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

Overview of bladder heating technology: matching capabilities with clinical requirements.

Moderate temperature hyperthermia (40-45°C for 1 h) is emerging as an effective treatment to enhance best available chemotherapy strategies for bladder cancer. A rapidly increasing number of clinical trials have investigated the feasibility and efficacy of treating bladder cancer with combined intravesical chemotherapy and moderate temperature hyperthermia. To date, most studies have concerned treatment of non-muscle-invasive bladder cancer (NMIBC) limited to the interior wall of the bladder. Following the promising results of initial clinical trials, investigators are now considering protocols for treatment of muscle-invasive bladder cancer (MIBC). This paper provides a brief overview of the devices and techniques used for heating bladder cancer. Systems are described for thermal conduction heating of the bladder wall via circulation of hot fluid, intravesical microwave antenna heating, capacitively coupled radio-frequency current heating, and radiofrequency phased array deep regional heating of the pelvis. Relative heating characteristics of the available technologies are compared based on published feasibility studies, and the systems correlated with clinical requirements for effective treatment of MIBC and NMIBC

Multiple antenna microwave ablation: impact of non-parallel antenna insertion

Master of ScienceDepartment of Electrical and Computer EngineeringPunit PrakashMicrowave ablation is a minimally invasive therapeutic modality used for the treatment of cancer in various organs. In this procedure, microwave energy is sent through a thin antenna placed inside the tumor. The microwave energy radiated from the antenna generates heat which kills the tumor cells by necrosis. During multiple-applicator microwave ablation, geometric estimates of treatment outcome are typically obtained by assuming parallel insertion of the applicators. This assumption is based on the guidelines provided in the brochures of antenna manufacturing companies. This assumption is flawed because it is rare to insert the antennas in parallel configuration due to the flexible nature of the antennas and the presence of intervening organs. Furthermore, movement of patients during the treatment procedure alters the position of the antennas. In order to see the effect of non-parallel insertion of antennas, model-based treatment planning may be instructive. Treatment planning can also determine the changes needed to be made for prospective ablation therapy if the antennas are not positioned in their ideal parallel configuration. This thesis provides a detailed computational comparison of the skewed configurations of microwave antennas to their closest parallel configurations. The metric used for com-paring the similarity between the cases is Dice Similarity Coefficient (DSC). Experimental results to validate the computational data are also discussed. Computations were done by using realistic cases of antenna positions obtained from Rhode Island Hospital

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

Wide Band Embedded Slot Antennas for Biomedical, Harsh Environment, and Rescue Applications

For many designers, embedded antenna design is a very challenging task when designing embedded systems. Designing Antennas to given set of specifications is typically tailored to efficiently radiate the energy to free space with a certain radiation pattern and operating frequency range, but its design becomes even harder when embedded in multi-layer environment, being conformal to a surface, or matched to a wide range of loads (environments). In an effort to clarify the design process, we took a closer look at the key considerations for designing an embedded antenna. The design could be geared towards wireless/mobile platforms, wearable antennas, or body area network. Our group at UT has been involved in developing portable and embedded systems for multi-band operation for cell phones or laptops. The design of these antennas addressed single band/narrowband to multiband/wideband operation and provided over 7 bands within the cellular bands (850 MHz to 2 GHz). Typically the challenge is: many applications require ultra wide band operation, or operate at low frequency. Low frequency operation is very challenging if size is a constraint, and there is a need for demonstrating positive antenna gain

Thermal ablation of biological tissues in disease treatment: A review of computational models and future directions

Percutaneous thermal ablation has proved to be an effective modality for treating both benign and malignant tumors in various tissues. Among these modalities, radiofrequency ablation (RFA) is the most promising and widely adopted approach that has been extensively studied in the past decades. Microwave ablation (MWA) is a newly emerging modality that is gaining rapid momentum due to its capability of inducing rapid heating and attaining larger ablation volumes, and its lesser susceptibility to the heat sink effects as compared to RFA. Although the goal of both these therapies is to attain cell death in the target tissue by virtue of heating above 50 oC, their underlying mechanism of action and principles greatly differs. Computational modelling is a powerful tool for studying the effect of electromagnetic interactions within the biological tissues and predicting the treatment outcomes during thermal ablative therapies. Such a priori estimation can assist the clinical practitioners during treatment planning with the goal of attaining successful tumor destruction and preservation of the surrounding healthy tissue and critical structures. This review provides current state-of- the-art developments and associated challenges in the computational modelling of thermal ablative techniques, viz., RFA and MWA, as well as touch upon several promising avenues in the modelling of laser ablation, nanoparticles assisted magnetic hyperthermia and non- invasive RFA. The application of RFA in pain relief has been extensively reviewed from modelling point of view. Additionally, future directions have also been provided to improve these models for their successful translation and integration into the hospital work flow