Open Access

Introduction Intracranial 4D flow MRI enables quantitative assessment of hemodynamics in patients with intracranial atherosclerotic disease (ICAD). However, quantitative assessments are still challenging due to the time-consuming vessel segmentation, especially in the presence of stenoses, which can often result in user variability. To improve the reproducibility and robustness as well as to accelerate data analysis, we developed an accurate, fully automated segmentation for stenosed intracranial vessels using deep learning. Methods 154 dual-VENC 4D flow MRI scans (68 ICAD patients with stenosis, 86 healthy controls) were retrospectively selected. Manual segmentations were used as ground truth for training. For automated segmentation, deep learning was performed using a 3D U-Net. 20 randomly selected cases (10 controls, 10 patients) were separated and solely used for testing. Cross-sectional areas and flow parameters were determined in the Circle of Willis (CoW) and the sinuses. Furthermore, the flow conservation error was calculated. For statistical comparisons, Dice scores (DS), Hausdorff distance (HD), average symmetrical surface distance (ASSD), Bland-Altman analyses, and interclass correlations were computed using the manual segmentations from two independent observers as reference. Finally, three stenosis cases were analyzed in more detail by comparing the 4D flow-based segmentations with segmentations from black blood vessel wall imaging (VWI). Results Training of the network took approximately 10 h and the average automated segmentation time was 2.2 ± 1.0 s. No significant differences in segmentation performance relative to two independent observers were observed. For the controls, mean DS was 0.85 ± 0.03 for the CoW and 0.86 ± 0.06 for the sinuses. Mean HD was 7.2 ± 1.5 mm (CoW) and 6.6 ± 3.7 mm (sinuses). Mean ASSD was 0.15 ± 0.04 mm (CoW) and 0.22 ± 0.17 mm (sinuses). For the patients, the mean DS was 0.85 ± 0.04 (CoW) and 0.82 ± 0.07 (sinuses), the HD was 8.4 ± 3.1 mm (CoW) and 5.7 ± 1.9 mm (sinuses) and the mean ASSD was 0.22 ± 0.10 mm (CoW) and 0.22 ± 0.11 mm (sinuses). Small bias and limits of agreement were observed in both cohorts for the flow parameters. The assessment of the cross-sectional lumen areas in stenosed vessels revealed very good agreement (ICC: 0.93) with the VWI segmentation but a consistent overestimation (bias ± LOA: 28.1 ± 13.9%). Discussion Deep learning was successfully applied for fully automated segmentation of stenosed intracranial vasculatures using 4D flow MRI data. The statistical analysis of segmentation and flow metrics demonstrated very good agreement between the CNN and manual segmentation and good performance in stenosed vessels. To further improve the performance and generalization, more ICAD segmentations as well as other intracranial vascular pathologies will be considered in the future.

Oxygen-containing plasmas are widely used in industry for a variety of applications, including thin-film deposition, etching, and other surface modification processes. Due to their high reactivity, oxygen atoms play a key role in most of these applications, and an accurate method for measuring their density is therefore of great importance, not only to gain a fundamental understanding of the chemistry in such plasmas, but also to improve industrial processes. However, existing techniques, such as two-photon absorption laser induced fluorescence (TALIF), vacuum ultraviolet (VUV) absorption spectroscopy, cavity ring-down spectroscopy (CRDS), and optical emission spectroscopy (OES), are all either bulky and expensive, experimentally challenging, or indirect and relying on a multitude of assumptions. This work presents the first implementation of absorption spectroscopy in the terahertz (THz) spectral region as a novel diagnostic technique for measuring atomic oxygen densities in plasmas. It is based on the detection of the 3P1←3P2 fine structure transition at approximately 4.745 THz, using a newly developed tunable THz quantum cascade laser (QCL) as the radiation source. THz absorption spectroscopy allows for direct measurements (i.e. no calibration is required) of absolute ground-state atomic oxygen densities, and its accuracy depends almost exclusively on the accuracy to which the line strength of the transition is known. In addition, the narrow laser linewidth of just a few MHz makes it possible to determine the translational temperature from the detected absorption profiles as well. Furthermore, the experimental setup is relatively compact (especially compared to TALIF setups that typically involve bulky laser systems), vacuum conditions are not essential (as opposed to when working in the VUV), and the requirements for the optical alignment are not as strict as for CRDS. These features make THz absorption spectroscopy an attractive alternative to existing diagnostic techniques. To confirm the accuracy of THz absorption spectroscopy, the obtained atomic oxygen densities were compared to those yielded by more established techniques. Therefore, TALIF and CRDS measurements of atomic oxygen densities were carried out as well. All measurements were performed in the same capacitively coupled radio frequency (RF) oxygen discharge for a variation of the applied RF power (20 W to 100 W) and the gas pressure (0.7 mbar and 1.3 mbar). The atomic oxygen densities obtained with the three different diagnostic techniques (all of the order of 1*10^14 cm^-3) were found to be in excellent agreement, both qualitatively and quantitatively. This demonstrated that these techniques all allow for accurate measurements and can be used interchangeably as long as no spatial resolution is required. The novel technique of THz absorption spectroscopy is thus well suited for measuring atomic oxygen densities. It has the potential to turn into a standard diagnostic technique once tunable THz QCLs become commercially available and is of particular interest for industrial applications due to the simplicity and compactness of the experimental setup.

Spintronic THz emitters have attracted much attention due to their desirable properties, such as affordability, ultra-wideband capability, high efficiency, and tunable polarization. In this study, we investigate the characteristics of THz signals, including their frequency, bandwidth, and amplitude, emitted from a series of heterostructures with ferromagnetic (FM) and nonmagnetic (NM) materials. The FM layer consists of a wedge-shaped CoFeB layer with a thickness of 0 to 5 nm, while the NM materials include various metals such as Pt, Au, W, Ru, Pt%92Bi%8, and Ag%90Bi%10 alloys. Our experiments show that the emitter with the Pt-NM layer has the highest amplitude of the emitted THz signal. However, the PtBi-based emitter exhibits a higher central THz peak and wider bandwidth, making it a promising candidate for broadband THz emitters. These results pave the way for further exploration of the specific compositions of Pt1−x Bix for THz emitter design, especially with the goal of generating higher frequency and wider bandwidth THz signals. These advances hold significant potential for applications in various fields such as high-resolution imaging, spectroscopy, communications, medical diagnostics, and more.

In this work, an overview of the neutral gas pressures and particle exhaust in the subdivertor of the stellarator Wendelstein 7-X during the last two experimental campaigns is presented for different magnetic field configurations. The particle exhaust, which depends on the neutral gas pressure as well as the available pumping capacity was analyzed regarding the newly installed cryo-vacuum pumping system after characterizing its pumping speed for different gases in the current neutral gas pressure regime of Wendelstein 7-X. The analysis of the neutral gas pressures shows that the pressures currently reached in the subdivertor of Wendelstein 7-X correspond to the molecular flow regime, in which the cryo-vacuum pumps are not operated efficiently. A variety of tools has been developed to simulate the neutral gas pressure and particle exhaust in the current divertor geometry and in the future, modified divertor geometries in order to improve the divertor geometry with respect to its particle exhaust properties. Direct Simulation Monte Carlo calculations as well as a ray-tracing approach used to simulate the neutral gas pressure in the molecular flow regime confirm the neutral gas pressures measured during plasma operation. A multi-chamber model of the subdivertor based on the conductance of the subdivertor space is presented as an analytical method to assess the neutral gas pressures in different parts of the subdivertor region without the need of a complete computer-aided design of the subdivertor geometry. Apart from modifications to the divertor geometry, the extended two-point model for stellarators has been used to explore how varying upstream plasma parameters affect the plasma density at the divertor targets—and consequently, the neutral gas pressure and particle exhaust in the subdivertor. These findings can be utilized to develop plasma scenarios that facilitate particle exhaust.

Shear Alfvén waves (SAWs) can be important in plasma magnetohydrodynam- ics (MHD) stability. In the Wendelstein 7-X (W7-X) stellarator plasmas, Alfvénic fluctuations have been identified in routine plasma scenarios of the opera- tional phase (OP) known as OP1.2b. Magnetic fluctuations between f = 100 − 200 kHz are measured using a system of Mirnov coils. The work presented in the thesis aims to contribute to the physics understanding of SAWs in fu- sion plasmas without externally driven fast ions. The dynamics of a broad frequency range of SAWs in W7-X plasmas are analyzed. The time variations of frequency, amplitude, and width of frequency bands of the magnetic fluc- tuations’ spectra are determined using a new analysis method, the so-called tracking method. The SAW variations are correlated to global plasma param- eters during W7-X plasmas. The new tracking method enabled determining the role of different plasma parameters in the observed variations of the SAW spectral properties. This thesis furthermore discusses potential couplings be- tween SAWs and ITG-driven turbulent modes to explain amplitude variations of SAWs.