Rapid safety assessment and mitigation of radiofrequency induced implant heating using small root mean square sensors and the sensor matrix Qs.

Berk Silemek,Oliver Speck,Johannes Petzold,Werner Hoffmann,Bernd Ittermann,Lukas Winter,Frank Seifert,Georg Rose,Harald Pfeiffer

doi:10.1002/mrm.28968

Abstract

Rapid detection and mitigation of radiofrequency (RF)-induced implant heating during MRI based on small and low-cost embedded sensors. A diode and a thermistor are embedded at the tip of an elongated mock implant. RF-induced voltages or temperature change measured by these root mean square (RMS) sensors are used to construct the sensor Q-Matrix (QS ). Hazard prediction, monitoring and parallel transmit (pTx)-based mitigation using these sensors is demonstrated in benchtop measurements at 300 MHz and within a 3T MRI. QS acquisition and mitigation can be performed in <20 ms demonstrating real-time capability. The acquisitions can be performed using safe low powers (<3 W) due to the high reading precision of the diode (126 µV) and thermistor (26 µK). The orthogonal projection method used for pTx mitigation was able to reduce the induced signals and temperatures in all 155 investigated locations. Using the QS approach in a pTx capable 3T MRI with either a two-channel body coil or an eight-channel head coil, RF-induced heating was successfully assessed, monitored and mitigated while the image quality outside the implant region was preserved. Small (<1.5 mm3 ) and low-cost (<1 €) RMS sensors embedded in an implant can provide all relevant information to predict, monitor and mitigate RF-induced heating in implants, while preserving image quality. The proposed pTx-based QS approach is independent of simulations or in vitro testing and therefore complements these existing safety assessments.

Highlights

Radiofrequency (RF) -induced heating of implants or interventional devices during an MRI exam is a well-k nown safety risk affecting a large patient population.[1,2] The current approach to safely scan patients with implants relies on off- line testing as described in ISO/TS 10974 or ASTM F2182.3,4 These standards provide reliable procedures to ensure the safety of patients with implants in most scenarios
RF-induced voltages or temperature change measured by these root mean square (RMS) sensors are used to construct the sensor Q-M atrix (QS)
Since the benefits of MRI frequently outweigh the risks even in patients with active implants, studies are pushing the limits toward MRI of patients with cardiac implantable electronic devices,6-10 or deep brain stimulators (DBS).11-20 reliable techniques are still needed, mitigating RF-induced implant heating with minimum detrimental effect on diagnostic image quality

Summary

Introduction

Radiofrequency (RF) -induced heating of implants or interventional devices during an MRI exam is a well-k nown safety risk affecting a large patient population.[1,2] The current approach to safely scan patients with implants relies on off- line testing as described in ISO/TS 10974 or ASTM F2182.3,4 These standards provide reliable procedures to ensure the safety of patients with implants in most scenarios. Since the benefits of MRI frequently outweigh the risks even in patients with active implants, studies are pushing the limits toward MRI of patients with cardiac implantable electronic devices,6-10 or deep brain stimulators (DBS).11-20 reliable techniques are still needed, mitigating RF-induced implant heating with minimum detrimental effect on diagnostic image quality Many such approaches are based on parallel transmission (pTx) systems, which were introduced two decades ago, increasing the degree of freedom to shape the RF excitation field in the target region of interest.21-24 Adjusting the amplitudes and phases of RF pulses transmitted to individual channels of a pTx coil properly, the background E-field along and the RF-induced currents within an implant can be minimized. RF-induced heating is substantially reduced while simultaneously the overall image quality can be largely preserved.1,25-32 To optimize the RF excitation for heating reduction, the induced current, field or temperature needs to be known

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