In magnetic resonance examination, the interaction between implants and the radio frequency (RF) fields induces heating in human tissue and may cause tissue damage. To assess the RF-induced heating of implants, three steps should be executed, including electromagnetic model construction, electromagnetic model validation, and virtual human body simulations. The crucial step of assessing RF-induced heating involves the construction of a test environment for electromagnetic model validation. In this study, a hardware environment, comprised of a RF generation system, electromagnetic field measurement system, and a robotic arm positioning system, was established. Furthermore, an automated control software environment was developed using a Python-based software development platform to enable the creation of a high-precision automated integrated test environment. The results indicate that the electric field generated in this test environment aligns well with the simulated electric field, making it suitable for assessing the RF-induced heating effects of implants.
在磁共振检查中，植入物与射频场相互作用会引起周围人体组织发热，甚至造成组织损伤。评估植入物射频热效应需要进行植入物电磁建模、电磁模型验证和虚拟人体仿真。其中，构建用于电磁模型验证的测试环境是射频热效应评估的重要环节。研究建立了包括射频发生系统、电磁场量测系统和机械臂定位系统的硬件环境，使用基于Python的软件开发平台实现了自动化控制软件环境，形成了高精度、自动化的集成测试环境。研究结果表明，该测试环境形成的电场与计算机仿真得到的电场具有较好的一致性，可以用于植入物射频热效应评估。.

A direct comparison of the impact of RF coil design under specific absorption rate andB1+rmslimitations are investigated and quantified using RF coils of different geometries and topologies at 64 MHz and 128 MHz. The RF-inducedin vivoelectric field and power deposition of a 50 cm long pacemaker and 55 cm long deep brain stimulator (DBS) are evaluated within two anatomical models exposed with these RF coils. The associated uncertainty is quantified and analyzed under a fixedB1+rmsincident and normal operating mode. For a fixedB1+rmsincident, thein vivoincident field shows a much higher uncertainty (>5.6 dB) to the RF coil diameter compared to other design parameters (e.g. <2.2 dB for coil length and topology), while the associated uncertainty reduced greatly (e.g. <1.5 dB) under normal operating mode exposure. Similar uncertainties are observed in the power deposition near the pacemaker and DBS electrode. Compared to the normal operating mode, applying a fixedB1+rmsfield to the untested implant will lead to a large variation in the induced incident and power deposition of the implant, as a result, a larger safe margin when different coil designs (e.g. coil diameter) are considered.

BACKGROUND: The imaging of patients with implanted electrically-conductive devices via magnetic resonance imaging at ultra-high fields is hampered by uncertainties relating to the potential for inducing tissue heating adjacent to the implant due to coupling of energy from the incident electromagnetic field into the implant. Existing data in the peer-reviewed literature of comparisons across field strengths of tissue heating and its surrogate, the specific absorption rate (SAR), is scarce and contradictory, leading to further doubts pertaining to the safety of imaging patients with such devices.
OBJECTIVE: The radiofrequency-induced SAR adjacent to orthopedic screws of varying length and at frequencies of 64 to 498 MHz was investigated via full-wave electromagnetic simulations, to provide an accurate comparison of SAR across MRI field strengths.
METHODS: Dipole antennas were used for RF transmission to achieve a uniform electric field tangential to the screws located 120 mm above the antenna midpoints, embedded in a bone-mimicking material. The input power to the antennas was constrained to achieve the following targets without the screw present: (i) E = 100 V/m, (ii) B1 +  = 2 μT, and (iii) global-average-SAR = 3.2 W/kg. Simulations were performed with a spatial resolution of 0.2 mm in the volume surrounding the screws, resulting in 76-137 MCells, noting the maximum 1 g-averaged SAR value in each case. Simulations were repeated at 128 and 297 MHz for screws embedded in muscle tissue.
RESULTS: The peak SAR, occurring at the resonant screw length, substantially increased as the frequency decreased when the input power to the dipole antenna was constrained to achieve constant electric field in background tissue at the screws\' locations. A similar pattern was observed when constraining input power to achieve constant B1 + and global-average-SAR. The dielectric properties of the tissue in which the screws were embedded dominated the SAR comparisons between 297 and 128 MHz.
CONCLUSIONS: The study design allowed for a direct comparison to be performed of SAR across frequencies and implant lengths without the confounding effect of variable incident electric field. Lower frequencies produced substantially larger SAR values for implants approaching the resonant length for the worst-case uniform incident electric field along the screws\' length. The data may inform risk-benefit assessments for imaging patients with orthopedic implants at the new clinical field strength of 7 Tesla.

OBJECTIVE: Radiofrequency (RF) exposure during MR examination is limited by IEC 60601-2-33 to prevent thermal hazards to patients. These limits are also the basis to derive the maximum induced field for the demonstration of MR safety of implants per ISO/TS 10974 (2018). One limit is the head-averaged specific absorption rate (SAR), for which the head extent is defined differently by MR and implant vendors. The purpose of this technical note is to inform MR safety stakeholders on the sensitivity of safety evaluations due to different head extent definitions.
METHODS: RF distributions from the validated MRIxViP exposure libraries of 12 high-resolution human anatomical models were scaled to the normative SAR limits for different definitions of the head extent to compare the corresponding induced SAR and electric (E-)field levels.
RESULTS: The definitions of the head extent used by major implant vendors and defined in ISO/TS 10974 (2018) are larger than those introduced in IEC 60601-2-33 (2022), resulting in lower RF head exposure by up to 2.4 dB (factor 1.7). Other proposed definitions of the head result in intermediate values.
CONCLUSIONS: The different head extents result in different maximum RF exposures affecting the risk assessment by up to a factor of 1.7. The results of this study can be used to estimate the additional uncertainty in safety assessments. Future revisions of MR standards should eliminate this inconsistency.

Purpose. The goal of this study was to develop and validate a computational model that can accurately predict the influence of flow on the temperature rise near a peripheral vascular stent during magnetic resonance imaging (MRI).Methods. Computational modeling and simulation of radio frequency (RF) induced heating of a vascular stent during MRI at 3.0 T was developed and validated with flow phantom experiments. The maximum temperature rise of the stent was measured as a function of physiologically relevant flow rates.Results. A significant difference was not identified between the experiment and simulation (P > 0.05). The temperature rise of the stent during MRI was over 10 °C without flow, and was reduced by 5 °C with a flow rate of only 58 ml min-1, corresponding to a reduction of CEM43from 45 min to less than 1 min.Conclusion. The computer model developed in this study was validated with experimental measurements, and accurately predicted the influence of flow on the RF-induced temperature rise of a vascular stent during MRI. Furthermore, the results of this study demonstrate that relatively low flow rates significantly reduce the temperature rise of a stent and the surrounding medium during RF-induced heating under typical scanning power and physiologically relevant conditions.

To improve intraoral transverse loop coil design for high-resolution dental MRI.
The transverse intraoral loop coil (tLoop) was modified (mtLoop) by overlapping the feed port conductors, bending the posterior section, introducing a parallel plate capacitor, optimizing the insulation thickness, and using it in receive-only mode. In addition, an MR-silent insulation was introduced. The performances of the mtLoop and tLoop coils were compared in terms of sensitivity, image SNR, and eddy currents using electromagnetic simulations and MRI measurements at 3T.
The receive-only mode of the mtLoop increases the sensitivity at the apices of the roots, and the overlapped feed port design eliminated signal voids along the incisors. The bent posterior section with the parallel plate capacitor reduced the unwanted signal of the tongue by a factor of 2.3 in the selected region off interest and lowered the eddy currents by 10%. The proposed new coil provided higher SNR by elevenfold and 2.5-fold at the incisors and apices of the molar roots within the selected regions of interest, respectively, in the experiments, as well as improved comfort. Optimal insulation thickness was determined as 1 mm. With the mtLoop, a (250 μm)3 isotropic resolution of the dental arch could be realized using a UTE sequence within 2 min total acquisition time. A T2 -SPACE protocol with (350 μm)2 in-plane resolution was also demonstrated.
The proposed new coil offers higher SNR at the incisors and apices of the molar roots, less unwanted signals from tongue, lower eddy currents, and improved patient comfort.

OBJECTIVE: Low-field MRI systems are expected to cause less RF heating in conventional interventional devices due to lower Larmor frequency. We systematically evaluate RF-induced heating of commonly used intravascular devices at the Larmor frequency of a 0.55 T system (23.66 MHz) with a focus on the effect of patient size, target organ, and device position on maximum temperature rise.
METHODS: To assess RF-induced heating, high-resolution measurements of the electric field, temperature, and transfer function were combined. Realistic device trajectories were derived from vascular models to evaluate the variation of the temperature increase as a function of the device trajectory. At a low-field RF test bench, the effects of patient size and positioning, target organ (liver and heart) and body coil type were measured for six commonly used interventional devices (two guidewires, two catheters, an applicator and a biopsy needle).
RESULTS: Electric field mapping shows that the hotspots are not necessarily localized at the device tip. Of all procedures, the liver catheterizations showed the lowest heating, and a modification of the transmit body coil could further reduce the temperature increase. For common commercial needles no significant heating was measured at the needle tip. Comparable local SAR values were found in the temperature measurements and the TF-based calculations.
CONCLUSIONS: At low fields, interventions with shorter insertion lengths such as hepatic catheterizations result in less RF-induced heating than coronary interventions. The maximum temperature increase depends on body coil design.

OBJECTIVE: During MR scans, abandoned leads from active implantable medical devices (AIMDs) can experience excessive heating at the lead tip, depending on the type of termination applied to the proximal contacts (proximal end treatment). The influence of different proximal end treatments (ie, [1] freely exposed in the tissue, [2] terminated with metal in contact with the tissue, or [3] capped with plastic, and thereby fully insulated, on the RF-induced lead-tip heating) are studied. A technique to ensure that MR Conditional AIMD leads remain MR Conditional even when abandoned is recommended.
METHODS: Abandoned leads from three MR Conditional AIMDs ([1] a sacral neuromodulation system, [2] a cardiac rhythm management pacemaker system, and [3] a deep brain stimulator system) were investigated in this study. The computational lead models (ie, the transfer functions) for different proximal end treatments were measured and used to assess the in vivo lead-tip heating for four virtual human models (FATS, Duke, Ella, and Billie) and compared with the lead-tip heating of the complete MR Conditional AIMD system.
RESULTS: The average and maximum lead-tip heating for abandoned leads proximally capped with metal is always lower than that from the complete AIMD system. Abandoned leads proximally insulated could lead to an average in vivo temperature rise up to 3.5 times higher than that from the complete AIMD system.
CONCLUSIONS: For the three investigated AIMDs under 1.5T MR scanning, our results indicate that RF-induced lead-tip heating of abandoned leads strongly depends on the proximal lead termination. A metallic cap applied to the proximal termination of the tested leads could significantly reduce the RF-induced lead-tip heating.

OBJECTIVE: To assess RF-induced heating hazards in 1.5T MR systems caused by body-loop postures.
METHODS: Twelve advanced high-resolution anatomically correct human body models with different body-loop postures are created based on poseable human adult male models. Numerical simulations are performed to assess the radiofrequency (RF)-induced heating of these 12 models at 11 landmarks. A customized phantom is developed to validate the numerical simulations and quantitatively analyze factors affecting the RF-induced heating, eg, the contact area, the loop size, and the loading position. The RF-induced heating inside three differently posed phantoms is measured.
RESULTS: The RF-induced heating from the body-loop postures can be up to 11 times higher than that from the original posture. The RF-induced heating increases with increasing body-loop size and decreasing contact area. The magnetic flux increases when the body-loop center and the RF coil isocenter are close to each other, leading to increased RF-induced heating. An air gap created in the body loop or generating a polarized magnetic field parallel to the body loop can reduce the heating by a factor of three at least. Experimental measurements are provided, validating the correctness of the numerical results.
CONCLUSIONS: Safe patient posture during MR examinations is recommended with the use of insulation materials to prevent loop formation and consequently avoiding high RF-induced heating. If body loops cannot be avoided, the body loop should be placed outside the RF transmitting coil. In addition, linear polarization with magnetic fields parallel to the body loop can be used to circumvent high RF-induced heating.

This paper studies the RF-induced heating for modular external fixation devices applied on the leg regions of the human bodies. Through numerical investigations of RF-induced heating related to different patient orientations, landmark positions, and device positions under 1.5T and 3T MRI systems, simple and practical methods to reduce RF-induced heating are recommended.
Numerical simulations using a full-wave electromagnetic solver based on the finite-difference time-domain method were performed to characterize the effects of patient orientations (head-first/feet-first), landmark positions (the scanning area of the patient), and device positions (device on left or right leg) on the RF-induced heating of the external fixation devices. The G32 coil design and three anatomical human models (Duke model, Ella model, and Fats model) were adopted to model the MRI RF coil and the patients.
The relative positions of the patient, device, and coil can significantly affect the RF-induced heating. With other conditions remaining the same, changing the device position or patient orientation can lead to a peak 1-g averaged spatial absorption ratio variation of a factor around four. By changing the landmark position and the patient orientation, the RF-induced heating can be reduced from 1323.6 W/kg to 217.5 W/kg for the specific scanning situations studied.
Patient orientations, landmark positions, and device positions influence the RF-induced heating of modular external fixation devices at 1.5 T and 3 T. These features can be used to reduce the RF-induced heating during MRI simply and practically.