Orthopedic implants affect the electric field induced by switching gradients in MRI.

Purpose: To investigate whether the risk of peripheral nerve stimulation increases in the presence of bulky metallic prostheses implanted in a patient's body.

Methods: A computational tool was used to calculate the electric field (E-field) induced in a realistic human model due to the action of gradient fields. The calculations were performed both on the original version of the anatomical model and on a version modified through "virtual surgery" to incorporate knee, hip, and shoulder prostheses.

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 filamentary closed loops (e.g., fixation grids, stents) when exposed to time varying magnetic fields, as those generated during certain diagnostic and therapeutic biomedical treatments.

Computational dosimetry in MRI in presence of hip, knee or shoulder implants: do we need accurate surgery models?

To quantify the effects of different levels of realism in the description of the anatomy around hip, knee or shoulder implants when simulating, numerically, radiofrequency and gradient-induced heating in magnetic resonance imaging. This quantification is needed to define how precise the digital human model modified with the implant should be to get realistic dosimetric assessments..

Heating of metallic biliary stents during magnetic hyperthermia of patients with pancreatic ductal adenocarcinoma: an study.

Objective: To investigate the eddy current heating that occurs in metallic biliary stents during magnetic hyperthermia treatments and to assess whether these implants should continue to be an exclusion criterion for potential patients.

Methods: Computer simulations were run on stent heating during the hyperthermia treatment of local pancreatic tumors (5-15 mT fields at 300 kHz for 30 min), considering factors such as wire diameter, type of stent alloy, and field orientation. Maxwell's equations were solved numerically in a bile duct model, including the secondary field produced by the stents.

Classification Scheme of Heating Risk during MRI Scans on Patients with Orthopaedic Prostheses.

Due to the large variety of possible clinical scenarios, a reliable heating-risk assessment is not straightforward when patients with arthroplasty undergo MRI scans. This paper proposes a simple procedure to estimate the thermal effects induced in patients with hip, knee, or shoulder arthroplasty during MRI exams. The most representative clinical scenarios were identified by a preliminary frequency analysis, based on clinical service databases, collecting MRI exams of 11,658 implant carrier patients.

A contribution to MRI safety testing related to gradient-induced heating of medical devices.

Purpose: To theoretically investigate the feasibility of a novel procedure for testing the MRI gradient-induced heating of medical devices and translating the results into clinical practice.

Methods: The concept of index of stress is introduced by decoupling the time waveform characteristics of the gradient field signals from the field spatial distribution within an MRI scanner. This index is also extended to consider the anisotropy of complex bulky metallic implants.

In silico assessment of collateral eddy current heating in biocompatible implants subjected to magnetic hyperthermia treatments.

Bearing partially or fully metallic passive implants represents an exclusion criterion for patients undergoing a magnetic hyperthermia procedure, but there are no specific studies backing this restrictive decision. This work assesses how the secondary magnetic field generated at the surface of two common types of prostheses affects the safety and efficiency of magnetic hyperthermia treatments of localized tumors. The paper also proposes the combination of a multi-criteria decision analysis and a graphical representation of calculated data as an initial screening during the preclinical risk assessment for each patient.

Heating of hip joint implants in MRI: The combined effect of RF and switched-gradient fields.

Purpose: To investigate how the simultaneous exposure to gradient and RF fields affects the temperature rise in patients with a metallic hip prosthesis during an MRI session.

Methods: In silico analysis was performed with an anatomically realistic human model with CoCrMo hip implant in 12 imaging positions. The analysis was performed at 1.

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.

Dynamic Simulation of a Fe-Ga Energy Harvester Prototype Through a Preisach-Type Hysteresis Model.

This paper presents the modeling of an Fe-Ga energy harvester prototype, within a large range of values of operating parameters (mechanical preload, amplitude and frequency of dynamic load, electric load resistance). The simulations, based on a hysteretic Preisach-type model, employ a voltage-driven finite element formulation using the fixed-point technique, to handle the material nonlinearities. Due to the magneto-mechanical characteristics of Fe-Ga, a preliminary tuning must be performed for each preload to individualize the fixed point constant, to ensure a good convergence of the method.

CSI-EPT in Presence of RF-Shield for MR-Coils.

Contrast source inversion electric properties tomography (CSI-EPT) is a recently developed technique for the electric properties tomography that recovers the electric properties distribution starting from measurements performed by magnetic resonance imaging scanners. This method is an optimal control approach based on the contrast source inversion technique, which distinguishes itself from other electric properties tomography techniques for its capability to recover also the local specific absorption rate distribution, essential for online dosimetry. Up to now, CSI-EPT has only been described in terms of integral equations, limiting its applicability to homogeneous unbounded background.

Assessment of exposure to MRI motion-induced fields based on the International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines.

Purpose: The goal of this study was to conduct an exposure assessment for workers moving through the stray stationary field of common MRI scanners, performed according to the recent International Commission on Non-Ionizing Radiation Protection (ICNIRP) Guidelines, which aim at avoiding annoying sensory effects.

Theory And Methods: The analysis was performed through numerical simulations, using a high-resolution anatomical model that moved along realistic trajectories in proximity to a tubular and open MRI scanner. Both dosimetric indexes indicated by ICNIRP (maximum variation of the magnetic flux density vector and exposure index for the motion-induced electric field) were computed for three statures of the human model.

Numerical prediction of temperature elevation induced around metallic hip prostheses by traditional, split, and uniplanar gradient coils.

Purpose: The paper presents a computational study for the estimation of the temperature elevation occurring in a human subject carrying metallic hip prostheses when exposed to the magnetic field produced by gradient coils.

Methods: The simulations are performed through validated numerical codes, which solve the electromagnetic and thermal equations applied to a high-resolution anatomical human model. Three different sets of gradient coils (traditional, split and uniplanar) are considered to evaluate the maximum steady-state temperature elevation in the human body.

Parametric analysis of transient skin heating induced by terahertz radiation.

This paper investigates the effect of relevant physical parameters on transient temperature elevation induced in human tissues by electromagnetic waves in the terahertz (THz) band. The problem is defined by assuming a plane wave, which, during a limited time interval, normally impinges on the surface of a 3-layer model of the human body, causing a thermal transient. The electromagnetic equations are solved analytically, while the thermal ones are handled according to the finite element method.

A numerical survey of motion-induced electric fields experienced by MRI operators.

This paper deals with the electric field generated inside the bodies of people moving in proximity to magnetic resonance scanners. Different types of scanners (tubular and open) and various kinds of movements (translation, rotation, and revolution) are analyzed, considering the homogeneous human model proposed in some technical Standards. The computations are performed through the Boundary Element Method, adopting a reference frame attached to the body, which significantly reduces the computational burden.

A procedure to estimate the electric field induced in human body exposed to unknown magnetic sources.

The paper proposes and discusses a boundary element procedure able to predict the distribution of the electric field induced in a human body exposed to a low-frequency magnetic field produced by unknown sources. As a first step, the magnetic field on the body surface is reconstructed starting from the magnetic field values detected on a closed surface enclosing the sources. Then, the solution of a boundary value problem provides the electric field distribution inside the human model.

Influence of probe size on the measurement accuracy of non-uniform ELF magnetic fields.

The accuracy of extremely low frequency (ELF) magnetic field measurements can be sensitively varied by the meter probe size when the spatial distribution of the magnetic flux density is highly non-uniform, e.g. if the measurement point is close to the field source.