M. I. Gutierrez

Finite element modeling of acoustic field of physiotherapy ultrasonic transducers and the comparison with measurements

This paper presents the modeling of the acoustic field of a physiotherapy ultrasonic transducer by using the finite element method. An ideal emission is presented obtained by using the approach developed by Rayleigh, called “piston in a baffle”. Simulations with FEM also are presented to compare them with the analytical results. The results of the model are also compared to those of the measurements in a physiotherapy transducer. The results show the efficacy of modeling the real transducers with ideal models in relation to the overlapping in the Fresnel zone and the characteristic parameters. A necessity is evident after this analysis: more appropriated models using more real boundary conditions. Results indicate that Fresnel zone is not correctly modeled using the ideal considerations.

Modeling the acoustic field of physiotherapy ultrasound transducers using non uniform acoustic pressure distributions

In ultrasonic physiotherapy, the therapeutic effect is produced in the first 5 cm, into the Fresnel zone. Models are focused principally in the Fraunhofer zone which is too far from the therapeutic region. More realistic models are required to express the overlapping commonly produced near the transducer face. This paper presents the modeling of the acoustic field of physiotherapy ultrasonic transducers under non-uniform pressure distributions by using the finite element method. Theoretical approaches are compared with measurements to determine their accuracy. Although the nonuniform vibration models were presented years ago, their validation is still not complete. The results show the efficacy of modeling real transducers under free vibrating, simply supported and clamped conditions in relation to the overlapping in the Fresnel zone. In was concluded that the Fresnel zone is not correctly modeled with any of the approaches of this paper which were proposed in the literature.

Modeling of acoustic field patter in rat's bone with a HIFU transducer for medical experiments in delivery drugs in brain

Currently the controlled release of drugs in specific areas is of great importance for the treatment of diseases such as cancer, Parkinson, Alzheimer and others. This project works with focused ultrasound (HIFU) in rats, more specifically in the application of HIFU in a specific area of the brain called the substantia nigra, in order to generate opening in the membrane of this area called blood-brain barrier (BBB) with the purpose of introducing vectors containing the drug used to treat Parkinson, proposed by the Department of Physiology, Biophysics and Neuroscience Research Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional. This work is the beginning of the investigation based on the modeling of a HIFU transducer in bone tissue of rats weighing approximately 400 gr. In order to obtain the sound field pattern, focus size, pressure and ultrasound path in a time dependent study. All of the above in order to get a clear idea of what you will get when it comes to experimentation.

Influence of the surrounding tissues in the radiation pattern of microcoaxial antenna for the treatment of breast tumors

This paper presents a model of microwave radiation into the surrounding tissues in the therapy of ablation of breast using the finite element method (FEM). Three designs of antennas with different diameters have been proposed to carry out simulations with the surrounding tissues: adipose tissue, connective and glandular tissue and tumor tissue. The tissues of the breast should be considered in the study of the effectiveness of the antennas to observe the change of patterns of the Specific Absortion Rate (SAR). The FEM provides information for designing of microcoaxial antennas and its effects. The antennas of 0.95 and 1.3 mm of diameter have a Standing Wave Ratio (SWR) lower than the antenna with a diameter of 2.30 mm, so the smaller diameter antennas have a better response to a depth of 50 mm with a SWR of 1.5.

Modeling a conical applicator for high intensity focused ultrasound with the finite element method

The high-intensity focused ultrasound (HIFU) has shown positive results in the treatment of prostate cancer and its possible application in neurodegenerative diseases. This paper describes the design of a HIFU applicator with a conical geometry, for modeling the software COMSOL® (COMSOL Inc., Burlington, MA) was used, which bases its operation on the finite element method (FEM). The equations that govern the phenomenon were used to optimize the dimensions of the applicator and their use in the application surface. The design condition to be met is that the applicator should not alter the focus and acoustic field generated by the transducer source of ultrasound. The designed model allows that the system does not depend on a water container for the application of HIFU, reducing the dimensions and facilitating interaction with the system. It was obtained a design where the conical applicator has a radius at the apex of 5.2 mm, a radius at the base of 11.5 mm, a thickness of 2 mm walls, an operating frequency of 2 MHz and a focus of 20 mm operation. Water was considered as a means of propagation between the application surface and the applicator.

Simulation and experimental verification of the acoustic field produced by an annular piezoelectric array

In the field of ultrasound transducers to generate frozen rings waves generated a tendency for use in biomedical applications. This paper proposes simulate through Finite Element Method (FEM) simulation of a transducer with 10 rings to find patterns of acoustic field and get a model attached to the real transducer designed by Castellanos et al [3]. This work focuses on obtained results of acoustic field as close as possible to those real measurements, with all this process entails: geometry, the definition of materials piezoelectric, the FEM equations governing the physics of problem and the resolution of the mesh. Whereby are obtained results showing patterns similar to those obtained in the real acoustic field measurements, in order to propose to extend this work to find the combination of this transducer modeling using the FEM.

Therapy ultrasound equipment characterization: Comparison of three techniques

Methods for characterizing ultrasonic therapy equipment rapidly and easily have to be implemented in order to avoid damages to patients; the existent methods measure different parameters in the ultrasonic beam that can be used to determine if the equipment works appropriately. In this paper, a comparison of three methods to characterize the ultrasonic beam is presented. The first one is a C-scan with microprobe which uses a hydrophone for measuring the signal and a positioning system. The second method is the IR-thermography which uses a phantom to absorb the ultrasonic energy and to convert it into heat. Here, the heat distribution is obtained with an IR camera. The third method uses a sheet of thermochromic liquid crystals (TLC) as sensor and a phantom to absorb the energy. The heat distribution is obtained with a normal camera because the TLCs change their color as a function of temperature. The results indicate that each technique has its own benefits, but the most important parameters can be obtained with only one of them.

Acoustic field simulation for focused ultrasound on skull with craniotomy for drug delivery in rat brain

The drugs delivered in the central nervous system can be used for the treatment of Parkinson or Alzheimer by acting directly in the affected zones. The problem is that the Blood Brain Barrier (BBB), only allows the passage of certain substances to protect the brain. The focused ultrasound (FUS) with ultrasonic contrast agents (UCA), have demonstrated the reversible opening of the BBB. This paper presents the modeling of the acoustic field for a Focused Ultrasound transducer by finite element method (FEM), that allows evaluating a craniotomy with the less dimension to improve the propagation of ultrasonic waves without deforming the transducers focus and cause minimal damage to the rat. For this work the head of a rat of 300 gr, following the dimensions and actual geometry of the murine models and a concave monoelemental transducer with a frequency of 2 MHz a focal length of 20 mm and a radius of 10 mm were taken as reference.

Modeling of electromagnetic and temperature distributions of an intersticial coaxial-based choked antenna for hepatic tumor microwave ablation

Liver tumors can be treated by microwave ablation, a therapy in which the goal is to increase the tissues temperature in order to induce necrosis. For the application of microwave ablation interstitial antenna applicators have been developed with the objective of delivering and focusing energy deposition on the tissue in an effective manner. Choked antennas propose an ideal solution, as they are capable of producing a punctual hot-spot that prevents backward heating of the antenna. In this work designs for a choked and un-choked metal-tip monopole antenna are presented and modeled using finite element method to obtain their heating patterns, power deposition and temperature distributions, compare them and analyze the effects of the metallic choke over the same base antenna design at a frequency of 2.45 GHz and a power of 10 W.

Modulation of the driven signal applied to an ultrasound therapy transducer

Studying the wave propagation and the cellular effects of the ultrasound energy emitted by physiotherapy equipments has been the direction of the past research issues. There are many publications about the effectiveness of ultrasound to treat some physical diseases but there is still few dealing with temperature distributions produced by this kind of equipment. Some companies have developed commercial ultrasonic devices for physiotherapy that have the following classical features: stable output of either continuous or pulsated mode (duty cycle), delivered power smaller than 3 W/cm2, and frequencies of either 1 or 3 MHz. Each equipment has its own protections and limits, and it is not easy to adjust some parameters; hence, using commercial equipment for research purposes limits research work. We propose a physiotherapy ultrasonic apparatus to induce heating in phantoms. Temperature distribution results in phantoms, obtained from these experiments, can be compared with those obtained from mathematical modeling. The device can deliver a modulated ultrasonic signal at frequencies and waveforms chosen by the user. This paper shows the first approximation to the complete design.