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).
Taylor, E., Cavuoto, S., Beal, D., Caujolle, S., Podoleanu, A. and Serpell, C. (2019). Development of Gold–Page: Towards the Electrophoretic Analysis of Sulphurous Biopolymers. Journal of Materials Chemistry B [Online]. Available at: https://doi.org/10.1039/C9TB00665F.
The prevalence; distinctive reactivity and biological significance of sulphur-based groups in proteins and nucleic acids means that analysis of sulphur is of prime importance in biochemistry, biotechnology, and medicine. We report steps in the development of a method to aid in the detection of these moieties using gold nanoparticles as adjuncts in polyacrylamide gel electrophoresis (Gold-PAGE).
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.
Manwar, R., Zafar, M., Podoleanu, A. and Avanaki, M. (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.
Podoleanu, A., Izatt, J., Lumbroso, B., Pircher, M., Rosen, R. and Weitz, R. (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.
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
Jensen, M., Gonzalo, I., Engelsholm, R., Maria, M., Israelsen, N., Podoleanu, A. and Bang, O. (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.
Israelsen, N., Petersen, C., Barh, A., Jain, D., Jensen, M., Hannesschläger, G., Tidemand-Lichtenberg, P., Pedersen, C., Podoleanu, A. and Bang, O. (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., 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.
Hu, Y., Podoleanu, A. and Dobre, G. (2019). Photothermal optical coherence tomography for investigation and imaging photothermal trapping of gold nano-rods in clear media and biological tissue. Journal of Optics [Online]. Available at: https://doi.org/10.1088/2040-8986%2Fab295d.
A quantitative spectrometer-based photothermal optical coherence tomography (PT-OCT) system is employed to investigate and image the photothermal trapping of gold nano-rods (GNRs) in clear and biological media. The PT-OCT system is calibrated through dynamic phase measurements of piezo motion with known driving parameters. We measure and compare the displacement sensitivities of the PT-OCT system at different camera exposure time settings in two configurations: with a distinct reference path; and with a common path. The displacement sensitivity of the system in the common path configuration is improved from 1.5 nm to 0.17 nm by performing Fourier analysis on the output phase. The minimum Ti:Sa power capable of inducing a detectable photothermal response of GNRs is measured to be 0.5 mW. This value agrees with the latest reported minimum Ti:Sa power for photothermal trapping GNRs. The PT-OCT system is used to generate en-face images of photothermal trapped GNRs in the water solution and in the biological tissue. By displaying the difference between successive en-face phase images, spatial distribution patterns of the aggregated GNRs, resulted from the photothermal trapping, are clearly outlined with great contrast. The photothermal trapping of GNRs in tissue shows a greater complexity than in the clear media. The limitation of the PT-OCT technology is discussed. The study proves the potential of PT-OCT for imaging the photothermal trapping of GNRs.
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.
Mogensen, M., Bojesen, S., Israelsen, N., Maria, M., Jensen, M., Podoleanu, A., Bang, O. and Haedersdal, M. (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.
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
Jensen, M., Israelsen, N., Maria, M., Feuchter, T., Podoleanu, A. and Bang, O. (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.
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.
Petersen, C., Israelsen, N., Barh, A., Jain, D., Jensen, M., Hannesschläger, G., Tidemand-Lichtenberg, P., Pedersen, C., Podoleanu, A., Bang, O., Alfano, R., Demos, S. and Seddon, A. (2019). Mid-infrared OCT imaging in highly scattering samples using real-time upconversion of broadband supercontinuum covering from 3.6-4.6 μm. In: Alfano, R. R., Demos, S. G. and Seddon, A. B. eds. Optical Biopsy XVII: Toward Real-Time Spectroscopic Imaging and Diagnosis. SPIE. Available at: https://doi.org/10.1117/12.2519444.
We present a mid-infrared spectral-domain optical coherence tomography system operating at 4.1 μm central wavelength with a high axial resolution of 8.6 μm enabled by more than 1 μm bandwidth from 3.58-4.63 μm produced by a mid-infrared supercontinuum laser. The system produces 2D cross-sectional images in real-time enabled the high-brightness of the supercontinuum source in combination with broadband upconversion of the signal to the range 820-865 nm, where a standard 800 nm array spectrometer can be used for fast detection. We discuss the potential applications within nondestructive testing in highly scattering materials and within biomedical imaging for achieving the in-vivo optical biopsy.
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.
Marcauteanu, C., Sinescu, C., Toader, M., Negrutiu, M., Mitroi, F., Stoica, E., Topala, F., Duma, V., Bradu, A. and Podoleanu, A. (2018). Biomimetic Tizian ’table tops’ analyzed with swept source 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/10.1117/12.2282407.
"Table tops"/occlusal veneers are partial crowns that allow for the biomimetic restoration of worn teeth in bruxing patients. The biomimetic approach includes a CAD/CAM composite resin for the manufacturing of the "table tops", for example Tizian, which is a zirconia reinforced composite (Schutz Dental Group). In the present study we prove the value of a fast swept source optical coherence tomography (SS-OCT) system in the evaluation of the marginal fit of Tizian "table tops". 12 maxillary first premolars were extracted and minimally invasive prepared for "table tops". The Tizian "table tops" were then adhesively cemented on the natural teeth with Variolink II (Ivoclar Vivadent). A SS-OCT system has been used to evaluate the marginal and internal adaptation of Tizian "table tops". The SS (Axsun Technologies, Billerica, MA) has a central wavelength of 1060 nm, a sweeping range 106 nm (quoted at 10 dB), and a 100 kHz line rate. The depth resolution of the system, measured experimentally in air was 10 ?m. The same samples were analyzed by using a micro-computed tomography (?CT) system, in order cu confirm the data obtained by using OCT. The imaging system used for this study offers good spatial resolutions in both directions, transversal and longitudinal, of around 10 ?m, a high sensitivity, and it is also able to acquire entire 3D volumes as fast as 2.5 s. Once the full dataset has been acquired, rendered high resolutions en-face projections could be produced. With it, the "table top" – cement – abutment tooth interfaces were observed on both C-scans as 2D images and also in the 3D reconstructions; the system was able to detect several open interfaces, which were confirmed on the ?CT i mages. The SS-OCT system allows for an efficient evaluation of biomimetic Tizian "table tops".
Abuabboud, L., Nica, L., Orel, L., Puscu, S., Negrutiu, M., Sinescu, C. and Podoleanu, A. (2018). Modern evaluation of the quality of the techniques of root canal dental obturation. In: Todea, C., Podoleanu, A. G. and Duma, V.-F. eds. Seventh International Conference on Lasers in Medicine. SPIE. Available at: https://doi.org/10.1117/12.2286452.
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).
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.
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).
Conclusions: OCT represents a valuable method for investigation with high depth resolution which can be used for evaluation of endodontic fillings.
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.
Dobre, G. (2018). Design considerations for ease of access and maneuverability of OCT imaging platforms in the oral cavity. In: Todea, C., Podoleanu, A. G. and Duma, V.-F. eds. Seventh International Conference on Lasers in Medicine. SPIE. Available at: https://doi.org/10.1117/12.2282646.
With the advent of instruments incorporating elements of confocal microscopy and Optical Coherence Tomography (OCT), optical technologies are steadily gaining usage in a variety of biomedical applications. They offer tissue visualization both at the superficial level and in depth, they operate using non-ionizing radiation at a relatively low cost, and the procedures are easily tolerated by patients. One area of growth for optical imaging is in the oral cavity, which has excellent optical accessibility both for dental tissue and oral mucosa. The need to image features accurately (both as part of oral mapping and as part of investigating the underlying tooth structure) requires producing 3-D volumes with sufficiently good lateral and vertical resolution to allow identification of features of interest. Although a few types of intraoral optical scanners are already successfully commercialized for surface mapping, the examination of optical design considerations that are particular to scanned beam imaging in the oral cavity does highlight particular challenges of interest to both clinicians and optical engineers engaged in designing the next generation of optical instruments.
Everson, M., Duma, V. and Dobre, G. (2018). Aspects of vignetting in a polygon mirror-based spectral filter for swept source optical coherence tomography (SS-OCT). In: Todea, C., Podoleanu, A. G. and Duma, V.-F. eds. Seventh International Conference on Lasers in Medicine. SPIE. Available at: https://doi.org/10.1117/12.2282284.
Optical Coherence Tomography (OCT) is a technology capable of producing 3-D volumes of microscopic structures with micron-scale resolution. Its main area of application remains ophthalmology and in particular retinal imaging. The quality and usability of the images depends upon the frame rate and the properties of the light being used. Swept source OCT (SS-OCT) can offer a speed advantage; variants using polygon mirrors (PMs) as spectral filters in SS-OCT have resulted in a variety of different arrangements. Although their application has been successfully demonstrated, a more detailed study of the particular aspects and requirements of beam propagation through the filter and their overall impact on the system performance have not been reported. Examining aspects related to vignetting at the PM facet leads to maximizing light throughput and system performance, which is the aim of this work. A swept source spectral filter consisting of a transmission grating, a two-lens telescope, an off-axis PM, and an end reflector mirror has been evaluated in terms of the beam width at the PM facet and how this parameter varies across the entire width of the spectrum at the input of the spectral filter.
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.
Israelsen, N., Petersen, C., Rodrigo, P., Barh, A., Jain, D., Woyessa, G., Jensen, M., Hannesschläger, G., Tidemand-Lichtenberg, P., Pedersen, C., Podoleanu, A. and Bang, O. (2020). Spectrometer-based mid-infrared optical coherence tomography operating at multi-kHz line rate speed (Conference Presentation). In: SPIE OPTO. SPIE. Available at: https://doi.org/10.1117/12.2542564.
In this work we demonstrate spectrometer based mid-infrared (MIR) optical coherence tomography (OCT) at 4 µm but with an increase of state-of-the-art imaging speed by at least 10 times. The improvement is based on exploiting a chirped periodically poled lithium niobate crystal. We show more than 3 kHz line rate OCT imaging. With this significant increase in imaging speed we hope to expand the efficacy of mid-infrared OCT.
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.
Kropáč, V., Marques, M. and Podoleanu, A. (2020). Transparent media thickness measurement employing low-coherence interferometry and a multi-element array. In: Optical Coherence Tomography and Coherence Domain Optical Methods in Biomedicine XXIV. SPIE. Available at: http://dx.doi.org/10.1117/12.2548564.
In this communication, we present a method to measure the thickness of transparent media employing a low coherence interferometer and a multi-element array as a photo-detector. The multi-element array employed is part of a consumer-grade digital camera (< 0.5 k$). A two-beam interferometer is created by inserting the slab of a transparent material half-way through into the measurement beam. The method is evaluated on a thin microscope cover slip and on a thicker microscope glass slide.
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.
Podoleanu, A. (2019). Advances in the optical sources and signal processing for Optical Coherence Tomography. In: International Microwave and Optoelectronics Conference (IMOC). Available at: https://imoc2019.com/workshops/.
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.
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.
Raymond, J., Everbach, E., Roy, R., Marques, M., Hughes, M. and Podoleanu, A. (2019). HIFU tissue lesion quantification by optical coherence tomography. In: Acoustical Society of America. Available at: http://dx.doi.org/10.1121/1.5101623.
Heating of tissue by high-intensity focused ultrasound (HIFU) can result in sufficient temperature elevation to cause irreversible changes in the tissue structure. The contiguous volume occupied by these changes, a lesion, and the extent of the tissue changes may be quantified histologically or estimated through techniques such as ultrasonic elastography. We have shown that changes in tissue optical scattering could be used as a proxy to improve sensing and imaging of HIFU lesion formation as an alternative to thermometry. Optical coherence tomography (OCT) is a light-based method appropriate for optically accessible tissues, which we have used to quantify lesion volume, shape, and quality based upon the irreversible changes in optical scattering that occurs with protein denaturation. We have adapted OCT to take into account changes in optical polarization of the tissue, providing sensitivity to changes in the collagen orientation of skin with heating. This technique has potential in detecting antecedents of skin burn during HIFU exposures, thereby increasing safety and reducing treatment times.
Alesbrook, L., Marques, M., New, J., Harriss, K. and Podoleanu, A. (2019). Analysis of Impact Craters Using Optical Coherence Tomography. In: 50th Lunar and Planetary Science Conference. Available at: http://adsabs.harvard.edu/abs/2019LPI. 50.1830A.
We present our work on the use of OCT to analyse impact samples.
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.
Jensen, M., Gonzalo, I., Engelsholm, R., Maria, M., Israelsen, N., Podoleanu, A. and Bang, O. (2019). Supercontinuum sources in optical coherence tomography: how pulse-to-pulse fluctuations affect the noise performance. In: Design and Quality for Biomedical Technologies XII. SPIE. Available at: https://doi.org/10.1117/12.2509374.
Supercontinuum sources are increasingly applied to spectral domain optical coherence tomography (OCT) due to their high power across octave-spanning bandwidths from the visible to the mid-infrared, enabling ultra-high resolution imaging with great flexibility in choice of operating wavelength region . However, one of the main drawback of supercontinuum sources in OCT imaging is the large pulse-to-pulse fluctuations which often acts as the limiting factor in terms of sensitivity rather than the shot noise . The theoretical noise description widely used by the OCT community assumes that the light source operation is based on spontaneous emission, which is not the case for supercontinuum laser source . As a result, the optimal operating conditions must be evaluated experimentally without a reliable prediction [2,4]. Without a reliable theoretical noise model, optimization can be challenging. We present a new and simple noise model that allows prediction of the noise performance of an OCT system driven by a supercontinuum sources, without any assumptions regarding the type of light source. We show that the predictions are in excellent agreement with the experimental results obtained by employing a widely used commercial supercontinuum source. We further investigate the shape of the noise floor in an A-scan obtained with a commercial supercontinuum source, which is not flat, as expected for shot-noise limited light sources. We demonstrate that this shape is predicted solely by the spectral correlations of the supercontinuum, which therefore must be taken into account when characterizing the sensitivity of the OCT system driven by a supercontinuum source.
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. (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.
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.