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Purpose: 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. Conclusion: 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. [ABSTRACT FROM AUTHOR]

Purpose: 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. Result: 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. Conclusion: 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. [ABSTRACT FROM AUTHOR]

Purpose: The paper presents a novel method to reduce the RF‐induced heating of active implantable medical devices during MRI. Methods: With the addition of an energy decoying and dissipating structure, RF energy can be redirected toward the dissipating rings through the decoying conductor. Three lead groups (45 cm‐50 cm) and 4 (50 cm‐100 cm) were studied in 1.5 Tesla MR systems by simulation and measurement, respectively. In vivo modeling was performed using human models to estimate the RF‐induced heating of an active implantable medical device for spinal cord treatment. Result: In the simulation study, it was shown that the peak 1g‐averaged specific absorption rate near the lead‐tips can be reduced by 70% to 80% compared to those from the control leads. In the experimental measurements during a 2‐min exposure test in a 1.5 Telsa MR system, the temperature rises dropped from the original 18.3℃, 25.8℃, 8.1℃, and 16.1℃ (control leads 1‐4) to 5.4℃, 6.9℃, 1.6℃, and 3.3℃ (leads 1‐4 with the energy decoying and dissipation structure). The in vivo calculation results show that the maximum induced temperature rise among all cases can be substantially reduced (up to 80%) when the energy decoying and dissipating structures were used. Conclusion: Our studies confirm the effectiveness of the novel technique for a variety of scanning scenarios. The results also indicate that the decoying conductor length, number of rings, and ring area must be carefully chosen and validated. [ABSTRACT FROM AUTHOR]

This paper proposes a locally modified phantom model to numerically assess the worst-case configuration of orthopedic implants under magnetic resonance imaging (MRI). The proposed model is developed based on the standard American Society for Testing and Materials (ASTM) phantom and bone models with cancellous or cortical materials. Three orthopedic implant families, metallic rods, a nail and screw system, and a plate and screw system, are studied. The worst-case configurations of orthopedic implants are identified inside the proposed model and ASTM phantom. These worst-case heating configurations are then implanted in a human body model to evaluate the RF-induced heating in terms of peak SAR1g. For the orthopedic implants fully inside the bone, like the rod and the nail and screw systems, the peak SAR1g values of worst-case configurations obtained from the proposed phantom model are higher than those obtained inside the ASTM phantom. For the orthopedic implants that are mainly outside the bone, such as the plate and screw system, similar worst-case configurations lead to a negligible variation of peak SAR1g inside the human body model.Clinical Relevance- The new phantom model leads to more accurate predictions of the worst-case configuration of orthopedic implants for MR conditional labeling.

The RF-induced lead-tip heating of AIMDs is related to the tangential electric field distribution along the AIMD lead paths in patients and the electromagnetic behavior (represented by the transfer function model) of the AIMDs. To evaluate the in-vivo RF-induced lead-tip heating of AIMDs using in-vitro methods, the electric field distribution is critical. In this paper, we proposed a Volume-Weighed Tissue-Cluster Model, a feasible bench method, to simplify the evaluation of the in-vivo RF-induced lead-tip heating of AIMDs. The incident electric field distribution inside this simplified model is highly correlated to that of the original inhomogeneous human body model. Compared to the RF-induced lead-tip heating results in the original model, the maximum error of the lead-tip heating in this Volume-Weighed Tissue-Cluster Model is less than 1 °C. The correlation coefficients of the temperature rise between the two models are higher than 0.997.Clinical Relevance- Simplified and accurate anatomical models can be used to emulate the in-vivo heating assessment for MRI safety.

During Magnetic Resonance (MR) scans, abandoned leads from active implantable medical devices (AIMDs) can experience excessive heating near the lead-tip, depending on the types of termination applied to the proximal end. The influence of different proximal treatments, i.e., (i) freely exposed in the tissue, (ii) capped with metallic material, and (iii) capped with plastic material on the RF-induced heating are studied. Abandoned leads from a sacral neuromodulation (SNM) system were investigated in this study. The device models, i.e., the transfer functions, for different proximal treatments were developed. These models are then used to assess the in-vivo lead-tip heating inside four virtual human models (FATS, Duke, Ella, and Billie). The RF-induced heating from these abandoned leads with different proximal end treatments are compared with the lead-tip heating of the original AIMD system. The maximum lead-tip heating for abandoned leads using metal cap at the proximal end is lower than that from the original intact AIMD system. Abandoned leads with plastic cap treatment at the proximal end will lead to an average in-vivo temperature that is 3.5 times higher than that from the original intact AIMD system. Therefore, from this study and in terms of the RF-induced heating, the abandoned leads with metallic cap treatment at the proximal end can maintain the MR conditionality of the original AIMD system.Clinical Relevance- The different treatments of proximal end of the abandoned leads from AIMD are studied to ensure that MR Conditional AIMD leads remain MR Conditional when the leads are abandoned in the patients.