In this article, we discuss numerical aspects related to the accuracy and the computational efficiency of numerical dosimetric simulations, performed in the context of human exposure to static inductive charging systems of electric vehicles. Two alternative numerical methods based on electric vector potential and electric scalar potential formulations, respectively, are here considered for the electric field computation in highly detailed anatomical human models. The results obtained by the numerical implementation of both approaches are discussed in terms of compliance assessment with ICNIRP guidelines limits for human exposure to electromagnetic fields. In particular, different strategies for smoothing localized unphysical outliers are compared, including novel techniques based on statistical considerations. The outlier removal is particularly relevant when comparison with basic restrictions is required to define the safety of electromagnetic fields exposure. The analysis demonstrates that it is not possible to derive general conclusions about the most robust method for dosimetric solutions. Nevertheless, the combined use of both formulations, together with the use of an algorithm for outliers removal based on a statistical approach, allows to determine final results to be compared with reference limits with a significant level of reliability.

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. This result is then rescaled to take into account the waveform of the signal, the duty‐cycle and the duration of the scan.Results: Several exposure situations obtained by changing the patient's position are analyzed, finding temperature elevations on the order of some degrees.Conclusion: The results are of possible concern and provide evidence of the need for further specific investigations aimed at assuring the safety of potential patients carrying metallic hip implants. Magn Reson Med 74:272–279, 2015.

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 1.5 T and 3 T, considering four clinical sequences: turbo spin‐echo, EPI, gradient‐echo, and true fast imaging sequence with steady precession. The exposure to gradient and RF fields was evaluated separately and superposed, by adopting an ad hoc computational algorithm. Temperature increase within the body, rather than specific absorption rate, was used as a safety metric.Results: With the exception of gradient‐echo, all investigated sequences produced temperature increases higher than 1 K after 360 seconds, at least for one body position. In general, RF‐induced heating dominates the turbo spin‐echo sequence, whereas gradient‐induced heating prevails with EPI; the situation with fast imaging sequence with steady precession is more diversified. The RF effects are enhanced when the implant is within the RF coil, whereas the effects of gradient fields are maximized if the prosthesis is outside the imaging region. Cases for which temperature‐increase thresholds were exceeded were identified, together with the corresponding amount of tissue mass involved and the exposure time needed to reach these limits.Conclusion: The analysis confirms that risky situations may occur when a patient carrying a hip implant undergoes an MRI exam and that, in some cases, the gradient field heating may be significant. In general, exclusion criteria only based on whole‐body specific absorption rate may not be sufficient to ensure patients’ safety.

Magnetic resonance imaging scanners can provide, by B1-mapping techniques, measurements of the transmit sensitivity magnitude, whereas the transmit sensitivity phase can only be approximated under symmetry assumptions of the coil and the sample. Nonetheless, up to now many techniques have been proposed to retrieve electric properties maps at radiofrequency by elaborating the complex transmit sensitivity. These techniques are usually referred to as magnetic resonance-based electric properties tomography. In order to perform imaging of the elecric properties also in cases which do not fulfill the assumptions for transmit sensitivity phase estimation, a variation of the phaseless contrast source inversion technique is proposed in this paper to retrieve the electric properties by elaborating only the magnitude of the transmit sensitivity. The novel technique is described in a generalised framework based on a functional viewpoint and it is specialised to deal with two-dimensional boundary-free problems. Moreover, a phaseless back propagation solution is proposed as initial guess. The proposed technique and initial guess have been tested on realistic two-dimensional model problems, which show that the numerical minimisation converges towards the global minimum when multiple linearly independent measurements are provided and that it is quite robust with respect to noisy inputs.

Patients with implanted orthopaedic devices represent a growing number of subjects undergoing magnetic resonance imaging (MRI) scans each year. MRI safety labelling is required for all implants under the EU Medical Device Regulations to ensure regulatory compliance, with each device assessed through standardised testing procedures. In this paper, we employ parametric studies to assess a range of clinically relevant factors that cause tissue heating, performing simulations with both radiofrequency (RF) and gradient coil (GC) switching fields, the latter of which is often overlooked in the literature. A series of ‘worst-case’ scenarios for both types of excitation field is discussed. In the case of GC fields, large volume implants and large plate areas with the field orientated perpendicular to the plane cause the highest heating levels, along with sequences with high rates of field switching. Implant heating from RF fields is driven primarily from the ‘antenna effect’, with thin, linear implants of resonant length resulting in the highest temperature rises. In this work, we show that simplifications may be made to the field sequence and in some cases the device geometry without significantly compromising the accuracy of the simulation results, enabling the possibility for generic estimates of the implant heating for orthopaedic device manufacturers and opportunities to simplify the safety compliance process.

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