Numerically simulated exposure of children and adults to pulsed gradient fields in MRI

Exposure of Infants to Gradient Fields in a Baby MRI Scanner

In pediatric magnetic resonance imaging (MRI), infants are exposed to rapid, time-varying gradient magnetic fields, leading to electric fields induced in the body of infants and potential safety risks (e.g. peripheral nerve stimulation). In this numerical study, the in situ electric fields in infants induced by small-sized gradient coils for a 1.5 T MRI scanner were evaluated. The gradient coil set was specially designed for the efficient imaging of infants within a small-bore (baby) scanner. The magnetic flux density and induced electric fields by the small x, y, z gradient coils in an infant model (8-week-old with a mass of 4.3 kg) were computed using the scalar potential finite differences method. The gradient coils were driven by a 1 kHz sinusoidal waveform and also a trapezoidal waveform with a 250 µs rise time. The model was placed at different scan positions, including the head area (position I), chest area (position II), and body center (position III). It was found that the induced electric fields in most tissues exceeded the basic restrictions of the ICNIRP 2010 guidelines for both waveforms. The electric fields were similar in the region of interest for all coil types and model positions but different outside the imaging region. The y-coil induced larger electric fields compared with the x- and z- coils. © 2022 Bioelectromagnetics Society.

Sensitivity Analysis of a Numerical Model for Percutaneous Auricular Vagus Nerve Stimulation

Background: Less-invasive percutaneous stimulation of the auricular branch of the vagus nerve (pVNS) gained importance as a possible nonpharmacological treatment for various diseases. The objective is to perform a sensitivity analysis of a realistic numerical model of pVNS and to investigate the effects of the model parameters on the excitation threshold for single and bundled axons. Methods: Sim4Life electrostatic solver and neural tissue models were combined for electromagnetic and neural simulation. The numerical model consisted of a high-resolution model of a human ear, blood vessels, nerves, and three needle electrodes. Investigated parameters include the axon diameter and number, model temperature, ear conductivity, and electrodes’ penetration depth and position. Results: The electric field distribution was evaluated. Model temperature and ear conductivity are the non-influential parameters. Axons fiber diameter and the electrodes’ penetration depth are the most influential parameters with a maximum threshold voltage sensitivity of 32 mV for each 1 μm change in the axon diameter and 38 mV for each 0.1 mm change in the electrodes’ penetration depth. Conclusions: The established sensitivity analysis allows the identification of the influential and the non-influential parameters with a sensitivity quantification. Results suggest that the electrodes’ penetration depth is the most influential parameter.

Risk assessment of copper-containing contraceptives: the impact for women with implanted intrauterine devices during clinical MRI and CT examinations

OBJECTIVES

To assess the risks for implant users with copper-containing intrauterine devices (IUDs) during MR and CT examinations.

METHODS

A tissue-mimicking phantom suitable for all experiments within this study was developed. Seven different types of copper IUDs were evaluated. Heating and dislocation of each IUD were investigated at two clinically relevant positions in 1.5 T and 3 T MR scanners. Artifacts in the field of view caused by each tested IUD were determined for clinical MR and CT imaging.

RESULTS

No significant heating of any tested IUD was detected during MR measurements. The temperature increase was less than 0.6 K for all IUDs. Neither angular deflection nor translation of any IUD was detected. Artifacts in MR images were limited to the very vicinity of the IUDs except for one IUD containing a steel-visualizing element. Streaking artifacts in CT were severe (up to 75.5%) in the slices including the IUD.

CONCLUSION

No significant risk possibly harming the patient was determined during this phantom study, deeming MR examinations safe for women with an implanted copper IUD. Image quality was more impaired for CT than for MR imaging and needs careful consideration during diagnosis.

KEY POINTS

• Risk assessment of copper-containing IUDs with regard to heating, dislocation, and artifacts during MR and CT imaging. • Neither significant heating nor dislocation was determined in MR; image quality was more impaired for CT than for MR imaging and needs careful consideration during diagnosis. • The tested IUDs pose no additional risks for implant users during MR and CT examinations.

Deep Transcranial Magnetic Stimulation: Improved Coil Design and Assessment of the Induced Fields Using MIDA Model

Stimulation of deep brain structures by transcranial magnetic stimulation (TMS) is a method for activating deep neurons in the brain and can be beneficial for the treatment of psychiatric and neurological disorders. To numerically investigate the possibility for deeper brain stimulation (electric fields reaching the hippocampus, the nucleus accumbens, and the cerebellum), combined TMS coils using the double-cone coil with the Halo coil (HDA) were modeled and investigated. Numerical simulations were performed using MIDA: a new multimodal imaging-based detailed anatomical model of the human head and neck. The 3D distributions of magnetic flux density and electric field were calculated. The percentage of volume of each tissue that is exposed to electric field amplitude equal or greater than 50% of the maximum amplitude of E in the cortex for each coil was calculated to quantify the electric field spread (V50). Results show that only the HDA coil can spread electric fields to the hippocampus, the nucleus accumbens, and the cerebellum with V50 equal to 0.04%, 1.21%, and 6.2%, respectively.