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

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

Three-dimensional quantification of the vascular cooling effect of hepatic vessels during high-energy microwave ablation ex vivo

EINLEITUNG: Die Mikrowellenablation (MWA) ist ein thermoablatives Verfahren, das in der Behandlung von Lebertumoren zunehmend an Bedeutung gewinnt. Kühleffekte durch lebereigene Gefäße beeinflussen den Ablationserfolg jedoch maßgeblich. Problematisch dabei ist, dass der Kühleffekt zu einer unvollständigen Tumordestruktion und folglich zu Tumorrezidiven führen kann. Ziel dieser Arbeit war es, den vaskulären Kühleffekt bei der MWA anhand eines dreidimensionalen Ex-vivo-Modells zu evaluieren. MATERIAL UND METHODEN: Es wurden MWA an Ex-vivo-Schweinelebern durchgeführt. Zur Simulation eines Gefäßflusses wurde eine mit Wasser perfundierte Glasröhre in die Leber eingeführt. Anschließend erfolgten die Ablationen bei verschiedenen Antennen-Gefäß-Abständen (2,5; 5; 10 mm) und Flussraten (0, 1, 2, 5, 10, 100, 500 ml/min) für jeweils fünf Minuten mit einer zugeführten Gesamtenergie von 100 W. Die Ablationen wurden im Anschluss bei –80 °C eingefroren, am Kryostat geschnitten und alle 2 mm fotografiert. Anhand dieser makroskopischen Bildreihen erfolgte eine dreidimensionale, qualitative und quantitative Analyse der Ablationen unter Berücksichtigung der Kühleffekte. ERGEBNISSE: Insgesamt wurden 132 MWA in 22 Versuchsreihen durchgeführt. Bei allen Versuchsreihen mit einer Gefäßflussrate ≥ 2 ml/min traten Kühleffekte auf und es wurde eine geringe, bis stark ausgeprägte Veränderung der Ablationsform beobachtet. Bei einem Antennen-Gefäß-Abstand von 2,5 mm zeigten sich bei Ablationen mit Flussraten bis zu 10 ml/min keine Kühleffekte im Ablationszentrum, allerdings traten diese in der Ablationsperipherie auf. Betrug der Antennen-Gefäß-Abstand 5 und 10 mm, so waren Kühleffekte in der gesamten Ablation, insbesondere in den zentralen Bereichen, zu beobachten. Ohne einen Kühlfluss konnten mit der MWA bei 100 W und fünf Minuten Ablationsgrößen bis zu 16 mm zuverlässig erreicht werden. Bei Vorhandensein eines Kühlflusses führten vaskuläre Kühleffekte allerdings zu einer Verringerung der Ablationsgröße um bis zu 56 %. SCHLUSSFOLGERUNG: Bei der MWA von Lebergewebe können bei Vorhandensein größerer Blutgefäße relevante Kühleffekte entstehen. Diese sind in Abhängigkeit von der Flussrate und dem Antennen-Gefäß-Abstand in verschiedenen Ablationsbereichen unterschiedlich stark ausgeprägt. In der Klinik sollten im Rahmen der Ablationsplanung Kühleffekte beachtet werden, da diese bereits bei geringen Flussraten zu relevanten Änderungen der Ablationsform führen können.INTRODUCTION: Microwave ablation (MWA) is a thermoablative procedure that is becoming increasingly important in the treatment of liver tumors. However, vascular cooling effects induced by hepatic vessels significantly influence the ablation success. Especially problematic is, that the cooling effect may lead to incomplete tumor destruction and consequently to tumor recurrence. The aim of this work was to evaluate the vascular cooling effect in MWA using a three-dimensional ex vivo model. MATERIAL AND METHODS: MWA was performed in ex vivo porcine livers. A glass tube perfused with water was inserted into the liver to simulate a vascular flow. Ablations were performed at various antenna-vessel-distances (2.5, 5, 10 mm) and flow rates (0, 1, 2, 5, 10, 100, 500 ml/min) for five minutes each with a total applied energy of 100 W. Afterwards the ablations were frozen at –80 °C, sectioned at the cryostat and photographed every 2 mm. Using these macroscopic image series, a three-dimensional, a qualitative and a quantitative analysis of the ablations were performed, taking into account the cooling effects. RESULTS: A total of 132 MWAs in 22 test series were performed. Cooling effects occurred in all test series with a vessel flow rate ≥ 2 ml/min and a mild to severe change in ablation shape was observed. At an antenna-vessel-distance of 2.5 mm, no cooling effects occurred in the ablation center for ablations with flow rates up to 10 ml/min, however some occurred in the ablation periphery. If the antenna-vessel-distance was 5 mm and 10 mm, cooling effects were observed throughout the ablation, especially in the central ablation areas. Without a vascular flow, ablations up to 16 mm could be reliably achieved with the MWA at 100 W and five minutes. However, in the presence of a vascular flow, vascular cooling effects resulted in a reduction in ablation size of up to 56%. CONCLUSION: Relevant cooling effects can occur during MWA of liver tissue in the pres- ence of larger blood vessels. Cooling effects vary in severity in different ablation areas depending on the flow rate and the antenna-vessel-distance. In the clinical setting, cooling effects should be considered as part of ablation planning, as these can lead to relevant changes in ablations shape even at low flow rates

A study of microwave TDFT applicator design for low power cancer ablation

The development of therapeutic thermal ablative techniques become viable alternative method to treat patients who cannot be treated by surgery because of high surgical risk or unfavourable tumour location. Microwave ablation is the least invasive technique recently developed for cancer treatment because of its low cost, smaller antenna size and shorter recovery time. However, there are shortcomings of microwave ablation therapy needed to be fulfilled. Unsuccessfully ablated tumour and destruction of large portion of surrounding healthy tissues due to usage of exceptionally high input power which yields lack of control over heating encountered with previously proposed applicator designs. This work investigates the efficacy of using low power ultra-wide band (UWB) microwave applicator in cancer ablation. A novel Tear Drop Flared tipped (TDFT) antenna was proposed as microwave applicator for treating focal malignant tumours using low input power by the means of directed axial radiation. TDFT antenna is modelled and analysed in different surroundings such as saline, healthy and malignant tissue models. Semi-analytical numerical model is introduced to calculate current distributions required on antenna and consequent near-field distribution for achieving homogenous heating conformal to the targeted lesion to overcome nonuniform field distribution of omni-directional radiation. Electromagnetic simulations showed that TDFT antenna achieved minimum reflection stability of -25.89 dB over ultra-wide bandwidth. Electromagnetic and thermal simulations proved that directed axial radiation within targeted lesion produce confined uniform heating at significantly low input power. Moreover, 60 ℃ temperatures were attained for successful ablation and provided more control over heating within the targeted lesion. Highest SAR value attained of 967.3 W/kg for only 3W input power. Thermal analysis revealed that TDFT antenna can achieve a successful ablation of spherical cancerous lesions of diameters of 15.5 mm in 3 minutes for input power of 3W. TDFT antenna was fabricated and tested in egg-white solution and bovine liver. A good agreement between the measured and simulated results were observed where overall efficiency of 99.99% was recorded at the operating frequency. Ablation experiments were conducted in egg-white solution and bovine liver for 1W input power. Feasibility of TDFT antenna as a microwave coagulator was clearly observed in creating confined heating manifested in ablated lesions of 16×19.5×19.5 mm3 for 15-min ablation. Highly-directed End-fire radiation of TDFT antenna noticeably achieves confined heating that facilitates using only 60% of the lowest input power recorded in literature to attain successful ablation in standard radiation exposure time of 15 mins. This reduces power consumption of microwave applicator by almost 40% of the lowest input power used in literature