In silico evaluation of the thermal stress induced by MRI switched gradient fields in patients with metallic hip implant

This work focuses on the in silico evaluation of the energy deposed by MRI switched gradient fields in bulk metallic implants and the consequent temperature increase in the surrounding tissues. An original computational strategy, based on the subdivision of the gradient coil switching sequences into sub-signals and on the time-harmonic electromagnetic field solution, allows to realistically simulate the evolution of the phenomena produced by the gradient coils fed according to any MRI sequence. Then, Pennes' bioheat equation is solved through a Douglas-Gunn time split scheme to compute the time-dependent temperature increase. The procedure is validated by comparison with laboratory results, using a component of a realistic hip implant embedded within a phantom, obtaining an agreement on the temperature increase better than 5%, lower than the overall measurement uncertainty. The heating generated inside the body of a patient with a unilateral hip implant when undergoing an Echo-Planar Imaging (EPI) MRI sequence is evaluated and the role of the parameters affecting the thermal results (body position, coil performing the frequency encoding, effects of thermoregulation) is discussed. The results show that the gradient coils can generate local increases of temperature up to some kelvin when acting without radiofrequency excitation. Hence, their contribution in general should not be disregarded when evaluating patients' safety

Thermal Analysis of Human Tissues Exposed to Focused Beam THz Radiations

The thermal response of human tissues exposed to a focused beam terahertz electromagnetic radiation is evaluated through a combined analytical electromagnetic wave solution and a step-by-step finite element numerical model, which solves Pennes’ bioheat equation. The computational procedure is applied to a three-layer model of the human tissues for wave frequencies ranging from 0.025 THz to 1 THz and compared with a more detailed five-layer model. The effects of the Gaussian beam parameters of the electromagnetic radiation on the temperature elevation are finally evaluated

Evaluation of the Electromagnetic Environment Around Underground HVDC Lines

This paper analyses the magnetic-field emissions of a high-voltage dc transmission line constituted by two couples of underground cables laid along a highway. The transmission system, including all its components (transformers, converters filters, and line), is modeled through a circuital approach, which provides the distribution of the current harmonics along the line length. The magnetic field produced in the environment is then estimated by a hybrid finite element/boundary element method. The electromagnetic interferences with existing appliances and the human exposure to magnetic fields are investigated considering different laying configurations, conductor dispositions, and supply conditions. Compliance with regulations limiting human exposure and technical standards ensuring electromagnetic compatibility of appliances and devices are assessed

A Near-Field Cloaking Study to Reduce MRI RF-Artefacts in Presence of Elongated Prostheses

The objective of this paper is to analyze a near-field electromagnetic cloaking to reduce the radiofrequency (RF) magnetic field inhomogeneities (responsible for the RF-artefacts onset) in magnetic resonance imaging (MRI) in the presence of an elongated metallic hardware. A lumped circuit is considered to explain the role that a dielectric coat has on hiding ametallic cylinder to the RF antenna. The theoretical assumptions are proved by means of full-wave simulations that are also applied to a realistic hip prosthesis considering a frequency equal to 64 and 128 MHz. The numerical results confirm the theoretical assumptions. Both the theoretical analysis and the numerical simulations highlight the different role that the coat thickness and electric permittivity have in the definition of a proper dielectric coat. A particular cloaking approach leads to a dielectric coat whose constitutive electrical parameters may be simple enough to fit the considered application reducing the interaction between an elongated prosthesis and the RF antenna. Furthermore, results obtained at 64 MHz suggest the possibility to employ an existing biocompatible material to achieve the envisaged purposes

Uncertainty propagation in phaseless electric properties tomography

Uncertainty propagation in a phaseless magnetic resonance-based electric properties tomography technique is investigated using the Monte Carlo method. The studied inverse method, which recovers the electric properties distribution at radiofrequency inside a scatterer irradiated by the coils of a magnetic resonance imaging scanner, is based on the contrast source inversion technique adapted to process phaseless input data.Comment: 4 pages, 6 figures. 2019 International Conference on Electromagnetics in Advanced Applications (ICEAA

Simplified modeling of implanted medical devices with metallic filamentary closed loops exposed to low or medium frequency magnetic fields

Background and objectives: Electric currents are induced in implanted medical devices with metallic fila-mentary closed loops (e.g., fixation grids, stents) when exposed to time varying magnetic fields, as those generated during certain diagnostic and therapeutic biomedical treatments. A simplified methodology to efficiently compute these currents, to estimate the altered electromagnetic field distribution in the bio-logical tissues and to assess the consequent biological effects is proposed for low or medium frequency fields.Methods: The proposed methodology is based on decoupling the handling of the filamentary wire and the anatomical body. To do this, a circuital solution is adopted to study the metallic filamentary implant and this solution is inserted in the electromagnetic field solution involving the biological tissues. The Joule losses computed in the implant are then used as a forcing term for the thermal problem defined by the bioheat Pennes' equation. The methodology is validated against a model problem, where a reference solution is available.Results: The proposed simplified methodology is proved to be in good agreement with solutions provided by alternative approaches. In particular, errors in the amplitude of the currents induced in the wires re-sult to be always lower than 3%. After the validation, the methodology is applied to check the interactions between the magnetic field generated by different biomedical devices and a skull grid, which represents a complex filamentary wire implant.Conclusions: The proposed simplified methodology, suitable to be applied to closed loop wires in the low to intermediate frequency range, is found to be sufficiently accurate and easy to apply in realistic exposure scenarios. This modeling tool allows analyzing different types of small implants, from coronary and biliary duct stents to orthopedic grids, under a variety of exposure scenarios.(c) 2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/