Monitoring thermal ablation via microwave tomography. An ex vivo experimental assessment

Thermal ablation treatments are gaining a lot of attention in the clinics thanks to their reduced invasiveness and their capability of treating non-surgical patients. The effectiveness of these treatments and their impact in the hospital's routine would significantly increase if paired with a monitoring technique able to control the evolution of the treated area in real-time. This is particularly relevant in microwave thermal ablation, wherein the capability of treating larger tumors in a shorter time needs proper monitoring. Current diagnostic imaging techniques do not provide effective solutions to this issue for a number of reasons, including economical sustainability and safety. Hence, the development of alternative modalities is of interest. Microwave tomography, which aims at imaging the electromagnetic properties of a target under test, has been recently proposed for this scope, given the significant temperature-dependent changes of the dielectric properties of human tissues induced by thermal ablation. In this paper, the outcomes of the first ex vivo experimental study, performed to assess the expected potentialities of microwave tomography, are presented. The paper describes the validation study dealing with the imaging of the changes occurring in thermal ablation treatments. The experimental test was carried out on two ex vivo bovine liver samples and the reported results show the capability of microwave tomography of imaging the transition between ablated and untreated tissue. Moreover, the discussion section provides some guidelines to follow in order to improve the achievable performances

Radiofrequency applicator concepts for thermal magnetic resonance of brain tumors at 297 MHz (7.0 Tesla)

PURPOSE: Thermal intervention is a potent sensitizer of cells to chemo- and radiotherapy in cancer treatment. Glioblastoma multiforme (GBM) is a potential clinical target, given the cancer's aggressive nature and resistance to current treatment options. The annular phased array (APA) technique employing electromagnetic waves in the radiofrequency (RF) range allows for localized temperature increase in deep seated target volumes (TVs). Reports on clinical applications of the APA technique in the brain are still missing. Ultrahigh field magnetic resonance (MR) employs higher frequencies than conventional MR and has potential to provide focal temperature manipulation, high resolution imaging and noninvasive temperature monitoring using an integrated RF applicator (ThermalMR). This work examines the applicability of RF applicator concepts for ThermalMR of brain tumors at 297 MHz (7.0 Tesla). METHODS: Electromagnetic field (EMF) simulations are performed for clinically realistic data based on GBM patients. Two algorithms are used for specific RF energy absorption rate based thermal intervention planning for small and large TVs in the brain, aiming at maximum RF power deposition or RF power uniformity in the TV for 10 RF applicator designs. RESULTS: For both TVs , the power optimization outperformed the uniformity optimization. The best results for the small TV are obtained for the 16 element interleaved RF applicator using an elliptical antenna arrangement with water bolus. The two row elliptical RF applicator yielded the best result for the large TV. DISCUSSION: This work investigates the capacity of ThermalMR to achieve targeted thermal interventions in model systems resembling human brain tissue and brain tumors

Wideband Self‐Grounded Bow‐Tie Antenna for Thermal MR

The objective of this study was the design, implementation, evaluation and application of a compact wideband self‐grounded bow‐tie (SGBT) radiofrequency (RF) antenna building block that supports anatomical proton (1H) MRI, fluorine (19F) MRI, MR thermometry and broadband thermal intervention integrated in a whole‐body 7.0 T system. Design considerations and optimizations were conducted with numerical electromagnetic field (EMF) simulations to facilitate a broadband thermal intervention frequency of the RF antenna building block. RF transmission (B1+) field efficiency and specific absorption rate (SAR) were obtained in a phantom, and the thigh of human voxel models (Ella, Duke) for 1H and 19F MRI at 7.0 T. B1+ efficiency simulations were validated with actual flip‐angle imaging measurements. The feasibility of thermal intervention was examined by temperature simulations (f = 300, 400 and 500 MHz) in a phantom. The RF heating intervention (Pin = 100 W, t = 120 seconds) was validated experimentally using the proton resonance shift method and fiberoptic probes for temperature monitoring. The applicability of the SGBT RF antenna building block for in vivo 1H and 19F MRI was demonstrated for the thigh and forearm of a healthy volunteer. The SGBT RF antenna building block facilitated 19F and 1H MRI at 7.0 T as well as broadband thermal intervention (234‐561 MHz). For the thigh of the human voxel models, a B1+ efficiency ≥11.8 μT/√kW was achieved at a depth of 50 mm. Temperature simulations and heating experiments in a phantom demonstrated a temperature increase ΔT >7 K at a depth of 10 mm. The compact SGBT antenna building block provides technology for the design of integrated high‐density RF applicators and for the study of the role of temperature in (patho‐) physiological processes by adding a thermal intervention dimension to an MRI device (Thermal MR).BMBF, 13GW0102A, KMU-Innovativ - Verbundprojekt: Forschung für Tumortherapie mit lokalisierter Hochfrequenz-Hyperthermie: Diagnostik, Therapiesteuerung und -kontrolle mit ultrahochfeld MRT (3-IN-1:THERAHEAT) - Teilvorhaben: Erforschung von Hochfrequenzantennen für Tumortherapie mittels kontrollierter Hochfrequenz-HyperthermieEC/H2020/EU/743077/Thermal Magnetic Resonance: A New Instrument to Define the Role of Temperature in Biological Systems and Disease for Diagnosis and Therapy/ThermalM

Antenna Design, Radiobiological Modelling, and Non-invasive Monitoring for Microwave Hyperthermia

The death toll of cancers is on the rise worldwide and surviving patients suffer significant side effects from conventional therapies. To reduce the level of toxicity in patients treated with the conventional treatment modalities, hyperthermia (HT) has been investigated as an adjuvant modality and shown to be a potent tumor cell sensitizer for radio- and chemotherapy. During the past couple of decades, several clinical radiofrequency HT systems, aka applicators, have been developed to heat tumors. Systems based on radiative applicators are the most widely used within the hyperthermic community. They consist of a conformal antenna array and need a beamforming method in order to focus EM energy on the tumor through constructive interference while sparing the healthy tissue from excessive heating. Therefore, a hyperthermia treatment planning (HTP) stage is required before each patient\u27s first treatment session to optimize and control the EM power deposition as well as the resultant temperature distribution. Despite the vast amount of effort invested in HTP and the progress made in this regard during recent years, the clinical exploitation of HT is still hampered by technical limitations and patients can still experience discomfort during clinical trials. This, therefore, calls for a more efficient hardware design, better control of EM power deposition to minimize unwanted hotspots, and more accurate quantification and monitoring of the treatment outcome. Given these demands, the present report tries to address some of the above-mentioned challenges by proposing - A new antenna model customized for HT applications that surpasses previously proposed models from several points of view.- A hybrid beamforming method for faster convergence and a versatile, robust thermal solver for handling sophisticated scenarios.- A radiobiological model to quantify the outcome of a combined treatment modality of the Gamma Knife radiosurgery and HT.- A differential image reconstruction method to assess the feasibility of using the same system for both heating and microwave thermometry

Systematic review of pre-clinical and clinical devices for magnetic resonance-guided radiofrequency hyperthermia

Clinical trials have demonstrated the therapeutic benefits of adding radiofrequency (RF) hyperthermia (HT) as an adjuvant to radio- and chemotherapy. However, maximum utilization of these benefits is hampered by the current inability to maintain the temperature within the desired range. RF HT treatment quality is usually monitored by invasive temperature sensors, which provide limited data sampling and are prone to infection risks. Magnetic resonance (MR) temperature imaging has been developed to overcome these hurdles by allowing noninvasive 3D temperature monitoring in the target and normal tissues. To exploit this feature, several approaches for inserting the RF heating devices into the MR scanner have been proposed over the years. In this review, we summarize the status quo in MR-guided RF HT devices and analyze trends in these hybrid hardware configurations. In addition, we discuss the various approaches, extract best practices and identify gaps regarding the experimental validation procedures for MR - RF HT, aimed at converging to a common standard in this process

Modeling the SIGMA-Eye Applicator for Hyperthermia via Multiple Infinitesimal Dipoles

Over the past several decades, cancer is still one of the leading causes of human deaths. Hyperthermia treatment, which is mostly performed in clinic as an assistant therapy method combined with chemotherapy or radiation therapy, has been also playing a more and more important role in tumor therapy. Driven by the developments of computing power and computational techniques, personalized hyperthermia treatment planning (HTP) is becoming possible and essential for clinical practice, aimed at achieving maximum treatment effects for tumor targets and minimal side effects for the surrounding tissues simultaneously. As an essential step of Electromagnetic Hyperthermia Treatment Planning, electromagnetic simulation with the phased-array applicator, SIGMA-Eye hyperthermia applicator, was explored. The approach of the basic-building-block-based modified Infinitesimal Dipole Model (IDM) as a virtual source model was developed and used for modeling the hyperthermia SIGMA-Eye applicator (BSD Medical Corporation) in this work. The basic idea of the IDM [1] is to replace the antenna with a series of infinitesimal dipoles which generates the same electric field as the antenna does. On the basis of the conventional IDM, a modified IDM is proposed, in which number and locations of dipoles are predefined. The reduced set of dipole parameters leads to a simpler objective function of the modified IDM in comparison to the conventional IDM concerning parameter fitting. In addition, the concept of a ‘basic building block’ [2] is introduced: the antenna under test (active antenna) and its neighboring antenna elements (passive antennas) are considered as a basic building block. The dipole model of the antenna under test will be fit by approximating the electric field of the block in order to correctly treat the mutual coupling between antenna elements. Therefore, electric fields generated by a phased-array applicator (with significant mutual coupling between elements) can be modeled. In this work, each antenna of the SIGMA-Eye applicator was modelled using the basic-building-block-based modified IDM. Taking the electric field data of the basic building block computed from the software COMSOL Multiphysics as reference, the global optimization algorithm OQNLP (OptQuest Nonlinear Programming) [3] was used for parameter fitting of the dipole models. And then the SIGMA-Eye applicator was simulated by the superposition of each simulated antenna. Electromagnetic simulations with different phase combinations of the antenna elements of the applicator were performed. The resulted electromagnetic energy deposition patterns were compared to the measurement data presented in the reference paper [4], where the electric field measurement within the phantom placed inside the SIGMA-Eye applicator was performed. The relative differences of energy deposition patterns ranged from 1.40% to 17.90% with an average at 5.07%. The agreement of energy deposition patterns between simulation data and measurement data justified the applicability of our virtual source model to hyperthermia forward planning and further to the commissioning of new systems. In addition, the frequency dependence and water-bolus permittivity and conductivity dependence of the block-based modified IDM was explored, and it was found that this approach is applicable for a narrow-band frequency, and is adaptable to the uncertainty of the water-bolus permittivity and conductivity. When operating at a frequency further away from the reference frequency, or the surrounding environment of the antennas changes a lot, the applicator needs to be simulated using a new equivalent model. [1] Mikki, S.M. and A.A. Kishk, Theory and applications of infinitesimal dipole models for computational electromagnetics. Ieee Transactions on Antennas and Propagation, 2007. 55(5): p. 1325-1337. [2] Mikki, S.M. and Y.M.M. Antar, Near-Field Analysis of Electromagnetic Interactions in Antenna Arrays Through Equivalent Dipole Models. Ieee Transactions on Antennas and Propagation, 2012. 60(3): p. 1381-1389. [3] Ugray, Z., Lasdon, L., Plummer, J., Glover, F., Kelly, J. and Martí, R, Scatter Search and Local NLP Solvers: A Multistart Framework for Global Optimization. INFORMS Journal on Computing, 2007. 19(3): p. 328-340. [4] F.Turner, P., Technical Aspect of the BSD-2000 and BSD-2000∙3D, in European Society for Hyperthermia Oncology and BSD Medical Corporation User’s Conference. 1997

Antenna Development for Radio Frequency Hyperthermia Applications

This thesis deals with the design steps, development and validation of an applicator for radio frequency hyperthermia cancer therapy. An applicator design to enhance targeted energy coupling is a key enabler for preferential temperature increments in tumour regions. A single-element, near-field approach requires a miniaturised solution, that addresses ergonomic needs and is tolerant to patient anatomy. The antenna war-field rriodality and the high-dielectric patient loading introduce significant analytical and computational resource challenges. The antenna input impedance has to be sufficiently insensitive to in-band resonant cletuning and the fields in the tissue can he targeted to selected areas in the patient. An introduction to the medical and biological background of hyperthermia is presented. The design requirements of antennas for medical and in particular for hyperthermia applications are highlighted. Starting from a conventional circular patch, the antenna evolved into a compact circular patch with a concentric annular ring and slotted groundplane, operating at the 434 MHz Industrial Scientific and Medical frequency band. Feed point location is optimized for an energy deposition pattern aligned with the antenna centre. The applicator is assessed with other published approaches and clinically used loop, dipole and square patch antennas. The antennas are evaluated for the unloaded condition and when loaded with a tri-layer body tissue numerical model. This model comprises skin, fat and transverse fiber of muscle of variable thicknesses to account for different body locations and patient. anatomy. A waterbolus containing de-ionized water is added at the skin interface for superficial tissue cooling aud antelina matching. The proposed applicator achieves a penetration depth that supersedes other approaches while remaining compact and an ergonomic fit to tumour areas on the body. To consider the inner and peripheral complex shapes of human bodies, the full human body numerical model developed by Remcom is used. This model was segmented from 1 mm step computed tomography (CT) and magnetic resonance imaging (MRI) cross-sections through and adult male and it comprises twenty-three tissue types with thermal and frequency-dependent dielectric properties. The applicator performance is evaluated at three anatomical body areas of the model to assess its suitability for treatment of tumours at different locations. These three anatomical regions present different aperture coupling and tissue composition. \u27Different conformal waterbolus and air gap thickness values are evaluated. The models used in this work are validated with measurements performed in a phantom containing a lossy liquid with dielectric properties representative of homogeneous human body tissue. The dosimetric assessment system (DASY) is used to evaluaxe the specific absorption rate (SAR) generated for the antenna into the liquid. The measurement setup with the antenna, phantom and liquid are simulated. Simulated and measured results in terrms of specific absorption rate and return loss are evaluated

Entwicklung von RF-Technologie für die Ultrahochfeld-MRT: Optimierung und Anwendung einer Self-Grounded-Bow-Tie-Dipolantenne

Magnetic resonance imaging (MRI) is an important diagnostic imaging modality free of ionizing radiation. Sensitivity gain, signal-to-noise ratio (SNR) considerations, and changes in the tissue dependent MRI properties. Together with technical and scientific developments further research into increasing the magnetic field strength is justified, culminating in human applications at ultrahigh magnetic field (UHF, B0 ≥ 7.0 T) MRI. Elevating the field strength results in an increased radiofrequency (RF) for signal transmission and reception in MRI (= Larmor frequency, f ≈ 298 MHz at B0 = 7.0 T). The wavelength of this RF signal becomes sufficiently short when passing through tissue relative to the size of the target anatomy of the brain, upper torso, or abdomen. This phenomenon leads to constructive and deconstructive interference of the electromagnetic field (EMF) distribution, which results in a high susceptibility for non-uniformities in the magnetic RF transmission field (B1+). This detrimental excitation field distribution can cause shading, massive signal drop-off or even signal voids, and potentially offset the benefits of UHF-MRI due to compromised image quality. UHF cardiovascular MR (CMR) benefits from SNR gains and changes in the tissue dependent MRI properties, but the B1+ distribution – in addition to the wavelength dependent non-uniformities – is further compromised by a dielectrically heterogeneous tissue environment. Research on UHF-CMR focuses on the improvement of the cardiac chamber morphology quantification, myocardial T1- and T2*-mapping, fat-water imaging, and vascular imaging (4D-flow). These applications benefit from a homogenous B1+ within the heart and the vascular structure. Several published reports on the development of RF antenna array technology tailored for UHF-CMR address this challenge with ideas and achievements to enable broad clinical UHF-CMR applications in the future. The primary objective of advancing this RF technology is to achieve a uniform B1+ distribution in the heart and the vascular structure with optimizing the magnetic field pattern. The second objective is the improvement of the RF antenna’s efficiency with the reduction of the specific absorption rate (SAR), which is achieved by an optimization of the electric field pattern. The control of the electric field is furthermore conceptually appealing beyond conventional MR imaging modalities and useful for localized and targeted RF induced thermal intervention. Combining MRI with a thermal intervention modality in an integrated Thermal MR system permits direct supervision of the treatment via MR-thermometry, as well as adapting and improving the focal point quality of the RF power deposition. The Thermal MR system is a platform for comprehensive investigation of the effects of temperature on molecular, biochemical, and physiological processes, ultimately yielding insights into temperature utilization for diagnosis and therapy in vivo. EMF control of an RF antenna array depends on the radiation pattern of the antenna elements. Electrical dipoles are promising for UHF-MRI due to a linear polarized current pattern and an energy deposition perpendicular to the antenna. However, the channel count and therefore the degree of freedom for EMF shaping of previously reported antenna concepts is limited by the geometric extent and the coupling between the elements. The first section of this work addresses the design, implementation, and validation of a novel small-sized Self-Grounded Bow-Tie (SGBT) antenna, in combination with a dielectrically filled housing. The narrowband SGBT antenna variant is used in a 32-channel transmit/receive array configuration for UHF-CMR at 7.0 T. The second section focuses on the development of a modified broadband SGBT concept for the Thermal MR system. The broadband antenna increases the degree of freedom with an adaptation of the intervention frequency to improve the focal point quality (size, homogeneity, and specificity). The third section presents the implementation and validation of a signal generator in conjunction with the broadband SGBT variant introduced in section two. The device allows the generation of the intervention signal with a time dependent, channel-wise adaptation of amplitude, phase, and frequency. The work of this thesis offers a technical and conceptual framework for an increased degree of freedom for EMF shaping for a multitude of applications ranging from UHF-MRI to interventional MRI.Die Magnetresonanztomographie (MRI) ist ein wichtiges bildgebendes Diagnoseverfahren mit der Anwendung in vielen medizinischen Disziplinen. Die Forschung zu ultrahohen Magnetfeldern (UHF, B0 ≥ 7.0 T) im humanen Bereich wird durch technische und wissenschaftliche Errungenschaften getrieben und basiert auf einer höheren Sensitivität, einem verbesserten Signal-Rausch-Verhältnisses (SNR) sowie eine Veränderung der gewebsspezifischen MR Eigenschaften. Die höhere Feldstärke resultiert auch in einer erhöhten Radiofrequenz (RF) für die MRI Signalübertragung (= Larmorfrequenz, f ≈ 298 MHz bei B0 = 7.0 T). Die Wellenlänge des RF Signals im Gewebe ist dabei bezogen zur Zielanatomie (e.g. Schädel, Oberkörper und Abdomen) verkürzt was zu konstruktiven und destruktiven Interferenzen des elektromagnetischen Feldes (EMF) führt. Diese Interferenzen ergeben ein heterogenes RF Transmissionsfeld (B1+) mit Abschattungen, massiven Signalabfällen oder Signalausfällen welche die Vorteile der UHF-MRI durch eine beeinträchtigte Bildqualität schmälert. Die UHF Herz MR (CMR) profitiert von einem SNR-Gewinn sowie von veränderten gewebsspezifischen MR Eigenschaften bei höheren Feldstärken. Jedoch wird die B1+ Verteilung, neben der gegebenen RF wellenlängenabhängigen Heterogenität, durch dielektrische Gradienten im Bereich des Thorax zusätzlich beeinträchtigt. Die anwendungsbezogene Forschung und Entwicklung auf dem Gebiet der UHF-CMR konzentriert sich auf die Verbesserung der Quantifizierung der Herzkammermorphologie, des myokardialen T1- und T2*-Mappings, der Fett-Wasser-Bildgebung und der Gefäßbildgebung inklusive der Flussbildgebung (4D-Flow). Die Weiterentwicklung dieser Methoden streben eine breite klinische Anwendung an und profitieren von einer homogenen B1+ Verteilung im Herzen und in der Gefäßstruktur. Das primäre Ziel der der Forschung und Entwicklung von RF Antennenarraytechnologie ist eine Optimierung der B1+ Verteilung. Das sekundäre Ziel ist die Verbesserung der Effizienz durch die Verringerung der spezifischen Absorptionsrate (SAR) mittels einer elektrischen Feldoptimierung. Die Kontrolle des elektrischen Feldes kann aber auch über die konventionelle MR Bildgebung hinaus genutzt werden und ermöglicht konzeptionell eine lokalisierte und gezielte RF induzierte thermische Intervention. Die Kombination von MRI und thermischen Interventionen in einem integrierten Thermal MR System ermöglicht die Anpassung und Verbesserung der lokalen Intervention durch eine Supervision der Behandlung mittels MR-Thermometrie. Das Thermal MR System stellt damit eine technologische Plattform dar, welche eine umfassende Untersuchung der Auswirkungen der Temperatur auf molekulare, biochemische und physiologische Prozesse erlaubt. Letztlich kann die Plattform Erkenntnisse darüber liefern, wie die Temperatur für Diagnosen und Therapien in vivo genutzt werden kann. Die Kontrolle der EMF Verteilung durch ein RF Antennen Array ist abhängig von den Abstrahlungseigenschaften der einzelnen Antennenelemente. Elektrische Dipole stellen durch eine linear polarisierte Stromverteilung und eine Abstrahlungsrichtung orthogonal zur Antenne eine vielversprechende Option dar. Allerdings ist die Kanalzahl und damit der Freiheitsgrad für die EMF Optimierung bei bisher vorgestellten Antennenkonzepten durch die Größe und die Kopplung zwischen den Elementen begrenzt. Der erste Abschnitt dieser Arbeit befasst sich mit dem Entwurf, der Implementierung und der Validierung einer Self-Grounded Bow-Tie (SGBT) Antenne in Kombination mit einem dielektrisch gefüllten Gehäuse. Eine schmalbandige Antennenvariante wird in einer 32-Kanal Sende-/Empfangs-Array Konfiguration für UHF-CMR bei 7,0 T vorgestellt. Der zweite Abschnitt befasst sich mit der Entwicklung eines modifizierten breitbandigen SGBT-Konzepts für das Thermal MR System. Diese Antennenvariante erhöht die Freiheitsgrade für die Optimierung der elektrischen Feldverteilung um die Interventionsfrequenz und erlaubt eine Verbesserung der lokalen Erwärmung (Größe, Homogenität und Spezifität). Im dritten Abschnitt dieser Arbeit wird die Implementierung und Validierung eines Signalgenerators in Verbindung mit der im zweiten Abschnitt vorgestellten Breitbandantennenvariante vorgestellt. Der Signalgenerator erzeugt einen Interventionssignal mit der zeitabhängigen Anpassung von Amplitude, Phase und Frequenz für jeden Kanal. Die Entwicklungen und Erkenntnisse dieser Arbeit bieten einen konzeptionellen Rahmen für eine Vielzahl von realen Anwendungen, welche von der konventionellen MRI bis zu einem integrierten interventionellen Thermal MR System reichen.EC/H2020/743077/EU/Thermal Magnetic Resonance: A New Instrument to Define the Role of Temperature in Biological Systems and Disease for Diagnosis and Therapy/ThermalM

Image-guidance and computational modeling to develop and characterize microwave thermal therapy platforms

Doctor of PhilosophyDepartment of Electrical and Computer EngineeringPunit PrakashThis dissertation focuses on the development of magnetic resonance imaging (MRI)-guided microwave thermal therapy systems for driving experimental studies in small animals, and to experimentally validate computational models of microwave ablation, which are widely employed for device design and characterization. MRI affords noninvasive monitoring of spatial temperature profiles, thereby providing a means to to quantitatively monitor and verify delivery of prescribed thermal doses in experimental studies and clinical use, as well as a means to validate thermal profiles predicted by computational models of thermal therapy. A contribution of this dissertation is the development and demonstration of a system for delivering mild hyperthermia to small animal targets, thereby providing a platform for driving basic research studies investigating the use of heating as part of cancer treatment strategies. An experimentally validated 3D computational model was employed to design and characterize a non-invasive directional water-cooled microwave hyperthermia applicator for MRI guided delivery of hypethermia in small animals. Following a parametric model-based design approach, a reflector aperture angle of 120°, S-shaped monopole antenna with 0.6 mm displacement, and a coolant flow rate of 150 ml/min were selected as applicator parameters that enable conformal delivery of mild hyperthermia to tumors in experimental animals. The system was integrated with real-time high-field 14.1 T MRI thermometry and feedback control to monitor and maintain target temperature elevations in the range of 4 – 5 °C (hypethermic range). 2 - 4 mm diameter targets positioned 1 – 3 mm from the applicator surface were heated to hyperthermic temperatures, with target coverage ratio ranging between 76 - 93 % and 11 – 26 % of non-targeted tissue heated. Another contribution of this dissertation is using computational models to determine how the fibroids altered ablation profile of a microwave applicator for global endometrial ablation. Uterine fibroids are benign pelvic tumors located within the myometrium or endometrium,and may alter the profile of microwave ablation applicators deployed within the uterus for delivering endometrial ablation. A 3D computational model was employed to investigate the effect of 1 – 3 cm diameter uterine fibroids in different locations around the uterine cavity on endometrial ablation profiles of microwave exposure with a 915 MHz microwave triangular loop antenna. The maximum change in simulated ablation depths due to the presence of fibroids was 1.1 mm. In summary, this simulation study suggests that 1 – 3 cm diameter uterine fibroids can be expected to have minimal impact on the extent of microwave endometrial ablation patterns achieved with the applicator studied in this dissertation. Another contribution of this dissertation is the development of a method for experimental validation of 3D transient temperature profiles predicted by computational models of MWA. An experimental platform was developed integrating custom designed MR-conditional MWA applicators for use within the MR environment. This developed platform was employed to conduct 30 - 50 W, 5 - 10 min MWA experiments in ex vivo tissue. Microwave ablation computational models, mimicking the experimental setting in MRI, were implemented using the finite element method, and incorporated temperature-dependent changes in tissue physical properties. MRI-derived Arrhenius thermal damage maps were compared to Model-predicted ablation zone extents using the Dice similarity coefficient (DSC). Mean absolute error between MR temperature measurements and fiber-optic temperature probes, used to validate the accuracy of MR temperature measurements, during heating was in the range of 0.5 – 2.8 °C. The mean DSC between model-predicted ablation zones and MRI-derived Arrhenius thermal damage maps for 13 experimental set-ups was 0.95. When comparing simulated and experimentally (i.e. using MRI) measured temperatures, the mean absolute error (MAE %) relative to maximum temperature change was in the range 5 % - 8.5 %

Sodium MRI of the human heart at 7.0 T: preliminary results

The objective of this work was to examine the feasibility of three-dimensional (3D) and whole heart coverage 23Na cardiac MRI at 7.0 T including single-cardiac-phase and cinematic (cine) regimes. A four-channel transceiver RF coil array tailored for 23Na MRI of the heart at 7.0 T (f = 78.5 MHz) is proposed. An integrated bow-tie antenna building block is used for 1H MR to support shimming, localization and planning in a clinical workflow. Signal absorption rate simulations and assessment of RF power deposition were performed to meet the RF safety requirements. 23Na cardiac MR was conducted in an in vivo feasibility study. 3D gradient echo (GRE) imaging in conjunction with Cartesian phase encoding (total acquisition time TAQ = 6 min 16 s) and whole heart coverage imaging employing a density-adapted 3D radial acquisition technique (TAQ = 18 min 20 s) were used. For 3D GRE-based 23Na MRI, acquisition of standard views of the heart using a nominal in-plane resolution of (5.0 x 5.0) mm2 and a slice thickness of 15 mm were feasible. For whole heart coverage 3D density-adapted radial 23Na acquisitions a nominal isotropic spatial resolution of 6 mm was accomplished. This improvement versus 3D conventional GRE acquisitions reduced partial volume effects along the slice direction and enabled retrospective image reconstruction of standard or arbitrary views of the heart. Sodium cine imaging capabilities were achieved with the proposed RF coil configuration in conjunction with 3D radial acquisitions and cardiac gating. Cardiac-gated reconstruction provided an enhancement in blood-myocardium contrast of 20% versus the same data reconstructed without cardiac gating. The proposed transceiver array enables 23Na MR of the human heart at 7.0 T within clinical acceptable scan times. This capability is in positive alignment with the needs of explorations that are designed to examine the potential of 23Na MRI for the assessment of cardiovascular and metabolic diseases