Juan Córcoles, Aiping Yao, and Niels Kuster, Applied Mathematical Modelling 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: