Transducer Arrays for 3D Ultrasound Computed Tomography

Die Ultraschall-Computertomographie (USCT) ist ein vielversprechendes medizinisches Bildgebungsverfahren zur Früherkennung von Brustkrebs. Am Karlsruher Institut für Technologie wird derzeit ein Gerät der dritten Generation (3D-USCT-III) für 3D-Aufnahmen entwickelt. Unter den kritischsten und technologisch anspruchsvollsten Komponenten dieses Geräts sind die Schallwandlerarrays. Diese müssen eine pseudozufällige Positionierung einzelner Wandler ermöglichen, hohe Bandbreiten, große Öffnungswinkel und eine isotrope Schallabstrahlung aufweisen sowie den Vorschriften für Medizinprodukte genügen. In dieser Arbeit wird die Realisierung neuer Schallwandlerarrays (TAS-III) für die 3D-USCT-Bildgebung umfassend vorgestellt. Dies beinhaltet die Definition von Anforderungen, Systemdesign, automatisierte Fertigung, Charakterisierung und Entwurfsoptimierung. Kernelement des TAS-III-Designs sind Scheiben aus piezoelektrischen Verbundwerkstoffen, die 18 in Polymer eingebettete, räumlich verteilte piezokeramische Fasern enthalten. Zusätzliche Scheiben zur akustischenAnpassung und Dämpfung werden auf beiden Seiten angebracht, um die Arrays zu finalisieren. Für die Herstellung der benötigten 256 TAS-III wurde ein teilautomatisierter Fertigungsprozess entwickelt. Quantitative Qualitätsprüfungen ergaben, dass mehr als 96% der produzierten Wandler voll funktionsfähig waren. Die Amplitude und der Phasenwinkel des akustischen Feldes von 54 Wandlern wurden gemessen und ausgewertet. Die meisten der definierten Anforderungen wurden erfüllt. Es wurde eine mittlere Mittenfrequenz von 2,6 MHz, mit einer fraktionellen Bandbreite von 134% bei -10 dB ermittelt. Die Bandbreite resultiert dabei aus zwei unterschiedlichen Schwingungsmoden. Messungen in 3D zeigten isotrope Abstrahlcharakteristiken mit einem mittleren Öffnungswinkel von 42,8°. Für die Analyse und Optimierung des Designs wurden verschiedene Modellierungsansätze entwickelt. Geringfügige Änderungen der Länge und des Durchmessers der piezoelektrischen Fasern sowie eine höhere laterale Dämpfung konnten die Leistung in gewissem Maße verbessern. Der Umfang weiterer möglicher Verbesserungen zeigte jedoch, dass das TAS-III Design nahe am erreichbaren Optimum liegt. Alternative Wandlertechnologien wurden untersucht, umdie grundsätzlichen Grenzen von Verbundwerkstoffen bestehend aus piezoelektrischen Fasern in Bezug auf Öffnungswinkel und Bandbreite zu überwinden. Der Ersatz der Fasern durch einkristalline piezoelektrische Materialien verspricht eine Erhöhung der Bandbreite um 35%, erfordert jedoch umfangreiche Anpassungen in den Herstellungsprozessen. Die Charakterisierung von mikromechanischen Ultraschallwandlern ergab eine signifikante Vergrößerung des Öffnungswinkels, aber geringere erzeugte Schalldrücke. Dennoch machen die Eigenschaften und die verfügbare Designfreiheit diese Schallwandlertechnologie sehr vielversprechend für zukünftige 3D-USCT- Generationen. Die entworfenen und realisierten TAS-III erwiesen sich als rundum geeignet für den vorgesehenen Einsatz und wurden in zwei 3D-USCT-III-Geräten integriert. Ausführliche klinische Tests werden in naher Zukunft durchgeführt, um die Sensitivität und Spezifität dieser neuartigen 3D-Ultraschallbildgebungsmethode zu bewerten

An Economy with Personal Currency: Theory and Experimental Evidence

Is personal currency issued by participants sufficient to operate an economy efficiently, with no outside or government money? Sahi and Yao (1989) and Sorin (1996) constructed a strategic market game to prove that this is possible. We conduct an experimental game in which each agent issues her personal IOUs, and a costless efficient clearinghouse adjusts the exchange rates among them so the markets always clear. The results suggest that if the information system and clearing are so good as to preclude moral hazard, any form of information asymmetry, and need for trust, the economy operates efficiently at any price level without government money. These conditions cannot reasonably be expected to hold in natural settings. In a second set of treatments when agents have the option of not delivering on their promises, a high enough penalty for non-delivery is necessary to ensure an efficient market; a lower penalty leads to inefficient, even collapsing, markets due to moral hazard

Start-up Funding via Equity Crowdfunding in Germany – A Qualitative Analysis of Success Factors

Entrepreneurs often struggle to find sufficient funding for their start-ups. A relatively new way for companies to attract capital is via an internet platform, locating investors who in return receive something in return for their ventures. Equity crowdfunding is one of several types of crowdfunding, and is also known as crowdinvesting in the German-speaking realm. This article predominantly advances the scientific knowledge regarding the success factors of equity crowdfunding for German start-ups. The study conducted nine qualitative interviews with start-ups and crowdinvesting platforms. Its first result is that German start-ups select crowdinvesting because (1) it is a funding opportunity and (2) it has an expected marketing effect. To organize the results of relevant success factors, the Crowdinvesting Success Model was designed by the researchers. This supports German entrepreneurs by presenting 20 important success elements that help to increase the capital collected during a campaign. The key finding is that an attractive business model, an appropriate preparation in the pre-campaign period, ongoing activities during the campaign, and corresponding advertising activities have a positive impact on a German start-up’s crowdinvesting campaign’s chances of success. The article closes with implications for theory and practice, as well as further research suggestions

Risk-reducing options in crowdinvesting: An experimental study

Financial constraints are a striking difficulty of entrepreneurial ventures in the early stages of their development. Recently, emerging crowdinvesting platforms try to fill this finance gap by involving an anonymous crowd into the funding process. Due to high information asymmetries, platform providers and start-ups alike try their best to reduce the risk for investors. We therefore examine existing and thinkable mechanisms of option-based risk reduction in crowdinvesting. We use a 2x3x3 mixed subject design to manipulate the availability and characteristics of risk-reducing options and the project attractiveness. With 210 participants, we are able to show that the introduction of different risk-reducing options in crowdinvesting solely favors high quality projects and increases capital concentration in a market that was originally built to make start-up funding available to a broader range of capital seekers. We also suggest reasonable prices for options and prove them to be accurate relatively to each other. Further implications for theory and practice are discussed

Method to Extract Frequency Dependent Material Attenuation for Improved Transducer Models

The time response of the ultrasound transducers used in our 3D ultrasound tomography device shows a slight reverberation. This may causes artifacts in the reconstructed images. Loss properties of materials used in the array fabrication have a big impact on their complex vibration behavior. Unfortunately, material parameters for accurate modeling are often not available in literature. Here, we present a method to derive loss properties of polymers and composites and how to include them in a finite element analysis (FEA). The method has three steps: First, an experiment to measure the frequency and thickness dependent sound attenuation. Second, a brute-force fit to a frequency-power law expression to obtain an analytic formulation. Third, a conversion of the sound attenuation to an equivalent structural loss factor. The last step is necessary as acoustic attenuation can not directly be implemented in structural mechanics FEA. We applied the method to derive loss properties of the filler and backing material which we use for our ultrasound transducer arrays. When including the loss factor in the simulation a reverberation is predicted, which matches the measurement well. Hence, considering loss properties allows more accurate modeling of complex vibration behavior. This aids in optimizing our ultrasound transducer array design towards better 3D ultrasound imaging

Model-Guided Manufacturing of Transducer Arrays Based on Single-Fibre Piezocomposites

For breast cancer imaging, ultrasound computer tomography (USCT) is an emerging technology. To improve the image quality of our full 3-D system, a new transducer array system (TAS) design was previously proposed. This work presents a manufacturing approach which realises this new design. To monitor the transducer quality during production, the electro-mechanical impedance (EMI) was measured initially and after each assembly step. To evaluate the measured responses, an extended Krimholtz–Leedom–Matthaei (KLM) transducer model was used. The model aids in interpreting the measured responses and presents a useful tool for evaluating parasitic electric effects and attenuation at resonance. For quality control, the phase angle at thickness resonance φt was found to be the most specific EMI property. It can be used to verify the functionality of the piezocomposites and allows reliable detection of faults in the acoustic backing. Evaluating the final response of 68 transducers showed 5% variance of the series resonance frequency. This indicates good consistency of derived ultrasound performance parameters

Arbitrage bots in experimental asset markets

While algorithmic trading robots are a proliferating presence in asset markets, there is no consensus whether their presence improves market quality or benefits individual investors. We examine the impact of robots seeking arbitrage in experimental laboratory markets. We find that the presence of algorithmic arbitrageurs generally enhances market quality. However, the wealth of human traders suffers from the presence of algorithmic traders. These social costs can be mitigated as we find high latency algorithms harm investors less than low latency algorithms; while the improvements in market quality are indistinguishable between algorithm latency levels and whether they provide liquidity or not

Arbitrage bots in experimental asset markets

While algorithmic trading robots are a proliferating presence in asset markets, there is no consensus whether their presence improves market quality or benefits individual investors. We examine the impact of robots seeking arbitrage in experimental laboratory markets. We find that the presence of algorithmic arbitrageurs generally enhances market quality. However, the wealth of human traders suffers from the presence of algorithmic traders. These social costs can be mitigated as we find high latency algorithms harm investors less than low latency algorithms; while the improvements in market quality are indistinguishable between algorithm latency levels and whether they provide liquidity or not

Focusing on Quality Patient Care in the New Global Subsidy for Malaria Medicines

Tido von Schoen-Angerer and colleagues discuss the new Affordable Medicines Facility for malaria (AMFm), which subsidizes and facilitates access to artemisinin-based combination therapy, and what mechanisms are needed to ensure it stays focused on quality patient care