Antenna and system design for controlled delivery of microwave thermal ablation

Doctor of PhilosophyDepartment of Electrical and Computer EngineeringPunit PrakashMicrowave ablation is an established minimally invasive modality for thermal ablation of unresectable tumors and other diseases. The goal of a microwave ablation procedure is to deliver microwave power in a manner localized to the targeted tissue, with the objective of raising the target tissue to ablative temperatures (~60 °C). Engineering efforts in microwave applicator design have largely been focused on the design of microwave antennas that yield large, near-spherical ablation zones, and can fit within rigid needles or flexible catheters. These efforts have led to significant progress in the development and clinical application of microwave ablation systems, particularly for treating tumors in the liver and other highly vascular organs. However, currently available applicator designs are ill-suited to treating targets of diverse shapes and sizes. Furthermore, there are a lack of non-imaging-based techniques for monitoring the transient progression of the ablation zone as a means for providing feedback to the physician. This dissertation presents the design, implementation, and experimental evaluation of microwave ablation antennas for site-specific therapeutic applications with these issues in mind. A deployable 915 MHz loop antenna is presented, providing a minimally-invasive approach for thermal ablation of the endometrial lining of the uterus for treatment of heavy menstrual bleeding. The antenna incorporates a radiating loop, which can be deployed to adjustable shapes within the uterine cavity, and a passive element, to enable thermal ablation, to 5.7–9.6 mm depth, of uterine cavities ranging in size from 4–6.5 cm in length and 2.5–4.5 cm in width. Electromagnetic–bioheat transfer simulations were employed for design optimization of the antennas, and proof-of-concept applicators were fabricated and extensively evaluated in ex vivo tissue. Finally, feasibility of using the broadband antenna reflection coefficient for monitoring the ablation progress during the course of ablation was evaluated. Experimental studies demonstrated a shift in antenna resonant frequency of 50 MHz correlated with complete ablation. For treatment of 1–2 cm spherical targets, water-cooled monopole antennas operating at 2.45 and 5.8 GHz were designed and experimentally evaluated in ex vivo tissue. The technical feasibility of using these applicators for treating 1–2 cm diameter benign adrenal adenomas was demonstrated. These studies demonstrated the potential of using minimally-invasive microwave ablation applicators for treatment of hypertension caused by benign aldosterone producing adenomas. Since tissue dielectric properties have been observed to change substantially at elevated temperatures, knowledge of the temperature-dependence of tissue dielectric properties may provide a means for estimating treatment state from changes in antenna reflection coefficient during a procedure. The broadband dielectric properties of bovine liver, an established tissue for experimental characterization of microwave ablation applicators, were measured from room temperature to ablative temperatures. The measured dielectric data were fit to a parametric model using piecewise linear functions, providing a means for readily incorporating these data into computational models. These data represent the first report of changes in broadband dielectric properties of liver tissue at ablative temperatures and should help enable additional studies in ablation system development

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

Design by Numerical Methods of a Patch Antenna Array for Microwave Hyperthermia

L'ipertermia e' una tecnica medicale per il trattamento di tumori. Nella tesi e' stato analizzata una schiera di antenne patch che irradino una regione tumorale con campi elettromagnetici. Per effetto di polarizzazione dielettrica si induce un innalzamento localizzato della temperatura provocando l'apoptosi dei tessuti malati. Modelli numerici sono stati applicati per studiare la focalizzazione dei campi EM e l'analisi termica transitori

Desarrollo de un nuevo sistema de hipertermia de microondas para aplicaciones

[ENG]Microwave technology is now widely used in a variety of medical applications such as cancer treatment and diagnostics. This project describes the structure of a novel hyperthermia system for biomedical research. The software Ansoft HFSS was used to design a rectangular waveguide applicator. A closed-loop is presented in order to control the output power of the system by the temperature measured on the sample. Initial results from experimental testing are presented. In these results, it is shown that the water temperature can be increased from 21ºC to 40ºC in 12 minutes. Therefore, it has been tested that the system works properly. The next step would be to apply the system to melanoma cancer cells. [SPA]Ya existen tecnologías que implican el uso de microondas en una gran variedad de aplicaciones m édicas tales como el diagnóstico y el tratamiento del cáncer. Este proyecto describe la estructura de un nuevo sistema de hipertermia para ser usado en todo tipo de investigaciones biom édicas. El software Ansoft HFSS ha sido usado para diseñar una guía de onda rectangular que ser á el componente final al que se aplicar á nuestro sistema. Además, se dispone de un bucle cerrado en el propio sistema para poder controlar la potencia de salida en función de la temperatura medida en la muestra. Los resultados iniciales del experimento se han presentado en este documento. En estos resultados, se muestra que la temperatura del agua puede ser incrementada desde 21ºC hasta 40ºC en unos 12 minutos. Por lo tanto, se ha comprobado que el sistema funciona de forma adecuada. El siguiente paso ser a aplicar el sistema directamente sobre c elulas cancer genas.Escuela Técnica Superior de Ingeniería de TelecomunicaciónHeriot Watt UniversityUniversidad Politécnica de Cartagen

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

Investigations on the Use of Hyperthermia for Breast Cancer Treatment

Hyperthermia using electromagnetic energy has been proven to be an effective method in the treatment of cancer. Hyperthermia is a therapeutic procedure in which the temperature in the tumor tissue is raised above 42°C without causing any damage to the surrounding healthy tissue. This method has been shown to increase the effectiveness of radiotherapy and chemotherapy. Radio frequencies, microwave frequencies or focused ultrasound can be used to deliver energy to the tumor tissue to attain higher temperatures in the tumor region for hyperthermia application. In this dissertation the use of a near field focused (NFF) microstrip antenna array for the treatment of stage 1 cancer tumors in the breast is proposed. The antenna array was designed to operate at a resonant frequency of 2.45 GHz. A hemispherical two-layer model of breast consisting of fat and glandular tissue layer was considered. The tumor, of the size of a typical stage 1 cancer, was considered at different locations within the breast tissue. The permittivity and conductivity of the breast and tumor tissue were obtained from literature. For a specific location of the tumor, the NFF array is positioned outside the breast in front of the tumor. The radiation from the array is focused onto the tumor and raises the temperature of the tumor. Regardless of the position of the tumor, when placed at the right distance, the array produced a focused spot at the tumor without heating the surrounding healthy tissue. Different placement locations of the antenna array were studied to analyze the depth of the focused radiation region. The antenna array can be placed on a rotating arm allowing it to be moved around the breast based on the tumor location. Results for the power density distribution, specific absorption rate and temperature increase in the tumor and surrounding breast region are presented

Two 27 MHz Simple Inductive Loops, as Hyperthermia Treatment Applicators: Theoretical Analysis and Development

Background. Deep heating is still the main subject for research in hyperthermia treatment. Aim. The purpose of this study was to develop and analyze a simple loop as a heating applicator. Methods. The performance of two 27 MHz inductive loop antennas as potential applicators in hyperthermia treatment was studied theoretically as well as experimentally in phantoms. Two inductive loop antennas with radii 7 cm and 9 cm were designed, simulated, and constructed. The theoretical analysis was performed by using Green’s function and Bessel’s function technique. Experiments were performed with phantoms radiated by the aforementioned loop antennas. Results. The specific absorption rate (SAR) distributions were estimated from the respective local phantom temperature measurements. Comparisons of the theoretical, simulation, and experimental studies showed satisfying agreement. The penetration depth was measured theoretically and experimentally in the range of 2–3.5 cm. Conclusion. The theoretical and experimental analysis showed that current loops are efficient in the case where the peripheral heating of spherical tumor formation located at 2–3.5 cm depth is required

An Investigation of Radiometer and Antenna Properties for Microwave Thermography

Microwave thermography obtains information about the subcutaneous body temperature by a spectral measurement of the intensity of the natural thermally generated radiation emitted by the body tissues. At lower microwave frequencies the thermal radiation can penetrate through biological tissue for significant distances. The microwave thermal radiation from inside the body can be detected and measured non-invasively at the skin surface by the microwave thermography technique, which uses a radiometer to measure the radiation which is received from an antenna on the skin. In the microwave region the radiative power received from a volume of material has a dependence on viewed tissue temperature T(r) of the form, where k is the Boltzmann's constant, B the measurement bandwidth, c(r) is the relative contribution from a volume element dv (the antenna weighting function). The weighting function, c(r), depends on the structure and the dielectric properties of the tissue being viewed, the measurement frequency and the characteristics of the antenna. In any practical radiometer system the body microwave thermal signal has to be measured along with a similar noise signal generated in the radiometer circuits. The work described in this thesis is intended to lead to improvement in the performance of microwave thermography equipment through investigations of antenna weighting functions and radiometer circuit noise sources. All work has been carried out at 3.2 GHz, the central operating frequency of the existing Glasgow developed microwave thermography system. The effects of input circuit losses on the operation of the form of Dicke radiometer used for the Glasgow equipment have been investigated using a computational model and compared with measurements made on test circuits. Very good agreement has been obtained for modelled and measured behaviour. The losses contributed by the microstrip circuit structure, that must be used in the radiometer at 3.2 GHz, have been investigated in detail. Microwave correlation radiometry, by "add and square" method, has been applied to the received signals from a crossed-pair antenna arrangement, the antennas being arranged to view a common region at a certain depth. The antenna response has been investigated using a noise source and by the nonresonant perturbation technique. The received pattern formed by the product of the individual antenna patterns gives a maximum depth in phantom dielectric material. The depth can be adjusted by changing the spacing of the antennas and the phase in an antenna path. However, the pattern is modulated by a set of positive and negative interference fringes so that the complete receive pattern has a complicated form. On uniform temperature distributions the total radiometric signal is zero with the positive and negative contributions cancelling each other out. The fringe modulation can be removed by placing the antennas close enough together, The pattern is then simple and gives a modest maximum response at a known depth in a known material. The radiometer system remains sensitive to the temperature gradients only and the wide range of dielectric properties and tissue structures in the region being investigated usually makes the system response difficult to interpret. For crossed-pair antennas in phase the effective penetration depth in high-and medium-water content tissues is about 2.5 cm at a frequency of 3.2 GHz. The field pattern observed was of the form expected from the measurements of the individual antenna behaviour with the appropriate interference pattern superimposed. The nonresonant perturbation technique has been developed and applied to assist the development of the medical application of both microwave thermographic temperature measurement and microwave hyperthermia induction. These techniques require the electromagnetic field patterns of the special antennas used to be known. These antennas are often formed by short lengths of rectangular or cylindrical waveguide loaded with a low-loss dielectric material to achieve good coupling to body tissues. The high microwave attenuation in biological materials requires the field configurations to be measured close to the antenna aperture in the near-field wave. The nonresonant perturbation is a simple technique which can be used to measure electromagnetic fields in lossy material close to the antenna. It has been applied here to measure accurately the antenna weighting function and the effective penetration depth in tissue simulating dielectric phantom materials. (Abstract shortened by ProQuest.)