M. (2018). Towards an ultra-thin medical endoscope: multimode fibre as a wide-field image transferring medium. In: Podoleanu, A. G. and Bang, O. eds. 2nd Canterbury Conference on OCT With Emphasis on Broadband Optical Sources. Bellingham, Washington: SPIE. Available at: https://doi.org/10.1117/12.2283482.
    Multimode optical fibres are attractive for biomedical and industrial applications such as endoscopes because of the small cross section and imaging resolution they can provide in comparison to widely-used fibre bundles. However, the image is randomly scrambled by propagation through a multimode fibre. Even though the scrambling is unpredictable, it is deterministic, and therefore the scrambling can be reversed. To unscramble the image, we treat the multimode fibre as a linear, disordered scattering medium. To calibrate, we scan a focused beam of coherent light over thousands of different beam positions at the distal end and record complex fields at the proximal end of the fibre. This way, the inputoutput response of the system is determined, which then allows computational reconstruction of reflection-mode images. However, there remains the problem of illuminating the tissue via the fibre while avoiding back reflections from the proximal face. To avoid this drawback, we provide here the first preliminary confirmation that an image can be transferred through a 2x2 fibre coupler, with the sample at its distal port interrogated in reflection. Light is injected into one port for illumination and then collected from a second port for imaging.
  • Cernat, R., Bradu, A., Rivet, S. and Podoleanu, A. (2018). Time efficient Gabor fused Master Slave optical coherence tomography. In: Optical Coherence Tomography and Coherence Domain Optical Methods in Biomedicine XXII. Bellingham, Washington: 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.
  • Bradu, A., Chui, T., Costa, C., Dubra, A., Hood, D., Kapinchev, K., Podoleanu, A., Rosen, R., Wang, J., Werner, J. and Zawadski, R. (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

  • Fernández Uceda, A., Marques, M., Bradu, A. and Podoleanu, A. (2020). Non-mechanical Axial Motion Compensation Using Master-Slave Optical Coherence Tomography. In: Biophotonics Congress: Biomedical Optics 2020 (Translational, Microscopy, OCT, OTS, BRAIN). OSA. Available at: https://www.osapublishing.org/abstract.cfm?uri=OCT-2020-OTu1E.6.
    We present a novel technique for accurate, non-mechanical, axial motion compensation for OCT in both ophthalmology and dermal imaging through the combination of MS-OCT for imaging and a LCI for the motion detection.
  • Podoleanu, A., Bradu, A., Cernat, R., Marques, M., Kapinchev, K. and Rivet, S. (2020). Unconventional optical coherence tomography (Conference Presentation). In: SPIE Photonics Europe. SPIE. Available at: https://doi.org/10.1117/12.2559572.
    We have introduced the Master Slave (MS) interferometry method to address the limitations due to the use of conventional FTs or its derivatives in OCT data processing. The novel MS technology replaces the FT operator with a parallel batch of correlators. An electrical signal proportional to the channeled spectrum at the interferometer output is correlated with P masks producing P signals, a signal for each point out of P in the A-scan. In this way, it is possible to: (i) directly access the information from selected depths in the sample placed in the slave interferometer; (ii) eliminate the process of resampling, required by the FT based conventional technology, with immediate consequences in improving the decay of sensitivity with depth, achieving the expected axial resolution limit and reducing the time to display an en-face OCT image, while slightly lowering the cost of OCT assembly and (iii) tolerate the dispersion left unbalanced in the slave interferometer. The lecture will present several developments based on the MS-OCT technology, such as: (a) an equivalent OCT/SLO (scanning laser ophthalmoscopy), where no extra optical channel for the SLO is needed; (b) coherence revival swept source OCT employing the MS tolerance to dispersion: (c) Gabor filtering, where large number of repetitions with different focus adjustments can be performed more time efficiently than when employing FT based OCT; (d) MS phase processing, which opens novel avenues in phase- and polarization-sensitive modalities; (e) achieving the theoretical axial resolution when using a ultra wide broadband source such as a supercontinuum laser; (f) down-conversion OCT that can deliver an en-face OCT image from a sample in real-time, irrespective of the tuning speed of the swept source where the mask signals are generated in real time (by a physical master interferometer) while sweeping the frequency of the swept source.
  • Podoleanu, A., Bradu, A., Cernat, R. and Marques, M. (2020). From master-slave to down-conversion optical coherence tomography. In: Optical Coherence Tomography and Coherence Domain Optical Methods in Biomedicine XXIV. SPIE. Available at: http://dx.doi.org/10.1117/12.2548617.
    We present here advances on the Master Slave (MS) concept, applicable to spectral/Fourier/frequency-domain optical coherence tomography (OCT) technology. Instead of obtaining an A-scan from the sample investigated via a Fourier Transform (FT) or equivalent, the amplitude of the A-scan for each resolvable point along the depth is obtained along a separate output. A multiplier produces the product of the photo-detected signal from the OCT system with that generated by an Electrical or an Optical Master. This allows acquisition at a frequency comparable to that of the sweeping, much inferior to the frequency bandwidth of the channeled spectrum. 3 advantages of the down-conversion method are demonstrated here: (a) real time delivery of an en-face image; (b) axial optical path difference (OPD) range at the level of the source’s dynamic coherence length and (c): tolerance to fluctuations in the sweep of the swept source. The most important advantage of the down-conversion method is that it reduces the signal bandwidth considerably, to the level of the sweeping rate. This facilitates real-time operation. Conventional A-scan production can only be performed real-time if the FT processing is carried out in a time comparable to or less than the sweep time, which depending on the number of sampled points and dynamic range determines a limit of ∼ MHz sweep rate. Before even calculating a FT, acquisition may also be limited by the sampling rate of the digitiser. In conventional SS-OCT, the number of depth points can exceed 1,000, which for a sweeping time of 1 μs would determine signals in the GHz range. Using long coherence length swept sources, this number of depths could be even larger, hence the conventional FT-based method faces a bottleneck due to the time needed to calculate the FT, combined with the need to acquire data at many GS/s.
  • Marques, M., Cernat, R., Ensher, J., Bradu, A. and Podoleanu, A. (2020). Master-slave principle applied to an electrically tunable swept source-OCT system. In: Optical Coherence Tomography and Coherence Domain Optical Methods in Biomedicine XXIV. SPIE. Available at: http://dx.doi.org/10.1117/12.2548126.
    In this communication, we evaluate the suitability of Master-Slave (MS) optical coherence tomography (OCT) for processing of interferograms generated by an interferometer driven by an akinetic, electrically-tunable swept source from Insight with an ultra-large instantaneous coherence length. The akinetic source is programmed to sweep linearly, but within the sweep, at predictable times, the laser tuning introduces invalid regions in the interferogram, which are normally removed post-acquisition using a pre-calibration file. This makes sure that any optical frequency component is used once only and enables correct operation of a Fourier transform (FT). A FT applied to an unprocessed emitted spectrum leads to wide and numerous peaks in the A-scan. MS processing was introduced to avoid the necessary corrections demanded by conventional FT signal processing or its derivatives. The MS procedure consists of comparing photo-detected signals at the output of two interferometers, a Slave and a Master interferometer. The MS method was advanced along two avenues, either by using (i) electricallygenerated master signals (making use of the same interferometer twice) or (ii) optically-generated master signal via a recently introduced MS down-conversion procedure. We apply both avenues to the Insight source. Approach (i) tests the MS principle as an alternative to the Insight file correction while (ii) demonstrates near coherencelimited operation at a large axial range (>80 mm) for which a too-high sampling rate digitizer would have been needed. In this communication, we evaluate for the first time the suitability of the MS procedure to OCT measurements performed with the akinetic swept source commercialized by Insight. Two modalities are evaluated to implement the MS processing, based on: (i) digital generation of the master signals using the OCT interferometer and (ii) down conversion using a second interferometer driven by the swept source.
  • Erdelyi, R., Duma, V., Dobre, G., Bradu, A. and Podoleanu, A. (2019). Investigations of dental cavities: between x-ray radiography and OCT. In: Kovačičinová, J. ed. Optics and Measurement 2019 International Conference. SPIE. Available at: https://doi.org/10.1117/12.2542904.
    Nowadays in dentistry a correct diagnosis is given only after a clinical and radiological evaluation. Radiographs are also required for treatment assessments. The aim of this study is to present results obtained on evaluating dental cavities in a dental clinic in Western Romania, using both X-ray radiography and Optical Coherence Tomography (OCT). The most common methods for daily-basis clinical imaging are utilized, i.e. panoramic radiography and three-dimensional (3D) cone beam computed tomography (CBCT). Advantages of OCT as an imaging method in dentistry are discussed: it avoids exposing the patient to X-ray radiation, and image resolution of OCT is superior. This led us to test this technique for dental assessments and see how it can work in conjunction with radiography. The study also provides upsides and downsides of both medical imaging techniques. Panoramic radiographs and 3D CBCT to several extracted teeth are performed. Dedicated toolbars from Romexis software (Planmeca, Helsinki, Finland) are analyzed with regard to their capability to make precise measurements. Processing of images are made to obtain a high-quality; measurements are done and data are collected. The same teeth are scanned with an in-house developed SS-OCT system. Images from both investigations are presented, and clinical conclusions are drawn. For dental issues (i.e., cavities) that appear on the surface of the teeth, OCT proves to be more suitable than radiographs; it is also more accurate and radiation-free.
  • Hutiu, G., Duma, V., Demian, D., Dimb, A., Erdelyi, R., Bradu, A. and Podoleanu, A. (2019). Metallic fractures assessments: OCT versus SEM. In: Kovačičinová, J. ed. Optics and Measurement 2019 International Conference, 2019. SPIE. Available at: https://doi.org/10.1117/12.2542917.
    Metals can break either in a ductile or brittle manner if a static or dynamic load is applied to the same material. This depends on a variety of factors, such as the manner in which the load is applied, the shape of the mechanical part, the operating conditions, the nature and structure of the metallic material, and the working temperature. If subjected to variable loads, metallic materials break due to what is called fatigue. The microscopic analysis of fracture surfaces is currently carried out by using scanning electron microscopy (SEM). We have proposed, for the first time to our knowledge, a new method to analyze fracture surfaces, using a low coherence interferometry technique, Optical Coherence Tomography (OCT) [Gh. Hutiu, V.-F. Duma, et al., Surface imaging of metallic material fractures using optical coherence tomography, Appl. Opt. 53, 5912-5916 (2014); Gh. Hutiu, V.-F. Duma, et al., Assessment of ductile, brittle, and fatigue fractures of metals using optical coherence tomography, Metals 8, 117 (2018)]. The present paper presents the way we have demonstrated that OCT can replace the gold standard in such assessments, i.e. SEM, despite the fact that OCT has a resolution of 20 to 4 μm (in our investigations), while the SEM we employed has a 4 to 2 nm resolution. A few examples are given in this respect–for different types of fractures. The advantages of OCT versus SEM are discussed. This development opens the way for in situ investigations, for example in forensic sciences, where OCT can be applied (including with handheld scanning probes. as we have developed). In contrast, SEM, TEM, and AFM are lab-based techniques, more expensive, and they require trained operators.
  • Nteroli, G., Koutsikou, S., Moselund, P., Podoleanu, A. and Bradu, A. (2019). Real-time multimodal high resolution biomedical imaging instrument using supercontinuum optical sources. In: Frontiers in Optics 2019. OSA, p. JTu3A.99. Available at: https://doi.org/10.1364/FIO.2019.JTu3A.99.
    We present progress towards developing a multimodality imaging instrument, optical coherence tomography (OCT)/ photo-acoustic microscopy (PAM). By utilizing supercontinuum optical sources, that deliver wide spectral bandwidths and high energy densities, we devised a real-time imaging instrument which can be employed to image biological tissues. The OCT channel was devised to operate around 1300 nm. A custom built spectrometer ensures a constant axial resolution of 6 µm over an axial range of up to 1.5 mm. The PAM operates within the therapeutic window providing an axial resolution of 30 µm. The lateral resolution in both channels is 6 µm.
  • Rivet, S., Dubreuil, M., Bradu, A., Le Grand, Y., Pavone, F., Beaurepaire, E. and So, P. (2019). Fast Mueller Linear Polarization Modality at the Usual Rate of a Laser Scanning Microscope. In: EUROPEAN CONFERENCES ON BIOMEDICAL OPTICS. SPIE, p. 43. Available at: https://doi.org/10.1117/12.2526850.
    Mueller microscopes enable imaging of the optical anisotropic properties of biological or non-biological samples, in phase and amplitude, at sub-micrometer scale. However, the development of Mueller microscopes faces instrumental challenges: whilst adjusting the microscope, the operator needs a polarimetric image as guidance and the production of polarimetric parameters must be sufficiently quick to ensure fast imaging. To mitigate this issue, in this paper, a full Mueller scanning microscope based on spectral encoding of polarization is presented. The spectrum collected every 10 ms for each position of the optical beam on the specimen, incorporates all the information needed to produce the full Mueller matrix, which allows simultaneous images 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.
  • Marques, M., Pomeroy, J., Green, R., Deter, C., Bradu, A. and Podoleanu, A. (2019). Improved visualization of decomposing tattoos using optical coherence tomography. In: European Conferences on Biomedical Optics. SPIE. Available at: http://dx.doi.org/10.1117/12.2526757.
    Tattoos can be used in forensic human identification as a secondary means of identification (other means being, but not limited to, personal descriptions and artefacts) allowing the identification procedure to be strengthened in this way. Despite this, the decomposition of tattoos is a topic not extensively studied in taphonomic research (study of how organisms decay). In this communication, we assess optical coherence tomography (OCT) as a method to reliably identify tattoos before and after decomposition, by imaging tattooed porcine samples. OCT was able to penetrate up to 3mm below the surface and visualize parts of tattoos after 16 days of decomposition, which were no longer visible and recognizable using conventional photography-based methods. We believe this imaging modality has the potential to increase the reliability of tattoos in forensic human identification.
  • Nteroli, G., Bondu, M., Moselund, P., Podoleanu, A. and Bradu, A. (2019). Developments on Using Supercontinuum Sources for High Resolution Multi-Imaging Instruments for Biomedical Applications. In: European Conferences on Biomedical Optics. SPIE Society of Photographic Instrumentation Engineers, p. 22. Available at: https://doi.org/10.1117/12.2527111.
    We report on further progress made on enhancing the capabilities of a multi-imaging modality instrument capable of producing high resolution images of biological tissues. At the core of the instrument is a supercontinuum (SC) source. Two SC sources commercialized by NKT Photonics were employed for our experiments: SuperK COMPACT and SuperK Extreme (EXR9). Using these two sources, we assembled an instrument capable to simultaneously provide in real-time cross-section high-resolution Optical Coherence Tomography (OCT) and Photo-acoustic (PA) images in various spectral ranges. Currently, the OCT channel is operating in the IR range around 1300 nm to allow better penetration into the tissue using either the COMPACT or the EXR9. The measured optical power on the sample is in both cases above 9.5 mW. An in-house spectrometer equipped with a sensitive InGaAs camera capable of operating at 47 kHz and sampling data over a spectral range from 1205 to 1395 nm was developed. A constant axial resolution provided by the instrument in the OCT channel over a range of 1.5 mm was experimentally measured (4.96 µm), matching the theoretical prediction. The spectral range 500-800 nm was used for PA channel. The COMPACT, used in the PA channel, can select the central wavelength and the spectral bandwidth of operations. Typically, the optical energy per pulse on the sample is superior to 60 nJ when a bandwidth superior to 50 nm is employed. This make the instrument usable for PA imaging of tissues.
  • Podoleanu, A., Bradu, A., Marques, M. and Rivet, S. (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.
  • Fauchart, M., Marques, M., Bradu, A. and Podoleanu, A. (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.
  • Marques, M., Bradu, A., Rivet, S. and Podoleanu, A. (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.
  • 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.
  • Kapinchev, K., Bradu, A. and Podoleanu, A. (2019). Parallel Approaches to Digital Signal Processing Algorithms with Applications in Medical Imaging. In: Wysocki, T. and Wysocki, B. eds. 13th International Conference on Signal Processing and Communication Systems. IEEE. Available at: https://doi.org/10.1109/ICSPCS47537.2019.9008720.
    This paper reviews established and emerging parallel technologies, which are employed to enhance the performance of digital signal processing algorithms. Special attention is paid to algorithms with applications in medical imaging. Parallel implementations of some of the most commonly used algorithms, such as Fourier transforms, convolution and cross-correlation are discussed. Parallel optimization of a newly introduced method in optical coherence tomography is presented. Its performance, in terms of latency, is presented and discussed.
  • Podoleanu, A., Bang, O., Bojesen, S., Bondu, M., Bradu, A., Caujolle, S., Chin, C., Denninger, M., Feuchter, T., Fleischhauer, F., Hædersdal, M., Israelsen, N., Jensen, M., Gonzalo, I., Maria, M., Marques, M., Leick, L., Mogensen, M. and Moselund, P. (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.
  • Sinescu, C., Bradu, A., Duma, V., Topala, F., Negrutiu, M. and Podoleanu, A. (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://dx.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.
  • Marques, M., Rivet, S., Bradu, A. and Podoleanu, A. (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.
  • Hutiu, G., Duma, V., Demian, D., Bradu, A. and Podoleanu, A. (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.
  • Marques, M., Rivet, S., Bradu, A. and Podoleanu, A. (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. Bellingham, Washington: 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.
  • Bradu, A., Cernat, R., Istraelsen, N., Bang, O., Rivet, S., Keane, P., Garway-Heath, D., Rajendram, R. and Podoleanu, A. (2017). Master/slave: a better tool for Gabor filtering optical coherence tomography imaging instruments. In: European Conference of Biomedical Optics. SPIE, 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.
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