TY - JOUR A1 - Dawood, Peter A1 - Breuer, Felix A1 - Stebani, Jannik A1 - Burd, Paul A1 - Homolya, István A1 - Oberberger, Johannes A1 - Jakob, Peter M. A1 - Blaimer, Martin T1 - Iterative training of robust k‐space interpolation networks for improved image reconstruction with limited scan specific training samples JF - Magnetic Resonance in Medicine N2 - To evaluate an iterative learning approach for enhanced performance of robust artificial‐neural‐networks for k‐space interpolation (RAKI), when only a limited amount of training data (auto‐calibration signals [ACS]) are available for accelerated standard 2D imaging. Methods In a first step, the RAKI model was tailored for the case of limited training data amount. In the iterative learning approach (termed iterative RAKI [iRAKI]), the tailored RAKI model is initially trained using original and augmented ACS obtained from a linear parallel imaging reconstruction. Subsequently, the RAKI convolution filters are refined iteratively using original and augmented ACS extracted from the previous RAKI reconstruction. Evaluation was carried out on 200 retrospectively undersampled in vivo datasets from the fastMRI neuro database with different contrast settings. Results For limited training data (18 and 22 ACS lines for R = 4 and R = 5, respectively), iRAKI outperforms standard RAKI by reducing residual artifacts and yields better noise suppression when compared to standard parallel imaging, underlined by quantitative reconstruction quality metrics. Additionally, iRAKI shows better performance than both GRAPPA and standard RAKI in case of pre‐scan calibration with varying contrast between training‐ and undersampled data. Conclusion RAKI benefits from the iterative learning approach, which preserves the noise suppression feature, but requires less original training data for the accurate reconstruction of standard 2D images thereby improving net acceleration. KW - complex‐valued machine learning KW - data augmentation KW - deep learning KW - GRAPPA KW - parallel imaging KW - RAKI Y1 - 2023 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-312306 VL - 89 IS - 2 SP - 812 EP - 827 ER - TY - JOUR A1 - Kern, Christian S. A1 - Haags, Anja A1 - Egger, Larissa A1 - Yang, Xiaosheng A1 - Kirschner, Hans A1 - Wolff, Susanne A1 - Seyller, Thomas A1 - Gottwald, Alexander A1 - Richter, Mathias A1 - de Giovannini, Umberto A1 - Rubio, Angel A1 - Ramsey, Michael G. A1 - Bocquet, François C. A1 - Soubatch, Serguei A1 - Tautz, F. Stefan A1 - Puschnig, Peter A1 - Moser, Simon T1 - Simple extension of the plane-wave final state in photoemission: bringing understanding to the photon-energy dependence of two-dimensional materials JF - Physical Review Research N2 - Angle-resolved photoemission spectroscopy (ARPES) is a method that measures orbital and band structure contrast through the momentum distribution of photoelectrons. Its simplest interpretation is obtained in the plane-wave approximation, according to which photoelectrons propagate freely to the detector. The photoelectron momentum distribution is then essentially given by the Fourier transform of the real-space orbital. While the plane-wave approximation is remarkably successful in describing the momentum distributions of aromatic compounds, it generally fails to capture kinetic-energy-dependent final-state interference and dichroism effects. Focusing our present study on quasi-freestanding monolayer graphene as the archetypical two-dimensional (2D) material, we observe an exemplary E\(_{kin}\)-dependent modulation of, and a redistribution of spectral weight within, its characteristic horseshoe signature around the \(\bar {K}\) and \(\bar {K´}\) points: both effects indeed cannot be rationalized by the plane-wave final state. Our data are, however, in remarkable agreement with ab initio time-dependent density functional simulations of a freestanding graphene layer and can be explained by a simple extension of the plane-wave final state, permitting the two dipole-allowed partial waves emitted from the C 2p\(_z\) orbitals to scatter in the potential of their immediate surroundings. Exploiting the absolute photon flux calibration of the Metrology Light Source, this scattered-wave approximation allows us to extract E\(_{kin}\)-dependent amplitudes and phases of both partial waves directly from photoemission data. The scattered-wave approximation thus represents a powerful yet intuitive refinement of the plane-wave final state in photoemission of 2D materials and beyond. Y1 - 2023 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-350330 VL - 5 IS - 3 ER - TY - JOUR A1 - Stebani, Jannik A1 - Blaimer, Martin A1 - Zabler, Simon A1 - Neun, Tilmann A1 - Pelt, Daniël M. A1 - Rak, Kristen T1 - Towards fully automated inner ear analysis with deep-learning-based joint segmentation and landmark detection framework JF - Scientific Reports N2 - Automated analysis of the inner ear anatomy in radiological data instead of time-consuming manual assessment is a worthwhile goal that could facilitate preoperative planning and clinical research. We propose a framework encompassing joint semantic segmentation of the inner ear and anatomical landmark detection of helicotrema, oval and round window. A fully automated pipeline with a single, dual-headed volumetric 3D U-Net was implemented, trained and evaluated using manually labeled in-house datasets from cadaveric specimen (N = 43) and clinical practice (N = 9). The model robustness was further evaluated on three independent open-source datasets (N = 23 + 7 + 17 scans) consisting of cadaveric specimen scans. For the in-house datasets, Dice scores of 0.97 and 0.94, intersection-over-union scores of 0.94 and 0.89 and average Hausdorf distances of 0.065 and 0.14 voxel units were achieved. The landmark localization task was performed automatically with an average localization error of 3.3 and 5.2 voxel units. A robust, albeit reduced performance could be attained for the catalogue of three open-source datasets. Results of the ablation studies with 43 mono-parametric variations of the basal architecture and training protocol provided task-optimal parameters for both categories. Ablation studies against single-task variants of the basal architecture showed a clear performance beneft of coupling landmark localization with segmentation and a dataset-dependent performance impact on segmentation ability. KW - anatomy KW - bone imaging KW - diagnosis KW - medical imaging KW - software KW - three-dimensional imaging KW - tomography Y1 - 2023 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-357411 VL - 13 ER - TY - JOUR A1 - Vogel, P. A1 - Rückert, M. A. A1 - Greiner, C. A1 - Günther, J. A1 - Reichl, T. A1 - Kampf, T. A1 - Bley, T. A. A1 - Behr, V. C. A1 - Herz, S. T1 - iMPI: portable human-sized magnetic particle imaging scanner for real-time endovascular interventions JF - Scientific Reports N2 - Minimally invasive endovascular interventions have become an important tool for the treatment of cardiovascular diseases such as ischemic heart disease, peripheral artery disease, and stroke. X-ray fluoroscopy and digital subtraction angiography are used to precisely guide these procedures, but they are associated with radiation exposure for patients and clinical staff. Magnetic Particle Imaging (MPI) is an emerging imaging technology using time-varying magnetic fields combined with magnetic nanoparticle tracers for fast and highly sensitive imaging. In recent years, basic experiments have shown that MPI has great potential for cardiovascular applications. However, commercially available MPI scanners were too large and expensive and had a small field of view (FOV) designed for rodents, which limited further translational research. The first human-sized MPI scanner designed specifically for brain imaging showed promising results but had limitations in gradient strength, acquisition time and portability. Here, we present a portable interventional MPI (iMPI) system dedicated for real-time endovascular interventions free of ionizing radiation. It uses a novel field generator approach with a very large FOV and an application-oriented open design enabling hybrid approaches with conventional X-ray-based angiography. The feasibility of a real-time iMPI-guided percutaneous transluminal angioplasty (PTA) is shown in a realistic dynamic human-sized leg model. KW - biomedical engineering KW - electrical and electronic engineering KW - imaging KW - three-dimensional imaging Y1 - 2023 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-357794 VL - 13 ER - TY - JOUR A1 - Schade, A. A1 - Bader, A. A1 - Huber, T. A1 - Kuhn, S. A1 - Czyszanowski, T. A1 - Pfenning, A. A1 - Rygała, M. A1 - Smołka, T. A1 - Motyka, M. A1 - Sęk, G. A1 - Hartmann, F. A1 - Höfling, S. T1 - Monolithic high contrast grating on GaSb/AlAsSb based epitaxial structures for mid-infrared wavelength applications JF - Optics Express N2 - We demonstrate monolithic high contrast gratings (MHCG) based on GaSb/AlAs0.08Sb0.92 epitaxial structures with sub-wavelength gratings enabling high reflection of unpolarized mid-infrared radiation at the wavelength range from 2.5 to 5 µm. We study the reflectivity wavelength dependence of MHCGs with ridge widths ranging from 220 to 984 nm and fixed 2.6 µm grating period and demonstrate that peak reflectivity of above 0.7 can be shifted from 3.0 to 4.3 µm for ridge widths from 220 to 984 nm, respectively. Maximum reflectivity of up to 0.9 at 4 µm can be achieved. The experiments are in good agreement with numerical simulations, confirming high process flexibility in terms of peak reflectivity and wavelength selection. MHCGs have hitherto been regarded as mirrors enabling high reflection of selected light polarization. With this work, we show that thoughtfully designed MHCG yields high reflectivity for both orthogonal polarizations simultaneously. Our experiment demonstrates that MHCGs are promising candidates to replace conventional mirrors like distributed Bragg reflectors to realize resonator based optical and optoelectronic devices such as resonant cavity enhanced light emitting diodes and resonant cavity enhanced photodetectors in the mid-infrared spectral region, for which epitaxial growth of distributed Bragg reflectors is challenging. KW - monolithic grating KW - high contrast KW - mid-infrared wavelength applications KW - epitaxial structures KW - GaSb/AlAsSb Y1 - 2023 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-350346 VL - 31 IS - 10 ER - TY - JOUR A1 - Grüne, Jeannine A1 - Londi, Giacomo A1 - Gillett, Alexander J. A1 - Stähly, Basil A1 - Lulei, Sebastian A1 - Kotova, Maria A1 - Olivier, Yoann A1 - Dyakonov, Vladimir A1 - Sperlich, Andreas T1 - Triplet Excitons and Associated Efficiency‐Limiting Pathways in Organic Solar Cell Blends Based on (Non‐) Halogenated PBDB‐T and Y‐Series JF - Advanced Functional Materials N2 - The great progress in organic photovoltaics (OPV) over the past few years has been largely achieved by the development of non‐fullerene acceptors (NFAs), with power conversion efficiencies now approaching 20%. To further improve device performance, loss mechanisms must be identified and minimized. Triplet states are known to adversely affect device performance, since they can form energetically trapped excitons on low‐lying states that are responsible for non‐radiative losses or even device degradation. Halogenation of OPV materials has long been employed to tailor energy levels and to enhance open circuit voltage. Yet, the influence on recombination to triplet excitons has been largely unexplored. Using the complementary spin‐sensitive methods of photoluminescence detected magnetic resonance and transient electron paramagnetic resonance corroborated by transient absorption and quantum‐chemical calculations, exciton pathways in OPV blends are unravelled employing the polymer donors PBDB‐T, PM6, and PM7 together with NFAs Y6 and Y7. All blends reveal triplet excitons on the NFA populated via non‐geminate hole back transfer and, in blends with halogenated donors, also by spin‐orbit coupling driven intersystem crossing. Identifying these triplet formation pathways in all tested solar cell absorber films highlights the untapped potential for improved charge generation to further increase plateauing OPV efficiencies. KW - halogenation KW - non‐fullerene acceptors KW - organic photovoltaics KW - spin physics KW - triplet excitons Y1 - 2023 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-312164 VL - 33 IS - 12 ER - TY - JOUR A1 - Gram, Maximilian A1 - Albertova, P. A1 - Schirmer, V. A1 - Blaimer, M. A1 - Gamer, M. A1 - Herrmann, M. J. A1 - Nordbeck, P. A1 - Jakob, P. M. T1 - Towards robust in vivo quantification of oscillating biomagnetic fields using Rotary Excitation based MRI JF - Scientific Reports N2 - Spin-lock based functional magnetic resonance imaging (fMRI) has the potential for direct spatially-resolved detection of neuronal activity and thus may represent an important step for basic research in neuroscience. In this work, the corresponding fundamental effect of Rotary EXcitation (REX) is investigated both in simulations as well as in phantom and in vivo experiments. An empirical law for predicting optimal spin-lock pulse durations for maximum magnetic field sensitivity was found. Experimental conditions were established that allow robust detection of ultra-weak magnetic field oscillations with simultaneous compensation of static field inhomogeneities. Furthermore, this work presents a novel concept for the emulation of brain activity utilizing the built-in MRI gradient system, which allows REX sequences to be validated in vivo under controlled and reproducible conditions. Via transmission of Rotary EXcitation (tREX), we successfully detected magnetic field oscillations in the lower nano-Tesla range in brain tissue. Moreover, tREX paves the way for the quantification of biomagnetic fields. KW - functional magnetic resonance imaging KW - Rotary EXcitation (REX) KW - oscillating biomagnetic fields Y1 - 2022 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-300862 VL - 12 IS - 1 ER - TY - JOUR A1 - Müller, S. A1 - Spriestersbach, F. A1 - Min, C.-H. A1 - Fornari, C. I. A1 - Reinert, F. T1 - Molecular beam epitaxy of TmTe thin films on SrF\(_{2}\) (111) JF - AIP Advances N2 - The odd parity nature of 4f states characterized by strong spin–orbit coupling and electronic correlations has led to a search for novel topological phases among rare earth compounds, such as Kondo systems, heavy Fermions, and homogeneous mixed-valent materials. Our target system is thulium telluride thin films whose bandgap is expected to be tuned as a function of lattice parameter. We systematically investigate the growth conditions of TmxTey thin films on SrF\(_{2}\) (111) substrates by molecular beam epitaxy. The ratio between Te and Tm supply was precisely tuned, resulting in two different crystalline phases, which were confirmed by x-ray diffraction and x-ray photoemission spectroscopy. By investigating the crystalline quality as a function of the substrate temperature, the optimal growth conditions were identified for the desired Tm1Te1 phase. Additional low energy electron diffraction and reflective high energy electron diffraction measurements confirm the epitaxial growth of TmTe layers. X-ray reflectivity measurements demonstrate that homogeneous samples with sharp interfaces can be obtained for varied thicknesses. Our results provide a reliable guidance to prepare homogeneous high-quality TmTe thin films and thus serve as a basis for further electronic investigations. KW - thulium telluride KW - molecular beam epitaxy KW - thin films Y1 - 2022 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-300876 VL - 12 IS - 2 ER - TY - JOUR A1 - Gram, Maximilian A1 - Gensler, Daniel A1 - Albertova, Petra A1 - Gutjahr, Fabian Tobias A1 - Lau, Kolja A1 - Arias-Loza, Paula-Anahi A1 - Jakob, Peter Michael A1 - Nordbeck, Peter T1 - Quantification correction for free-breathing myocardial T1ρ mapping in mice using a recursively derived description of a T\(_{1p}\)\(^{*}\) relaxation pathway JF - Journal of Cardiovascular Magnetic Resonance N2 - Background Fast and accurate T1ρ mapping in myocardium is still a major challenge, particularly in small animal models. The complex sequence design owing to electrocardiogram and respiratory gating leads to quantification errors in in vivo experiments, due to variations of the T\(_{1p}\) relaxation pathway. In this study, we present an improved quantification method for T\(_{1p}\) using a newly derived formalism of a T\(_{1p}\)\(^{*}\) relaxation pathway. Methods The new signal equation was derived by solving a recursion problem for spin-lock prepared fast gradient echo readouts. Based on Bloch simulations, we compared quantification errors using the common monoexponential model and our corrected model. The method was validated in phantom experiments and tested in vivo for myocardial T\(_{1p}\) mapping in mice. Here, the impact of the breath dependent spin recovery time T\(_{rec}\) on the quantification results was examined in detail. Results Simulations indicate that a correction is necessary, since systematically underestimated values are measured under in vivo conditions. In the phantom study, the mean quantification error could be reduced from − 7.4% to − 0.97%. In vivo, a correlation of uncorrected T\(_{1p}\) with the respiratory cycle was observed. Using the newly derived correction method, this correlation was significantly reduced from r = 0.708 (p < 0.001) to r = 0.204 and the standard deviation of left ventricular T\(_{1p}\) values in different animals was reduced by at least 39%. Conclusion The suggested quantification formalism enables fast and precise myocardial T\(_{1p}\) quantification for small animals during free breathing and can improve the comparability of study results. Our new technique offers a reasonable tool for assessing myocardial diseases, since pathologies that cause a change in heart or breathing rates do not lead to systematic misinterpretations. Besides, the derived signal equation can be used for sequence optimization or for subsequent correction of prior study results. KW - T1rho KW - radial KW - cardiac KW - correction KW - quantitative MRI KW - mapping KW - spin-lock KW - T1ρ Y1 - 2022 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-300491 VL - 24 IS - 1 ER - TY - JOUR A1 - Guggenberger, Konstanze Viktoria A1 - Torre, Giulia Dalla A1 - Ludwig, Ute A1 - Vogel, Patrick A1 - Weng, Andreas Max A1 - Vogt, Marius Lothar A1 - Fröhlich, Matthias A1 - Schmalzing, Marc A1 - Raithel, Esther A1 - Forman, Christoph A1 - Urbach, Horst A1 - Meckel, Stephan A1 - Bley, Thorsten Alexander T1 - Vasa vasorum of proximal cerebral arteries after dural crossing - potential imaging confounder in diagnosing intracranial vasculitis in elderly subjects on black-blood MRI JF - European Radiology N2 - Objectives Vessel wall enhancement (VWE) may be commonly seen on MRI images of asymptomatic subjects. This study aimed to characterize the VWE of the proximal internal carotid (ICA) and vertebral arteries (VA) in a non-vasculitic elderly patient cohort. Methods Cranial MRI scans at 3 Tesla were performed in 43 patients (aged ≥ 50 years) with known malignancy for exclusion of cerebral metastases. For vessel wall imaging (VWI), a high-resolution compressed-sensing black-blood 3D T1-weighted fast (turbo) spin echo sequence (T1 CS-SPACE prototype) was applied post gadolinium with an isotropic resolution of 0.55 mm. Bilateral proximal intradural ICA and VA segments were evaluated for presence, morphology, and longitudinal extension of VWE. Results Concentric VWE of the proximal intradural ICA was found in 13 (30%) patients, and of the proximal intradural VA in 39 (91%) patients. Mean longitudinal extension of VWE after dural entry was 13 mm in the VA and 2 mm in the ICA. In 14 of 39 patients (36%) with proximal intradural VWE, morphology of VWE was suggestive of the mere presence of vasa vasorum. In 25 patients (64 %), morphology indicated atherosclerotic lesions in addition to vasa vasorum. Conclusions Vasa vasorum may account for concentric VWE within the proximal 2 mm of the ICA and 13 mm of the VA after dural entry in elderly subjects. Concentric VWE in these locations should not be confused with large artery vasculitis. Distal to these segments, VWE may be more likely related to pathologic conditions such as vasculitis. KW - vertebral artery KW - magnetic resonance imaging KW - vasa vasorum KW - large artery vasculitis KW - Atherosclerosis, intracranial arteries Y1 - 2022 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:bvb:20-opus-266524 SN - 1432-1084 VL - 32 IS - 2 ER -