Microwavephysics and Atmospheric Physics
Biomedizinische Photonik
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Ultrafast Science and Technology
Last update: 07.05.2018
FS 2018: Seminare über Biomedizinische Photonik
Wednesday 10-12
Vorträge, die innerhalb der nächsten Tage stattfinden, sind speziell markiert.
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Mittwoch, 28.03.2018

Clutter reduction methods in epi-optoacoustic imaging: a comparative study

Zeit: 10:15 Uhr
Hörsaal: A97
 
Tigran Petrosyan
Institute of Applied Physics
University of Bern

In epi-optoacoustic (OA) imaging, optical components are attached or directly integrated into the ultrasound probe, providing single-handed probe guidance for flexible imaging of the human body. Such a setup, however, generates clutter signals originating from optical absorption at the tissue irradiation site near the probe, reducing contrast and imaging depth. To increase imaging depth towards the noise limit, various clutter reduction techniques have been proposed: (1) Displacement compensated averaging (DCA) employs the clutter decorrelation that results from quasi-static tissue deformation when palpating the tissue with freehand probe motion. (2) Photoacoustic-guided focused ultrasound (PAFUSion) uses ultrasound pulse-echo acquisitions to mimic reflection artefacts, which can then be subtracted from the OA image for clutter reduction. (3) In localized vibration tagging (LOVIT), a focused ultrasonic pushing beam transiently displaces optical absorbers at its focus and thus leads to a phase shift of OA signal originating from this focal region inside the tissue. Subtraction of OA acquisitions before and after the push highlights OA signals from the focus but eliminates clutter from outside the pushing beam. In this study, we compare DCA, PAFUSion and LOVIT in terms of clutter reduction efficiency for different imaging depths and laser irradiation geometries, where all three techniques can be implemented in a single custom built setup.

 
Mittwoch, 04.04.2018

Understanding polarization and polarimetric patterns

Zeit: 10:15 Uhr
Hörsaal: A97
 
Arushi Jain
Institute of Applied Physics
University of Bern

Polarization is the the property of electromagnetic radiations in which the direction and magnitude of the vibrating electric field are related in a specified as opposed to unpolarized light vibrating in random direction. Though this being the encyclopedia meaning, there is more to that needs to be understood. In order to reach our goal of tissue diagnostics, it is crucial to understand what these patterns. We will discuss the analytical solutions and measurements, that were used to understand the said patterns. Further I will also briefly talk about our current work and future steps.

 
Mittwoch, 11.04.2018

Master Thesis: A novel approach to ultrasound computed tomography of the human breast

Zeit: 10:15 Uhr
Hörsaal: A97
 
Cyril Etter
Institute of Applied Physics
University of Bern

Ultrasound computed tomography (UCT) is a promising alternative to the standard X-ray mammography for breast cancer diagnosis due to its non invasive nature and non-ionizing radiation. The imaging principle is the same as in X-ray computed tomography (CT): Radiation transmitted through the breast is measured from different angles to reconstruct the spatial distribution of a physical tissue property, in case of UCT the speed of sound (SoS). But there is a fundamental difference to X-ray CT: The ultrasound wavelength is comparable to the spatial resolution of structures of interest, thus diffraction plays a role. In addition, spatial SoS variations are large enough to cause substantial refraction. To achieve a high-resolution and accurate SoS image by accounting for these effects, rather complex and computationally demanding reconstruction algorithms are typically needed. In this seminar, a computationally less demanding alternative method based on a modified filtered backprojection approach is presented. To compensate for diffraction and refraction, the measured signals are backpropagated into the tissue before time-of-flight determination. In phantom experiments using a setup based on two commercial linear array ultrasound transducers, we have shown that this fast reconstruction technique provides a diffraction-limited spatial resolution. The SoS could be accurately reconstructed in phantoms with size and SoS comparable to a human breast.

 
Mittwoch, 18.04.2018

Radiative Transfer Theory and Maxwell’s Equations

Zeit: 10:15 Uhr
Hörsaal: A97
 
Leonie Ulrich
Institute of Applied Physics
University of Bern

Since its first formulation more than a century ago, radiative transfer theory has extensively been used to describe the propagation of electromagnetic radiation in a variety of fields such as astrophysics, atmospheric physics and biomedical optics. Yet, the link between the heuristically derived radiative transfer equation and the fundamental laws of electrodynamics had remained unclear until very recently. In this seminar, we will look at the radiometric quantities commonly used in biomedical optics and examine their relation to the Poynting vector, by deriving the scalar radiative transfer equation from Maxwell’s equations. We will thereby identify the approximations inherent to the theory of radiative transfer and discuss their implications for the description of light propagation in tissue.

 
Mittwoch, 02.05.2018

Anthropomorphic oil/gel breast phantoms

Zeit: 10:15 Uhr
Hörsaal: A97
 
Patrick Stähli
Institute of Applied Physics
University of Bern

To examine new photoacoustic-based imaging modalities, anthropomorphic phantoms that mimic acoustic properties (speed of sound, echogenicity, attenuation), optical properties (absorption- and reduced scattering coefficient) and realistic tissue complexity are essential tools. Longevity, mechanically robustness, low manufacturing costs and non-toxicity are also important considerations. Among the available approaches, oil-in-gel emulsions provide realistic speed of sound and echogenicity where the former is determined by the oil/water ratio and the latter can be independently adapted by adding further ingredients such as cellulose or glass microspheres. To verify the phantom production process, breast phantoms based on a 2D and 3D design were produced, providing an undulated fat layer around a glandular inner core containing cylindrical inclusions. To acoustically characterize these breast phantoms, a 2D ultrasound tomography setup was used (1). In this seminar, I will give an overview on the phantoms production process and highlight the important steps to achieve acoustically realistic and stable phantoms. Further, first time of flight measurements (2), which characterize the optical properties of the phantom (absorption- and reduced scattering-coefficient) are shown.

1 T.Schweizer: Master’s Thesis (2017), C.Etter: Master’s Thesis (2018)

2 H. Günhan Akarçay et. Al: Determining the optical properties of a gelatin-TiO2 phantom at 780nm (2012)

 
Mittwoch, 09.05.2018

Carotid Plaque Characterization using Speed of Sound imaging: Recent Progresses

Zeit: 10:15 Uhr
Hörsaal: A97
 
Dr. Maju Kuriakose
Institute of Applied Physics
University of Bern

Noninvasive Determination of Carotid plaque morphology is highly appreciated in order to assess the need for removal of plaques by risky operation. Present state of the art imaging techniques like MRI or CT are still lacking specificity and accuracy in assessing the plaque morphology. So, in this seminar, I will talk about the potential and our recent progresses in using classical ultrasound based speed of sound reconstruction techniques to image plaques.

 
Mittwoch, 16.05.2018

Discrete sampling and interpolation within and “beyond” the sampling theorem

Zeit: 10:15 Uhr
Hörsaal: A97
 
Dr. Michael Jaeger
Institute of Applied Physics
University of Bern

During the Physics study, we may have (or not have) learnt about the Nyquist-Shannon sampling theorem (many other names exist) in the following way: Given a function s(t) with a frequency spectrum S(f) that is zero outside ±fmax, the adequate sampling rate to unambiguously capture this function must be at least 2fmax. Having a closer look, however, this requirement can often be slackened depending on the spectral characteristics of s(t). In my work this became important because we would want to increase ultrasound imaging depth without increasing data volume. Ultrasound signals are typically limited to a frequency band centred around a frequency f0 with a bandwidth B, and the minimum sampling rate is determined by B rather than by fmax = f0 + B/2. In this talk I will revisit the conventional sampling theorem and then discuss the generalisation therefrom, illustrated on some examples taken from my current work.

 
Mittwoch, 23.05.2018

Quantitative Comparison of Time-domain and Frequency-domain Approaches in Optoacoustic Microscopy Image Reconstruction

Zeit: 10:15 Uhr
Hörsaal: A97
 
Florentin Spadin
Institute of Applied Physics
University of Bern

Image reconstruction is important in optoacoustic microscopy as it allows the region of acceptable resolution around the plane of focus to be increased, which in turn greatly improves the usability of the technique. In this seminar, time-domain and frequency-domain reconstructions are quantitatively compared in terms of their ability to improve the true resolution of the final image.

 
Donnerstag, 07.06.2018

Master Thesis: Speed of Sound reconstruction combining Echo and Transmission Mode

Zeit: 10:15 Uhr
Hörsaal: A97
 
Louis Wyss
Institute of Applied Physics
University of Bern

Ultrasound computed tomography (UCT) is a promising alternative to the standard X-ray mammography for breast cancer diagnosis due to its non-invasive nature and non-ionizing radiation. Conventional through-transmission tomography reconstructs the spatial distribution of speed of sound (SoS) based on the arrival time of ultrasound outside the tissue after having propagated through the tissue from various different angles. This technique suffers from artefacts associated with diffraction and refraction of ultrasound that propagates parallel to the skin surface. Computed Ultrasound Tomography in Echo mode (CUTE), on the other hand, is based on the phase shift of internal echoes measured under reflection mode perpendicular to the skin surface. My research is focused on investigating whether a combination of these two complementing techniques results in an over-all improved SoS image. In my talk I will explain how a SoS reconstruction combining the two data types can be implemented and will show recent results from phantom studies.

 
Mittwoch, 13.06.2018

Polarimetric measurements in the realm of biomedical applications: whereto?

Zeit: 10:15 Uhr
Hörsaal: A97
 
Dr. Günhan Akarçay
Institute of Applied Physics
University of Bern

Optical polarization measurements have a rich history spanning more than three centuries and nowadays, Stokes-vector / Mueller-matrix polarimetry finds applications in a plethora of fields encompassing physical, chemical, and biological sciences, just to name a few. The questions seeking for an answer with such measurements can be succinctly summarized as follows: (ii) how does the probed system alter the light polarization? (ii) what information does this provide on the microscopic features of said system? The (Perrin-)Mueller matrix PM is certainly an elegant and fascinating mathematical object that encodes all the polarimetric properties of a physical system, but is notoriously difficult to decipher. More precisely, in the case of multiply scattering systems such as biological tissue, there seems to be a genuine struggle to extract and interpret polarimetric data contained in the PM matrix. Many research groups have been trying to solve this riddle but there exists no consensus yet. In this talk, we shall reflect, as experimentalists, on whether it is necessary at all to work with the PM matrix and on whether it would be more advantageous to return to the essence of a polarimetric measurement by working with the Stokes vectors. Moreover, the polarimetric parameters used to describe the behaviour of crystals are perhaps not suited for tissue. Can these parameters be re-defined for multiply scattering systems, so that polarimetric imaging can be used to better understand tissue? Or is this technique destined to remain a mere qualitative imaging modality?

 
Mittwoch, 20.06.2018

Optical Time-of-Flight Measurements on Tissue Phantoms: To what extent can the semi-infinite model be used to extract optical properties for biomedical applications?

Zeit: 10:15 Uhr
Hörsaal: A97
 
Thilo Ladner
Institute of Applied Physics
University of Bern and Department of Chemistry and Applied Biosciences, ETH Zurich

Time-of-flight measurements (TOF) have been routinely used in the Biomedical Photonics group to optically characterise tissue phantoms (i.e., to retrieve their absorption coefficient and mean free transport path). Such measurements have been thus far conducted invasively, in the "infinite geometry", for better accuracy. Bearing the potential clinical applications of this approach in mind, we have been interested in examining the accuracy of TOF measurements when performed at the surface of the material, in a minimally invasive manner. In this seminar, I will present the work I have carried out during my semester project and show first results obtained with liquid phantoms.