Parallel transmission medical implant safety testbed: Real‐time mitigation of RF induced tip heating using time‐domain E‐field sensors

Abstract

Purpose To implement a modular, flexible, open‐source hardware configuration for parallel transmission (pTx) experiments on medical implant safety and to demonstrate real‐time mitigation strategies for radio frequency (RF) induced implant heating based on sensor measurements. Methods The hardware comprises a home‐built 8‐channel pTx system (scalable to 32‐channels), wideband power amplifiers and a positioning system with submillimeter precision. The orthogonal projection (OP) method is used to mitigate RF induced tip heating and to maintain sufficient B1+ for imaging. Experiments are performed at 297MHz and inside a clinical 3T MRI using 8‐channel pTx RF coils, a guidewire substitute inside a phantom with attached thermistor and time‐domain E‐field probes. Results Repeatability and precision are ~3% for E‐field measurements including guidewire repositioning, ~3% for temperature slopes and an ~6% root‐mean‐square deviation between B1+ measurements and simulations. Real‐time pTx mitigation with the OP mode reduces the E‐fields everywhere within the investigated area with a maximum reduction factor of 26 compared to the circularly polarized mode. Tip heating was measured with ~100 μK resolution and ~14 Hz sampling frequency and showed substantial reduction for the OP vs CP mode. Conclusion The pTx medical implant safety testbed presents a much‐needed flexible and modular hardware configuration for the in‐vitro assessment of implant safety, covering all field strengths from 0.5‐7 T. Sensor based real‐time mitigation strategies utilizing pTx and the OP method allow to substantially reduce RF induced implant heating while maintaining sufficient image quality without the need for a priori knowledge based on simulations or in‐vitro testing.

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

“…With this information, two methods (OP 26,29 and NM 28 ) have been applied. The OP method is based on an orthogonal projection of a vector used for imaging (eg, CP) to a worst‐case (WC) voltage vector , ie, the eigenvector with the largest eigenvalue of :…”

“…28,49 A recent study proposed the so-called "orthogonal projection" (OP) method as a pTx mitigation tool based on time-domain E-field probe measurements outside the implant. 29 The OP method projects a homogeneous excitation vector, eg, the circular polarization mode (CP) or a B + 1 -shim, onto the subspace orthogonal to a worst-case excitation (producing maximum implant heating), thus preserving imaging quality while reducing tip heating. 29 So far, these existing methods were demonstrated by using bulky time-domain sensors that could not be placed directly at the hot-spot location (ie, the implant tip).…”

“…29 The OP method projects a homogeneous excitation vector, eg, the circular polarization mode (CP) or a B + 1 -shim, onto the subspace orthogonal to a worst-case excitation (producing maximum implant heating), thus preserving imaging quality while reducing tip heating. 29 So far, these existing methods were demonstrated by using bulky time-domain sensors that could not be placed directly at the hot-spot location (ie, the implant tip). Root mean square (RMS) sensors, on the other hand, offer more flexible design options as they have smaller footprint and simpler acquisition electronics.…”

An 8 channel parallel transmit system with current sensor feedback for MRI-guided interventional applications

“…To avoid these limitations, some have proposed manipulating the incident electric fields responsible for generating RF induced currents in implanted wires [6]- [8], such that the net electric field tangential to the wire is forced to zero consequently eliminating the potential for localized heating at the tip. This is only possible using parallel transmit (pTx) MR systems which allow spatial manipulation of RF fields by altering the relative amplitude/phase of the individual transmitters.…”

Minimally invasive optical sensors for microwave-electric-field exposure measurements

“…Active sensors, which modulate a laser diode or a light-emitting diode to convert the electric signal into an optical signal, can provide GHz bandwidth (BW) with a millimeter footprint. [8][9][10] But such active components are sensitive to magnetic-fields-induced perturbations. In addition, the active probe generates significant E-field perturbation.…”