Atherosclerotic plaques in the bifurcation of the carotid artery vessels can pose a significant stroke risk from stenosis, thrombosis and emboli, or plaque rupture. However, the possibility of the latter depends on the structure of the plaque and its stability. So far, the assessment of such depositions, and the evaluation of the risk they pose, is not satisfactory with 3 Tesla black blood imaging. It is expected that the SNR increase at 7 Tesla, together with an appropriate and patient-safe RF coil, will result in higher resolution images that would help in better assessing the composition of atherosclerotic plaques in vessel walls. A custom-built neck array was designed and constructed, with the aim of investigating the benefits of the higher field strength using DANTE-prepared black blood imaging. A 4-channel transmit array was designed to generate the required <b>B</b>^&plus;<sub style='position: relative; left: -.5em;'>1</sub> field for the DANTE module to be used. A separate close fitting 4-channel receive array was preferred for improved SNR and parallel (receive) imaging. Geometric, active, passive as well as preamp decoupling schemes were employed for adequate isolation between the arrays and their channels. Electromagnetic simulation software, Semcad X (SPEAG, Zurich), was used for safety assessment with human phantoms (Virtual population). The <b>E</b> fields for 1 W transmission per channel were calculated for each element for a worst case SAR estimation. The transmission power limits per channel were set according to the 10g SAR limit set in IEC 60601. For simulation validation, temperature measurements and surface heat mapping were performed on a meat phantom. Finally, a healthy male subject was scanned using a protocol consisting of <b>B</b>₁ mapping, RF shimming at an ROI, and 2D and 3D DANTE prepared Gradient Echo (GRE). The worst-case heating scenario, as defined in the methods section, generated a maximum local SAR of 7.65 W/kg for 1 Watt per channel input. Thus, for 1st level mode (20W/kg max), the power limit was set at 2.6 W per channel. The heating profile was similar to that simulated and the measured temperature increase was within a &plusmn;10&percnt; margin relative to the simulation. The global SAR power limit per channel was found to be higher (i.e. more allowed power) than the worst case local SAR power limit, and thus did not impose additional power penalty. The resolution achieved was 0.6 mm isotropic for the 3D protocol and 0.6 by 0.6 by 2.5 mm for the 2D protocol. The average SNR was measured within the vessel wall location of the two carotid arteries and found to be 27&plusmn;6 for the DANTE images and for the static tissue closer to the skin the SNR was 55&plusmn;2. In conclusion, a 4Tx/4Rx coil was designed to target the carotid arteries operating under pTx mode and a black blood imaging sequence was implemented for blood signal suppression and vessel wall imaging. The initial results from the subject and phantom imaging show satisfactory blood suppression and spatial resolution.

Um die Rolle der Temperatur in biologischen Systemen und bei Krankheiten zu definieren; für die Gewährleistung der Sicherheit, zur Erleichterung der Diagnose und zur sicheren Steuerung der Therapie, wird besseres Verständnis von durch hochfrequente (HF) elektromagnetische Wellen induzierter Erwärmung von Gewebe benötigt. Die Magnetresonanz (MR) ist eine unverzichtbare diagnostische Bildgebung. Eine zentrale Abgrenzung der Ultrahochfeld-MR (UHF-MR) ist der Einsatz höherer Frequenzen und kürzerer Wellenlängen. Die veränderte Ausbreitung von HF-Wellen im menschlichen Körper stellt die MR-Sicherheit vor Herausforderungen. Die kleinere Skala der HF-Erwärmung von elektrisch leitfähigen passiven Implantaten während der UHF-MRT wird in aktuellen Richtlinien nicht erfasst. Ich habe einen, auf die Untersuchung der Implantatsicherheit bei UHF-MRT zugeschnittenen, Ansatz entwickelt, der in Phantomen evaluiert, in Testobjekten validiert und zur Bewertung der MR-Sicherheit von kleinen okularen Tantalmarkern (OTMs) angewandt wurde. OTMs können für die MR bei Magnetfeldstärken ≤ 7,0 T als sicher angesehen werden. Die verkürzte Wellenlänge ermöglicht präzise lokalisierte Temperaturmanipulation wie Hyperthermie-Behandlungen von Krebs. Die Kombination der Stärken der UHF-MRT in diagnostischer Bildgebung und MR-Thermometrie (MRTh) mit spezieller Hardware und Algorithmen zur Hyperthermie-Behandlungsplanung ermöglicht eine einzigartige theranostische Herangehensweise mittels integrierten Geräts (HF-Applikator). Ziel ist die erstmalige Behandlung von Gehirntumoren mittels nicht-invasiver HF-Hyperthermie. Das Ergebnis dieser Arbeit ist die Etablierung des gesamten Arbeitsablaufs von grundlegenden EMF-Simulationen eines HF-Applikators mit patientenspezifischen Simulationsmodellen über die (Multifrequenz-)Behandlungsplanung und Leistungsbewertung bis zur Behandlungsüberwachung mittels MRTh. Für eine Anwendung der Erkenntnisse ist weitere Forschung im HF-Applikator-Design erforderlich. / To better define the role of temperature in biological systems and disease; for ensuring safety, facilitating diagnosis and safe-guiding therapy, we require approaches to study and manipulate temperature and characterize its effects. Better understanding of radiofrequency (RF) induced heating of tissue is therefore required. Magnetic resonance (MR) is an indispensable diagnostic imaging tool. A critical distinction of ultrahigh field MR (UHF-MR) is the use of higher RF and thus shorter wavelengths. The altered RF wave propagation in the human body poses challenges for MR safety considerations. The smaller scale, on which RF heating of electrically conductive passive implants occurs during UHF MRI, is not covered by current guidelines. I proposed a novel approach tailored to examining implant safety in UHF MRI. This approach was evaluated in phantoms, validated in test objects and applied to assess the MR safety of small ocular tantalum markers (OTMs). My results show that OTMs can be considered safe for MRI at magnetic field strengths ≤ 7.0 T. The short wave length regime supports precisely localized temperature manipulation, such as hyperthermia anticancer treatment. Combining the strength of UHF MRI in diagnostic imaging and MR thermometry (MRTh) with dedicated hardware and hyperthermia treatment planning algorithms affords a unique theranostic approach in a single integrated device (RF applicator). The goal is treatment of brain tumors, where to the best of our knowledge no focused RF hyperthermia treatment is currently available. The main accomplishment of this work is establishing the entire workflow from basic EMF simulations of an RF applicator with patient specific simulation models over (multifrequency) treatment planning and performance assessment, to treatment monitoring via MRTh. For translation of the insights obtained during this PhD thesis project, more research is warranted addressing remaining engineering challenges in the RF applicator design.