Methods and Strategies for Evaluating MRI Rf-Induced Heating for Implanted Metallic Stents



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Magnetic resonant imaging (MRI) is a widely-used medical imaging technology, which however incurs some safety hazards with the existence of conductive implanted medical devices, and radio frequency (RF)-induced heating is one of the major concerns. As one of the general metallic implanted devices, however stents are not fully studied on its RF-induced heating behavior. In this work, evaluations of MRI RF-induced heating are carried out on implanted medical stents. Due to various designs of stents, several typical generic stent models are developed and the effects of different design parameters, e.g. stent pattern, length, diameter, structural connectivity, etc., on the RF-induced heating are investigated in the American Society for Testing and Materials (ASTM) phantom. Experimental measurements are conducted for validation. Then four search strategies are introduced for assessing the worst-case RF-induced heating based on the simulated results of the multi-configuration devices, and a stent is studied to investigate the feasibility of the search strategies. It is found that the strategy called single sequential search with the second worst case validations and the multi-iteration sequential search strategy can decrease the probability of failed worst case from 45% to 34.8% and 0% for the stent respectively. Afterwards, a neural network is applied to fast predict the RF-induced heating of stents. Furthermore, the stents are implanted into the anatomically correct adult male model Duke for in-vivo evaluation. Stents are placed into the thoracic aorta, esophagus and colon respectively. The comparison between the in-vivo results and in-vitro results indicates the failure of utilizing the ASTM phantom to conservatively estimate the RF-induced SAR in the colon. It is found that the in-vivo incident electric field is different from that in the ASTM phantom. Moreover, we propose the inhomogeneous phantom to study the effects of the medium on RF-induced heating. Significant differences in SAR are observed for mediums having different physiological tubular structures. However, the inhomogeneous phantom can only better estimate the case of the stent in the esophagus than the ASTM phantom regarding the in-vivo situation representation. For the stent in the colon and the thoracic aorta, the inhomogeneous phantom shows similar results to the ASTM phantom, which makes no improvement in representing the in-vivo situations, and more studies are needed.



MRI, RF-induced heating, Stents


Portions of this document appear in: Jianfeng Zheng, Xiaohe Ji, Wolfgang Kainz, and Ji Chen. "Study on Search Strategies for Assessing the Worst-case RF-induced Heating for Multi-Configuration Implant System under MRI." IEEE Transactions on Electromagnetic Compatibility. (2018). And in: Xiaohe Ji, Jianfeng Zheng, Rui Yang, Wolfgang Kainz, and Ji Chen. "Evaluations of the MRI RF-Induced Heating for Helical Stents Under a 1.5 T MRI System." IEEE Transactions on Electromagnetic Compatibility (2018). And in: Xiaohe Ji, Jianfeng Zheng, and Ji Chen. "Numerical evaluation of RF-induced heating for various esophageal stent designs under MRI 1.5 Tesla system." Electromagnetic biology and medicine 36, no. 4 (2017): 379-386. And in: Li, Dawei, Xiaohe Ji, Jianfeng Zheng, Changwang Pan, Ji Chen, and Wolfgang Kainz. "A Novel Design of Implantable Esophageal Stent to Reduce the MRI RF-Induced Heating." IEEE Transactions on Electromagnetic Compatibility (2016).