Ітеративний метод корекції артефактів при оптоакустичній реконструкції

Optoacoustic tomography is effective in applying to the visualization of objects that have a high coefficient of light absorption relative to the environment. Such tasks include, for example, defectoscopy, vascular imaging, detection and monitoring of tumors, diagnosis of porosity of composite materials, high-precision local measurement of the modulus of elasticity. However, the quality of optoacoustic images largely depends on factors such as noise (equipment or environmental noise) and distortion due to the characteristics of the model and the calculation algorithm. The article proposes an iterative algorithm for improving the quality of optoacoustic images, based on the observation that artifacts increase in magnitude with each iteration, while the nature and location of the distortions remain unchanged. Numerical simulations of the propagation of ultrasonic waves in environments close to soft biological tissues have been performed. In terms of eliminating distortion and artefacts inherent to the method of image reconstruction, an iterative filter was found to be highly effective The effectiveness of the approach is manifested in the use of a small number of iterations. Pages of the article in the issue: 98 - 107 Language of the article: UkrainianОптоакустична томографія ефективна при її застосуванні до задач візуалізації об’єктів, що мають підвищений коефіцієнт поглинання світла по відношенню до навколишнього середовища. До таких задач належить, наприклад, дефектоскопія, візуалізація кровоносних судин, виявлення і моніторинг пухлин, діагностика пористості композиційних матеріалів, високоточне локальне вимірювання модулів пружності. При цьому, якість оптоакустичних зображень в значній мірі пов’язана з такими факторами, як шум (шуми апаратури і навколишнього середовища) і спотворення, зумовлені особливостями самої моделі і розрахунковим алгоритмом. У роботі пропонується ітеративний алгоритм поліпшення якості оптоакустичних зображень, заснований на припущенні про постійність модельних і численних спотворень і наступному порівнянні (і коригуванні) відновлених зображень з оригінальними даними. Проведено чисельне моделювання поширення хвиль ультразвукового діапазону в середовищах, наближених до м’яких біологічних тканин. Показана ефективність підходу вже при використанні незначної кількості ітерацій

Поліпшення якості оптоакустичної візуалізації: співставлення фізичного та числового експерименту

Optoacoustic imaging is based on the generation of thermoelastic waves by heating an object in an optically inhomogeneous medium with a short laser pulse. The generated ultrasonic waves contain information about the distribution of structures with predominant optical absorption. Detection of acoustic perturbations on the surface of the object and the application of the backprojection algorithm are used to create a picture of the absorbed energy inside the environment. Conventional reconstruction methods lead to artifacts due to the peculiarities of the recovery algorithm. This study proposes an iterative procedure to reduce these artifacts. The algorithm minimizes the error between the measured signals and the signals calculated from the recovered image. The paper compares the results of processing optoacoustic signals implemented in numerical experiments with the results of physical experiments. It is shown that the quality of the recovered images improves even with a small number of iterations. Pages of the article in the issue: 46 - 56 Language of the article: UkrainianОптоакустична візуалізація заснована на генерації термопружних хвиль шляхом нагрівання об’єкта в оптично неоднорідному середовищі коротким лазерним імпульсом. Сгенеровані ультразвукові хвилі містять інформацію про розподіл структур з переважним оптичним поглинанням. Виявленні на поверхні об’єкта акустичні збурення та застосування алгоритмів реконструкції дозволяють відтворити картину поглиненої енергії всередині середовища. Звичайні методи реконструкції призводять до артефактів через особливості алгоритму відновлення. Це дослідження пропонує ітераційну процедуру для зменшення цих шкідливих спотворень. Алгоритм мінімізує похибку між виміряними сигналами та сигналами, розрахованими з відновленого зображення. У роботі порівнюються результати обробки оптоакустичних сигналів, реалізованих у чисельних експериментах, з результатами фізичних експериментів. Показано, що якість відновлених зображень покращується навіть при невеликій кількості ітерацій

PSR B0329+54: Statistics of Substructure Discovered within the Scattering Disk on RadioAstron Baselines of up to 235,000 km

We discovered fine-scale structure within the scattering disk of PSR B0329+54 in observations with the RadioAstron ground-space radio interferometer. Here, we describe this phenomenon, characterize it with averages and correlation functions, and interpret it as the result of decorrelation of the impulse-response function of interstellar scattering between the widely-separated antennas. This instrument included the 10-m Space Radio Telescope, the 110-m Green Bank Telescope, the 14x25-m Westerbork Synthesis Radio Telescope, and the 64-m Kalyazin Radio Telescope. The observations were performed at 324 MHz, on baselines of up to 235,000 km in November 2012 and January 2014. In the delay domain, on long baselines the interferometric visibility consists of many discrete spikes within a limited range of delays. On short baselines it consists of a sharp spike surrounded by lower spikes. The average envelope of correlations of the visibility function show two exponential scales, with characteristic delays of $\tau_1=4.1\pm 0.3\ \mu{\rm s}$ and $\tau_2=23\pm 3\ \mu{\rm s}$, indicating the presence of two scales of scattering in the interstellar medium. These two scales are present in the pulse-broadening function. The longer scale contains 0.38 times the scattered power of the shorter one. We suggest that the longer tail arises from highly-scattered paths, possibly from anisotropic scattering or from substructure at large angles.Comment: 15 pages, 6 figures, 3 tables; accepted by Astrophysical journa

Evolved star water maser cloud size determined by star size

Cool, evolved stars undergo copious mass loss but the details of how the matter is returned to the ISM are still under debate. We investigated the structure and evolution of the wind at 5 to 50 stellar radii from Asymptotic Giant Branch and Red Supergiant stars. 22-GHz water masers around seven evolved stars were imaged using MERLIN, at sub-AU resolution. Each source was observed at between 2 and 7 epochs (several stellar periods). We compared our results with long-term Pushchino single dish monitoring. The 22-GHz emission is located in ~spherical, thick, unevenly filled shells. The outflow velocity doubles between the inner and outer shell limits. Water maser clumps could be matched at successive epochs separated by <2 years for AGB stars, or at least 5 years for RSG. This is much shorter than the decades taken for the wind to cross the maser shell, and comparison with spectral monitoring shows that some features fade and reappear. In 5 sources, most of the matched features brighten or dim in concert from one epoch to the next. One cloud in W Hya was caught in the act of passing in front of a background cloud leading to 50-fold, transient amplification. The masing clouds are 1-2 orders of magnitude denser than the wind average and contain a substantial fraction of the mass loss in this region, with a filling factor <1%. The RSG clouds are ~10x bigger than those round the AGB stars. Proper motions are dominated by expansion, with no systematic rotation. The maser clouds survive for decades (the shell crossing time) but the masers are not always beamed in our direction. Radiative effects cause changes in flux density throughout the maser shells on short timescales. Cloud size is proportional to parent star size; clouds have a similar radius to the star in the 22-GHz maser shell. Stellar properties such as convection cells must determine the clumping scale.Comment: Accepted by A&A 2012 July 10 Main text 29 pages, 62 figures Appendix 44 pages, 23 figure

VLBI observations of the G25.65+1.05 water maser superburst

This paper reports observations of a 22 GHz water maser 'superburst' in the G25.65+1.05 massive star-forming region, conducted in response to an alert from the Maser Monitoring Organisation (M2O). Very long baseline interferometry (VLBI) observations using the European VLBI Network (EVN) recorded a maser flux density of 1.2 × 104 Jy. The superburst was investipgated in the spectral, structural, and temporal domains and its cause was determined to be an increase in maser path length generated by the superposition of multiple maser emitting regions aligning in the line of sight to the observer. This conclusion was based on the location of the bursting maser in the context of the star-forming region, its complex structure, and its rapid onset and decay. © 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical SocietyRAB acknowledges support through the EACOA Fellowship from the East Asian Core Observatories Association. GO acknowledges support from the Australian Research Council Discovery project DP180101061 funded by the Australian Government, the CAS ‘Light of West China’ Program 2018-XBQNXZ-B-021, and the National Key R&D Program of China 2018YFA0404602. TH is financially supported by the MEXT/JSPS KAKENHI Grant Number 17K05398. BM acknowledges support from the Spanish Ministerio de Economía y Competitividad (MINECO) under grants AYA2016-76012-C3-1-P and MDM-2014-0369 of ICCUB (Unidad de Excelencia ‘María de Maeztu’). NS acknowledges support from Russian Science Foundation grant 18-12-00193. AMS is supported by the Ministry of Science and High Education (the basic part of the State assignment, RK No. AAAA-A17-117030310283-7) and by the Act 211 Government of the Russian Federation, contract No.02.A03.21.0006. JOC acknowledges support by the Italian Ministry of Foreign Affairs and International ..

H2O and OH masers associated with cold infrared sources

International audienc

Evolution of the H2O and OH Maser Emission in W75 N

International audienc

Long-term monitoring and interpretation of flares in the H2O maser emission of IRAS 16293–2422

International audienc

Investigation of the OH and H2O maser emission from the semiregular variable HU puppis

International audienc