Assessing radiofrequency safety of active implants by measuring induced radiofrequency currents using MRI.

Purpose: During MRI in the presence of active wire-like implants, such as deep brain stimulation leads, there is a risk of thermal lesions in tissues adjacent to implant contacts due to radiofrequency currents induced in the wire. Currently, there is no established method to evaluate the radiofrequency (RF) safety of an implant in situ, due to complex interactions between the implant and the electric field inside the patient during MRI. This article presents a method to quantify the RF current in an implant using MRI acquisitions at very low SAR.

Theory and methods: To measure RF current in situ, a modified $B1$ -mapping sequence is proposed to image the associated perturbation of the $B_1^{+}$ field. A forward signal model links the RF current intensity to the MRI signal and is used to fit the RF current from acquired data. Electromagnetic simulations and experiments on a homogeneous phantom are presented for simplified and real implant wires to validate the method.

Results: The presented model can correctly reconstruct RF current amplitudes from field maps obtained with detailed electromagnetic simulations, with a normalized RMS error of 4.7%. Phantom experiments show a good linearity between the square of the current measured by MRI and temperature increase ( $R^2>0.91$ ), demonstrating that the RF current measurements quantitatively represent the effective heating.

Conclusion: A method has been developed to quantify the RF current in situ from MRI signals. This method enables to predict the individual heating risk for other MRI sequences performed in the same scanning session.