Thermal Ablation

Tissue-mimicking phantoms that are currently available for routine biomedical applications may not be suitable for high-temperature experiments or calibration of thermal modalities. Therefore, design and fabrication of customized thermal phantoms with tailored properties are necessary for thermal therapy studies. A multitude of thermal phantoms have been developed in liquid, solid, and gel forms to simulate biological tissues in thermal therapy experiments. This article is an attempt to outline the various materials and techniques used to prepare thermal phantoms in the gel state. The relevant thermal, electrical, acoustic, and optical properties of these phantoms are presented in detail and the benefits and shortcomings of each type are discussed. This review could assist the researchers in the selection of appropriate phantom recipes for their in vitro study of thermal modalities and highlight the limitations of current phantom recipes that remain to be addressed in further studies.

High-intensity focused ultrasound guided by magnetic resonance imaging has been extensively evaluated during the past decade as a clinical alternative for thermal ablation of tumor tissue. However, the maximal ablation volume is limited by the extensive treatment duration resulting from the small size of the focal point as compared to the average tumor size. Volumetric sonication has been shown to efficiently enlarge the ablated volume per sonication, but remains limited by the temperature increase induced in the skin and fat layers. In this study, multiplane MR thermometry is proposed for monitoring the near-field temperature rise in order to prevent related unintended thermal damage.
The method was evaluated by performing sonications in the thigh muscle of 11 pigs maintained under general anesthesia. Volumetric ablations were performed by steering the focal point along trajectories consisting of multiple outward-moving concentric circles. Near-field heating was characterized with MR temperature maps and thermal dose maps. The results from the MR measurements were compared to simulations.
In this study, the measured maximum temperature rise was found to correlate linearly with the surface energy density within the near field of the beam path with a slope of 4.2 K mm2/J. This simple linear model appears to be almost independent of the trajectory pattern and the sonication depth. The safety limit to avoid lethal damage of the subcutaneous tissues of the porcine thigh was identified to be an absolute temperature of 50 degrees C, corresponding to a surface energy density of 2.5 J/mm2 at 1.2 MHz.
A linear relationship can be established to estimate the temperature increase based on the chosen power prior to ablation, thereby providing an a priori safety check for possible excessive near-field heating using a known surface energy density threshold. This method would also give the clinician the possibility to abort the sonication should excessive near-field temperature rise be seen before fat layer damage or skin burns are inflicted.

A primary motivation for this study comes from the need to improve the ability of polymer matrix composites to withstand lightning strikes. In particular, we are concerned with lightning strike damage in composite wind turbine blades. The problem is essentially multiphysic, as the electric-current-induced temperature in the composite blade subjected to a lightning strike may reach up to 1,200 °C, leading to extensive thermal damage including surface damage, delamination, cracks, matrix decomposition, fiber breakage and sublimation, and abrupt failure of the blade.   
In this work, the thermal response of the polymer-matrix composite laminate used in a Sandia 100-meter All-glass Baseline Wind Turbine Blade (SNL 100-00) subjected to lightning strike is studied. The tip region composite panel is considered, as it has been reported in the literature that the blade tip is more susceptible to lightning strike than the remaining parts of the blade.
A physical model to show the surface interaction between the lightning arc and the composite structure has been developed. The model provides time- and electric-current-dependent variation of the lightning arc radius and lightning-current-induced heat flux generated at the composite surface. The temperature-dependent thermal properties of the VectorPly E-LT 5500 unidirectional [0]2 E-glass fiber vinyl ester resin matrix fabric and SNL triaxial [±45]2[0]2 E-glass fiber vinyl ester resin matrix fabric used in the wind blade tip composite panel were derived using available experimental data. The formulated nonlinear transient heat transfer problem with moving boundary is solved using the finite element method. The solution procedure accounts for phase transitions in the materials, and the obtained results include temperature field profiles and evolution of the thermal damage.

This work is concerned with thermal ablation modeling in carbon fiber-reinforced polymer-matrix composite laminates subjected to the lightnig strike. Both direct heat injection and Joule heating produced by the lightning current are taken into account. First, a model describing interaction of the lightning current channel with a conductive structure is presented. The model includes channel expansion and spatial and temporal distribution of the lightning current and linghtning-induced heat flux. Second, anisotropic electrical and thermal properties of the CFRP composite laminates are determined in a wide temperature range (up to the sublimation temperature of the carbon fibers) using experimental data and micromechanics considerations. Third, a nonlinear thermo-electric coupled problem is formulated and solved for a CFRP composite laminate to determine the electric-current-induced temperature distribution and associated thermal ablation. Finally, the obtained predictions of thermal ablation in the CFRP composite laminate are compared to the reported experimental results. It is found that the predicted thermal ablation depths agree well with those reported in the experimental study.

This paper describes a method of determination of the optimal sizes and locations of cylindrical air gaps inside the multi-slot coaxial antenna operating at 2.45 GHz. The dimensions of the microwave antenna with a 50-ohm coaxial feed were revised with regard to antenna reflection coefficient distributions. It should be emphasized that there are some differences between the operating frequencies of such antennas described in the literature and the resonant frequencies obtained from the antenna characteristics. It may significantly affect the interstitial hyperthermia treatment. It is also possible to resize the antenna slots to minimalize the S11-parameter value and obtain the best antenna impedance matching inside various tissues commonly treated in this method. The numerical implementation of such multi-physical problem is based on the finite element method.

In this work, the influence of lightning-strike-induced electric current and surface heat flux on the thermal response and thermal ablation of a carbon fiber polymer matrix composite laminated panel is studied. A coupled electric-thermal model for an anisotropic plate exposed to the lightning-strike-induced electric current and surface heat flux is formulated.  Temperature-dependent material properties of the carbon fiber polymer matrix composites, including electrical conductivity, thermal conductivity, and specific heat, are used. A coupled electric-thermal finite element analysis (FEA) is conducted using a MATLAB-ABAQUS integrated numerical procedure, which enables progressive element deletion to accurately model the lightning-strike-induced continuous surface recession (thermal ablation) in the composite panel. The obtained thermal response and thermal ablation using the proposed numerical procedure are compared with those obtained using existing solution procedures without progressive element deletion.

For the localized treatment of a tumor in a more controlled fashion, several stimuli-responsive nanocarriers
and minimally or non-invasive techniques like photothermal therapy (PTT) have emerged. However, PTT is
limited to only treatment of small and superficial tumors due to the inability of NIR light to penetrate more
and kill the core cells of large and deep-seated tumors. As a preliminary step towards addressing the
problem, NIR light-triggered thermoresponsive theranostic nanoshell consisting of chitosan-grafted
poly(N-vinyl caprolactam) as core and biocompatible gold as shell (Au PNVCL NS) are synthesized and
well characterized by various techniques. PNVCL is polymerized from N-vinyl caprolactam using free
radical polymerization method, and chitosan is grafted to raise its lower critical solution temperature
(LCST) to hyperthermic temperature (
43 C). Surface plasmon resonant gold shell over PNVCL NPs
core is assembled by ascorbic acid-driven in situ reduction. Core to shell diameter ratio is controlled to
tune the peak in NIR region (750 nm). Therapeutic potential of Au PNVCL NS is determined over breast
cancer cells MCF-7, while diagnostic potential is compared with the commercial contrast agent-
Omnipaque.

Ablation is commonly used medical procedure for removal of liver tumors. It is minimally invasive technique that is based on the insertion of a thin needle antenna into malignant tissue and heating the region of interest by microwaves. The aim of this work is to optimize shape of a microwave antenna with 50-ohm feed and multiple air gaps. Fundamental sizes of cylindrical antenna should be changed, in particular dimensions of antenna slots and distances between them. Change of the geometry should be done in such a way as to obtain the best impedance matching of the antenna-tissue system. Therefore, S11-scattering parameter will be used in the objective function. Importantly, the antenna operates at working frequency equal to 2.45 GHz.

Introduction: Most brain surgeries involving deep tumors/cyst resections are performed using invasive craniotomies, the surgical removal of part of the bone from the skull to expose the brain. These invasive surgeries entail hours of blood loss, potential infections, and long recovery periods. While several devices using high-intensity focused ultrasound (HIFU), that is, high-energy ultrasonic waves focused to destroy brain targets through coagulation necrosis, are in various development phases, they are limited by the attenuation of ultrasound while passing through the skull, and the inability to reach targets deep in the brain. In
particular, the use of excessive power is undesirable from a regulatory standpoint. Here, we report the design, development, and
cadaveric testing of a novel HIFU device for brain tumor ablation. This device is designed to access the ventricular space via a minimally invasive burr hole in the skull (Fig. 1a). This approach allows ultrasound to reach targets deep in the brain, while eliminating the need for high-power transcranial ultrasound procedures. (Manbachi et al. 2018, PCT)

Materials and Methods: As shown in Fig. 1b a custom, transducer with a hybrid imaging/HIFU tip was designed and manufactured, based on prior simulation studies (Zhang et al. 2017, AAPM; Gamo et al. 2018, AANS). Once the transducer was verified to be functional in a phantom model, a cadaver trial was performed to demonstrate proof-of-concept in ablating a
predetermined region of interest. Following ablation, the brain was sectioned to verify that the transducer ablated the predicted location, and according to the criteria outlined in Fig. 1c.

Results and Discussion: The transducer was successfully inserted into the ventricular space through a burr hole in the skull of a cadaver, and was used to create single-point ablations in the regions of interest, 3 cm away from the surface of the transducer. These data showed similar accuracy relative to that of transcranial approaches, but with considerably less power (12-35 W vs. 800 W with the transcranial approach).

Conclusions: A hybrid imaging/therapeutic ultrasound transducer was designed and manufactured to be inserted into the ventricular space through a minimally invasive burr hole in the skull. The performance of this probe was validated through precise ablations of deep intracranial pathology at low power in a cadaver. Thus, our solution presents a compromise between quality of care (treatment of oncology lesion) and patient safety (amount of applied power). Based on these findings, a beta
prototype will be developed to ablate medium to large size tumors.

Acknowledgements: This work was supported by the TEDCO Maryland Innovation Initiative Technology Validation Grant and Johns Hopkins University, Whiting School of Engineering’s Cohen Translational Fund awarded to AM. The authors thank Maryland Development Center and SonicConcepts for fruitful collaborations.

References:

Zhang et al. 2017, AAPM
Gamo et al. 2018, AANS
Manbachi et al. 2018, PCT/US18/20159

The purpose of this prospective multicenter study was to assess the safety and technical feasibility of volumetric Magnetic Resonance-guided High Intensity Focused Ultrasound (MR-HIFU) ablation for treatment of patients with symptomatic uterine fibroids.
Thirty-three patients with 36 fibroids were treated with volumetric MR-HIFU ablation. Treatment capability and technical feasibility were assessed by comparison of the Non-Perfused Volumes (NPVs) with MR thermal dose predicted treatment volumes. Safety was determined by evaluation of complications or adverse events and unintended lesions. Secondary endpoints were pain and discomfort scores, recovery time and length of hospital stay.
The mean NPV calculated as a percentage of the total fibroid volume was 21.7%. Correlation between the predicted treatment volumes and NPVs was found to be very strong, with a correlation coefficient r of 0.87. All patients tolerated the treatment well and were treated on an outpatient basis. No serious adverse events were reported and recovery time to normal activities was 2.3 ± 1.8 days.
This prospective multicenter study proved that volumetric MR-HIFU is safe and technically feasible for the treatment of symptomatic uterine fibroids.
• Magnetic-resonance-guided high intensity focused ultrasound allows non-invasive treatment of uterine fibroids. • Volumetric feedback ablation is a novel technology that allows larger treatment volumes • MR-guided ultrasound ablation of uterine fibroids appears safe using volumetric feedback.

The present invention provides n ultrasonic treatment device (10) for heating a portion of a subject of interest, comprising a ultrasonic irradiation unit (12) for generating high-intensity focused ultrasonic irradiation, whereby a beam path of the ultrasonic irradiation is movable along a trajectory for depositing ultrasonic energy within a target zone (22) of the subject of interest, and a control unit (20) for controlling the ultrasonic irradiation unit (12) to move the beam path of the ultrasonic irradiation along the trajectory and to apply an ultrasonic dose to the target zone (22), wherein the control unit (20) is adapted to receive temperature information of the target zone (22) and to control the ultrasonic irradiation unit (12) based on the received temperature information, and the control unit (20) is adapted to control the ultrasonic irradiation unit (12) based on the temperature of a current (26) and at least one previous direction (28) of the beam path of the ultrasonic irradiation along the trajectory. The invention further provides a corresponding method for ultrasonic treatment and an ultrasonic treatment system, comprising the above ultrasonic treatment device (10) and a diagnostic imaging device for providing temperature information of a target zone (22) of the subject of interest, wherein the control unit (20) of the ultrasonic treatment device (10) is adapted to receive the temperature information from the diagnostic imaging device. By looking at the temperature or a thermal dose distribution along a previous direction of the beam path of the ultrasonic irradiation along the trajectory where the heating has been completed, an improved control of the treatment is achieved. Even if this previous sonication was different to the one currently applied, it represents a good indicator of the tissue and ultrasound system behavior.

Tissue-mimicking phantoms that are currently available for routine biomedical applications may not be suitable for high-temperature experiments or calibration of thermal modalities. Therefore, design and fabrication of customized thermal phantoms with tailored properties are necessary for thermal therapy studies. A multitude of thermal phantoms have been developed in liquid, solid, and gel forms to simulate biological tissues in thermal therapy experiments. This article is an attempt to outline the various materials and techniques used to prepare thermal phantoms in the gel state. The relevant thermal, electrical, acoustic, and optical properties of these phantoms are presented in detail and the benefits and shortcomings of each type are discussed. This review could assist the researchers in the selection of appropriate phantom recipes for their in vitro study of thermal modalities and highlight the limitations of current phantom recipes that remain to be addressed in further studies.

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.