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Accurate magnetic resonance fingerprinting-based B1+ mapping in the human body for ultra-high-field MRI
No full textUltra-high field (UHF) MRI (B0 >= 7 Tesla) offers unique advantages compared to lower field strengths, such as an increased signal-to-noise ratio, but it also faces critical challenges due to inhomogeneity of the transmit magnetic field (B1+). When the human body is targeted at UHF, a large B1+ variation becomes apparent, and many applications rely on accurate quantification of B1+ across this wide range. However, established methods exhibit limited sensitivity at low flip angles (FAs), and hardware constraints of the RF amplifier and transmit chain often preclude straightforward compensation strategies.
This thesis presents the development and investigation of a novel magnetic resonance fingerprinting (MRF)-based B1+ mapping method capable of accurately quantifying B1+ across a broad FA range (6° to 74°). Compared to existing techniques, this more than doubles the usable dynamic range, significantly improving performance in low FA regions, which are critical in body imaging at 7 T. The method was validated through phantom and in vivo studies, demonstrating strong agreement with reference methods and showing better consistency in subjects with higher body mass index.
A detailed investigation of the MRF signal model highlighted key factors affecting accuracy, including RF-pulse properties and signal spoiling efficacy. Hardware investigations revealed nonlinearities and amplifier imperfections as significant sources of error, emphasizing the importance of monitoring the actual RF output, which allowed correction and significant improvement in the resulting B1+ maps.
The method was further extended to a 3D hybrid channel-wise framework using a stack-of-stars trajectory, enabling full liver coverage within 10 min under free-breathing conditions. This hybrid approach, combining two absolute B1+ maps with low FA GRE acquisitions, allowed accurate channel-wise estimation and showed good agreement with the 3D method with one channel active, where the 3D approach was more consistent in low FA areas.
Overall, this work represents an important step toward achieving accurate, reference-standard B1+ mapping at UHF. Its improved sensitivity at low flip angles offers significant benefits for applications that demand precise estimation, potentially advancing the capabilities of UHF body imaging at 7 T and beyond.Ultra-Hochfeld (UHF) MRT (B0 >= 7 Tesla) bietet im Vergleich zu niedrigeren Feldstärken einzigartige Vorteile, etwa ein erhöhtes Signal-zu-Rausch-Verhältnis, steht jedoch vor erheblichen Herausforderungen, insbesondere durch die Inhomogenität des Anregungsmagnetfeldes B1+. Bei der Bildgebung des menschlichen Körpers zeigt sich eine große Variation des B1+-Feldes, und viele Anwendungen sind auf eine präzise Quantifizierung über diesen weiten Bereich angewiesen. Etablierte Methoden weisen jedoch eine eingeschränkte Sensitivität bei niedrigen Flipwinkeln (FAs) auf, und hardwarebedingte Einschränkungen des HF-Verstärkers und der HF Übertragungskette verhindern häufig eine einfache Kompensation.
Diese Arbeit stellt die Entwicklung und Untersuchung einer auf Magnetic Resonance Fingerprinting (MRF) basierenden Methode zur Bestimmung des B1+-Feldes vor, die eine präzise Quantifizierung über einen weiten Bereich von Flipwinkeln (6° bis 74°) ermöglicht. Im Vergleich zu bestehenden Techniken verdoppelt sich der Bereich zuverlässig quantifizierbarer Flipwinkel und ermöglicht eine deutlich höhere Genauigkeit, insbesondere bei niedrigen Flipwinkeln, die für die Körperbildgebung bei 7 T besonders relevant sind. Die Methode wurde durch Phantom- und In-vivo-Studien validiert, mit starker Übereinstimmung zu Referenzmethoden und besserer Konsistenz bei Probanden mit höherem Body-Mass-Index.
Eine detaillierte Untersuchung des MRF-Signalmodells identifizierte zentrale Faktoren, die die Genauigkeit beeinflussen, darunter die Eigenschaften der HF-Pulse und die Effizienz des Signal-Spoilings. Untersuchungen der HF-Hardware offenbarten Nichtlinearitäten als wesentliche Fehlerquellen und unterstrichen die Bedeutung der Überwachung des tatsächlichen HF-Outputs, was eine Korrektur und eine deutliche Verbesserung der resultierenden B1+-Karten ermöglichte.
Die Methode wurde zu einer hybriden 3D-Sequenz mit Stack-of-Stars-Trajektorie weiterentwickelt, wodurch eine vollständige Leberabdeckung innerhalb von 10 min unter freier Atmung erreicht werden kann. Durch Kombination von zwei absoluten B1+-Karten mit GRE-Aufnahmen wurde eine präzise kanalweise Kartierung erreicht, die besonders in niedrigen FA-Bereichen konsistente Ergebnisse lieferte.
Insgesamt stellt diese Arbeit einen wichtigen Schritt dar, um eine präzise, referenzfähige B1+-Kartierung bei UHF zu ermöglichen. Die verbesserte Empfindlichkeit bei niedrigen Flipwinkeln bietet Vorteile für präzise Anwendungen und könnte die UHF-Körperbildgebung bei 7 T und darüber hinaus voranbringen
Uncertainty estimation with prediction-error circuits
No full textNeural circuits continuously integrate noisy sensory stimuli with predictions that often do not perfectly match, requiring the brain to combine these conflicting feedforward and feedback inputs according to their uncertainties. However, how the brain tracks both stimulus and prediction uncertainty remains unclear. Here, we show that a hierarchical prediction-error network can estimate both the sensory and prediction uncertainty with positive and negative prediction-error neurons. Consistent with prior hypotheses, we demonstrate that neural circuits rely more on predictions when sensory inputs are noisy and the environment is stable. By perturbing inhibitory interneurons within the prediction-error circuit, we reveal their role in uncertainty estimation and input weighting. Finally, we link our model to biased perception, showing how stimulus and prediction uncertainty contribute to the contraction bias.TU Berlin, Open-Access-Mittel – 202
Effiziente monolithische Perowskit/Silizium-Tandemsolarzellen durch optimierten Perowskit-Frontschichtstapel, integriert mit industriell skalierbaren Silizium-Bottom-Cell-Technologien
No full textThis thesis examines the development of high-efficiency monolithic perovskite-silicon tandem solar cells by optimizing the integration of perovskite front-layer stacks with scalable silicon bottom cell technology. This study aims to improve power conversion efficiency and long-term stability by tackling critical issues in material deposition, device architecture, and fabrication methods. A crucial aspect of the research involves improving the atomic layer deposition (ALD) of tin oxide (SnOx) to function as an effective buffer layer against sputter-induced damage. Additionally, the use of multifunctional polymeric interlayers, such as polyethyleneimine, is investigated to facilitate a better growth of high-quality tin oxide films, reduce interfacial traps, and enhance overall device efficiency. Extensive collaboration with industrial partners enabled the fabrication of indutry-like silicon bottom cells utilizing TOPCon next to silicon heterojunction (SHJ) technologies to reach industry-relevant scalability. A comparative investigation of several perovskite compositions and interlayer materials identified strategies to diminish non-radiative recombination losses and optimize open-circuit voltage (VOC). The final tandem solar cell configuration attained a record power conversion efficiency of 30% for ALD-free monolithic perovskite/silicon tandem solar cells based on PERx/Topcon silicon bottom cells, illustrating the feasibility for extensive application of this technology.Diese Dissertation untersucht die Entwicklung hocheffizienter monolithischer Perowskit/Silizium Tandemsolarzellen durch die Optimierung der Integration von Perowskit-Frontschichten mit skalierbaren Silizium- Bottom-Cell-Technologien. Ziel der Arbeit ist es, die Effizienz und die Langzeitstabilität der Zellen zu verbessern, indem kritische Baustellen in den Bereichen Materialabscheidung, Bauelementarchitektur und Fertigungsmethoden angegangen werden. Ein wesentlicher Bestandteil der Forschung ist die Optimierung der Atomlagenabscheidung (ALD) von Zinnoxid (SnOx), um eine effektive Pufferfunktion gegen sputterinduzierte Schäden zu gewährleisten. Darüber hinaus wird der Einsatz multifunktionaler Polymerschichten, wie Polyethylenimin, untersucht, um ein besseres Wachstum hochwertiger Zinnoxidfilme zu ermöglichen, die Zahl der Grenzflächendefekte zu reduzieren und die Gesamteffizienz der Zellen zu steigern. Die enge Zusammenarbeit mit industriellen Partnern ermöglichte die Herstellung industrienaher Silizium-Bottom-Cells basierend auf TOPCon-Technologie neben der Silizium- Heteroschichten (SHJ)-Technologien nutzen, um eine skalierbare Produktion zu gewährleisten. Eine vergleichende Untersuchung mehrerer Perowskit-Zusammensetzungen und Zwischenschichtmaterialien identifizierte Strategien zur Verringerung der nichtstrahlenden Rekombinationsverluste und zur Optimierung der Leerlaufspannung (VOC). Die resultierende Tandemsolarzellen-Konfiguration erzielte eine Rekord-Energieumwandlungseffizienz von 30% für monolithische, ALD-freie Perowskit/Silizium- Tandemsolarzellen auf Basis von PERx/TOPCon-Silizium-Bottom-Cells und demonstriert damit das Potenzial für eine breite industrielle Anwendung dieser Technologie
Auto-ignition of dimethyl ether/air mixtures behind reflected shock waves
No full textThe data set contains measurement data of ignition of dimethyl ether/ air mixtures behind reflected shock waves. The data is used for the following publication: Zander, L.; Vinkeloe, J.; Djordjevic, N. Ignition Delay and Chemical–Kinetic Modeling of Undiluted Mixtures in a High-Pressure Shock Tube: Nonideal Effects and Comparative Uncertainty Analysis. International Journal of Chemical Kinetics 2021, 53 (5), 611–637. https://doi.org/10.1002/kin.21469.DFG, 200291049, SFB 1029: TurbIn - Signifikante Wirkungsgradsteigerung durch gezielte, interagierende Verbrennungs- und Strömungsinstationaritäten in Gasturbine
Climate governance experiments: current practices and their meta-governance embedding in Berlin’s solar energy transition
No full textCities are elective sites for experimenting and testing-out new technologies, infrastructures, social practices and governance arrangements in search for policy change. The article explores the relevance of experimental governance practices in climate change policy. The experimental nature of involved practices entails allowing for degrees of flexibility and the inclusion of novel actors, and highlights the searching, adaptive and open-ended nature of climate governance. Based on existing literature in transition and governance studies, the article advances a conceptualization of different models of experimentation – policy, socio-technical, strategic governance and grassroots governance experiments – that is further refined through the sensitizing concepts of uncertainty and learning and through an analysis of factors of interdependency to account for the contextual embedding of experimental actions. Grounded in the empirical context of Berlin’s transition to solar energy provision, the research analyzes three governance initiatives that are representative of the variety and simultaneity of experiments: the governmental approach of Masterplan Solarcity, the civil society cooperative of BürgerEnergie Berlin and the grassroots neighbourhood initiative of Reichenberger Kiez für alle. This contributes to a grounded and nuanced understanding of experimental governance processes and provides insights into the need for embedding diverse governance experiments in an experimental meta-governance ecology.TU Berlin, Open-Access-Mittel – 202
Exploring hand tracking, voice, and passthrough interactions
No full textWith the increasing adoption of Extended Reality (XR) in various fields, understanding how different interaction techniques influence user experience is of high importance. In this paper, the impact of multimodal interaction techniquesy—hand tracking, voice commands, and passthrough technologyy—on User Experience (UX) in XR environments is examined. These modalities are analyzed in relation to user engagement, immersion, and satisfaction in both virtual and passthrough reality settings. A hybrid methodology was employed, combining quantitative and qualitative data collection through user testing, self-assessment tools, and standardized questionnaires such as the Igroup Presence Questionnaire (IPQ), Self-Assessment Manikin (SAM), and the User Experience Questionnaire (UEQ-S). Results indicate that multimodal interactions significantly enhance both hedonic and pragmatic dimensions of UX, with passthrough combined with voice commands emerging as the most preferred configuration. Statistical analysis further reveals that passthrough is generally preferred over non-passthrough, while voice commands improve usability across both conditions. However, passthrough can introduce potential distractions, depending on the task context. These findings contribute to the design of more intuitive and immersive XR applications by providing insights into the opportunities and challenges of multimodal interaction. The results support the development of human-centered extended reality experiences and inform more effective implementations for future XR systems.TU Berlin, Open-Access-Mittel – 2025EC/HE/101092875/EU/DIgital DYnaMic and respOnsible twinS for XR/DIDYMOS-X
Dynamic kinetic C(sp2)–Si cross-coupling with Si–B reagents for the atroposelective construction of unsymmetrical heterobiaryls
No full textA dynamic kinetic C(sp2)–Si cross-coupling of heterobiaryl triflates and silylboronic acid esters as silicon pronucleophiles is reported. The deracemization reaction proceeds typically in high yields and with 99% ee when using Pd(acac)2 as the precatalyst and CuCl as a cocatalyst, and (R)-TolBINAP as the chiral ligand. The copper salt assists the transmetalation of the Si–B reagent, thereby accelerating the reaction and also improving enantioselectivity. The method tolerates a wide range of functional groups and heterocycles, and different silyl groups can be introduced.TU Berlin, Open-Access-Mittel – 202
Theory of nonlinear electron relaxation in thin gold films and their signatures in optical observables
No full textBased on the momentum-resolved Boltzmann equation, we provide self-consistent numerical calculations of the dynamics of conduction electrons in thin noble metal films after linear and nonlinear optical excitations with infrared and terahertz frequencies. Focusing exclusively on electron-phonon interaction, orientational relaxation is introduced and acts as dephasing of the optical excitation on a scale of tens of fs. In the linear regime, our numerical results agree with the field-strength-independent orientational relaxation rate and correspondingly fits of experimental data to a Drude model and predict for nonlinear excitations a field-strength-dependent increase of the orientational relaxation rate. In the THz regime, where the orientational relaxation proceeds faster than the oscillation cycle of the excitation THz field, a new high-order dissipative Kerr-type nonlinearity is predicted. This nonlinearity originates from the Pauli blocking included in the electron-phonon scattering and results in a nonlinearly increasing transmission of the film, detectable in experiments.TU Berlin, Open-Access-Mittel – 2025FG, 432266622, Active Plasmonics with Strong THz Field
Experimental evaluation of molecular communication in different fluids
No full textThe experimental appraisal of existing molecular communication (MC) testbeds and modeling frameworks in real blood is an important step for future Internet of Bio-Nano Things applications. In this article, we experimentally compare the MC flow characteristics of water, blood substitute, and real porcine blood for a previously presented superparamagnetic iron oxide nanoparticles (SPION) MC testbed. We perform an extensive analysis of the channel impulse response (CIR) behavior of the testbed for the different fluids. Based on the identified MC flow characteristics, we extend an existing mathematical framework for our SPION testbed to capture the flow properties of blood. We evaluate its applicability to the collected data in comparison to two existing theoretical CIR models for MC in blood. In our work, we see that the added complexity of the transmission in blood opens up promising new possibilities to improve communication through the human circulatory system.TU Berlin, Open-Access-Mittel – 2025BMFTR, 16KIS1986K, Verbundprojekt: Internet of Bio-Nano-Things - IoBNT -DFG, 419981515, Connecting in-body nano communication with body area networks (NaBoCom III
Eyes, mouth or full face?
No full textNon-manual facial features play a crucial role in sign language communication, yet their importance in automatic sign language recognition (ASLR) remains underexplored. While prior studies have shown that incorporating facial features can improve recognition, related work often relies on hand-crafted feature extraction and fails to go beyond the comparison of manual features versus the combination of manual and facial features. In this work, we systematically investigate the contribution of distinct facial regions—eyes, mouth, and full face—using two different deep learning models (a CNN-based model and a transformer-based model) trained on an SLR dataset of isolated signs with randomly selected classes. Through quantitative performance and qualitative saliency map evaluation, we reveal that the mouth is the most important non-manual facial feature, significantly improving accuracy. Our findings highlight the necessity of incorporating facial features in ASLR.TU Berlin, Open-Access-Mittel – 202