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24/03/2021

Experimental and Numerical Optimization Modelling to Reduce Radiofrequency-Induced Risks of Magnetic Resonance Examinations on Leaded Implants

Juan Córcoles, Aiping Yao, and Niels Kuster, Applied Mathematical Modelling, August 2021, Volume 96, pages 177-188, online 09 March 2021; doi: 10.1016/j.apm.2021.02.036

Convex formulations can be used to reduce the local specific absorption rate (SAR) enhancement by active medical implants of radiofrequency (RF) fields in magnetic resonance (MR) examinations while minimizing the loss of image quality. This paper demonstrates that such an optimization methodology, previously presented for strictly computational models, can be extended to a hybrid scheme using experimentally determined implant models and precomputed fields, which can enable quasi real-time exposure optimization. The methodology determines the optimum RF field shimming condition by considering both the reduction of specific absorption rate enhancement at the tip of the implant lead, created by the interaction of the RF fields tangential to the implant trajectory with the characteristic response of the implant, and the preservation of magnetic field homogeneity, which correlates to image quality. The inputs to this workflow are those required for each implant by standard ISO 10974 evaluation, namely the validated piecewise transfer function of the implant, the clinical routing within the patient, and the pre-computed numerical estimation of patient exposure without the implant. Optimized incident field conditions were computed to meet a range of numerical targets for specific absorption rate reduction, stepping down percentagewise from the maximum field homogeneity to the minimum exposure enhancement, for a generic implant with a flexible wire in a standard benchtop RF coil and phantom. Measurements of the corresponding specific absorption rate enhancements validated the predictions from the optimization approach within the combined confidence interval.

The scientific and technical impact of the study can be summarized as:

  • RF-induced deposited power by elongated implants during MR examination can be substantially reduced by modifying exposure conditions through RF-shimming while maintaining good magnetic field homogeneity with a hybrid numerical-experimental methodology
  • The novel approach combines the measured transfer function of the implant with the computationally determined fields induced by the MR coil, and applies numerical optimization to yield the desired exposure configurations and determine safe and risky scenarios, thus paving the way for future incorporation into MR technology, e.g. performing quasi real-time optimization for multi-transmit scanners
  • The methodology was experimentally validated showing that the measurement of the absorbed power at the electrode of the implant for the considered scenarios agreed to <± 0.7 dB with the numerically predicted values (well within the combined confidence interval)