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Real-time safety assessment and mitigation of RF induced implant heating with parallel transmission and low-cost RMS sensors.1Physikalisch-Technische Bundesanstalt, Berlin, Germany
Synopsis
In this work, we implement a fast and accurate safety monitoring and prediction for pTx based mitigation of implant heating using fast, cheap and easy to implement rms sensors. The proof of concept is demonstrated with a guidewire substitute in an 8-channel pTx RF coil setup at 300MHz. The signals from the field sensor and temperature sensor at the guidewire tip correlated well. The pTx mitigation obtained within 25.6ms showed significant heating reduction verified by temperature measurements. The method does not require additional simulations or in vitro testing and is potentially applicable in a patient and exam specific clinical setting.
Introduction
Simulation based Q-matrices have been introduced as a fast method to calculate and predict RF-induced heating in parallel transmission(pTx)1 and have been extended to pTx-based mitigation in implants2. While simulation studies made a strong case for utilizing pTx in this context3-5, translating these approaches to patient-and exam-specific scenarios proves challenging. Sensors in and around the implant, on the other hand, could provide fast, patient-specific safety information by measuring the currents6, fields7 or temperatures8 directly at the most critical locations. Real-time pTx mitigation using phase-sensitive E-field sensors was shown to be feasible7, but such sensors are bulky and expensive, preventing widespread uptake.In this work, we implement a fast and accurate safety monitoring and hazard prediction for pTx-based mitigation of implant heating using diodes as fast, cheap and easy to implement root-mean-square (rms), i.e. not phase sensitive, sensors. In analogy to the Q-matrix a so-called Diode-matrix (D-matrix) is presented, making a real-time mitigation and prediction tool available. The proof of concept is demonstrated with a guidewire substitute in an 8-channel pTx -coil setup at 300MHz.Methods
Instrumentation: An open source9 implant safety testbed with an 8-channel 7T pTx RF coil7 and a cylindrical PVP based phantom is used for the benchtop experiments (Fig.1a,b). A semi-rigid coaxial cable with an uninsulated tip is used as a guidewire substitute(Fig.1c). A Schottky diode (MMDL101T1G,ON-Semiconductor) is connected over the cathode to the inner conductor at the tip of the guidewire and a 5mm copper wire soldered to the anode serves as an antenna to pick up more E-field signal in the benchtop setup with 20W peak power (Fig. 1c,d). This configuration allows to pick up (positive) E-fields and measure the induced voltage. To measure corresponding temperature changes, an NTC thermistor (NCP18XH103F03RB, Murata) is connected between inner (diode cathode) and outer conductor at the guidewire tip (Fig.1d). The diode voltages are measured over a 50Ω resistor at the other end of the cable with a 14-bit ADC (4MHz sampling frequency)The thermistor measurements are performed where, ADC connection is exchanged with a digital multimeter and a high input resistance (Keithley 2000, Tektronix)(Fig.1).D-matrix: Induced tip voltages are recorded in response to constant-amplitude RF pulse applied to two of the eight coil elements. All combinations of two different pTx channels are probed by transmitting first two in-phase ($$$\Delta \varphi = 0^{o}$$$) and then two out-of-phase ($$$\Delta \varphi = 90^{o}$$$) pulses constant amplitude. The data is sorted in a Hermitian D-matrix, which is then used to calculate a “worst case”(WC) vector, defined as the eigenvector with the highest eigenvalue2,7. Knowledge of this WC vector is useful for general safety assessments or for practical mitigation strategies, e.g. by marking stay-away excitation conditions2,7.
Experiments: The D-matrix was acquired with the guidewire positioned inside the PVP phantom. For each of the n2=64 channel combinations 200µs in-phase and out-of-phase RF pulses are applied followed by a 200µs idle period. To correlate between the diode and thermistor readings, the same is repeated with 2s pulses to produce sufficient heating for temperature measurements with lower cool-down contributions. Consequently, WC and a low-risk “orthogonal projection mode”2,7(OP) were determined based on D-matrix (diode). Then, RF-heating experiments (thermistor) were performed using the same voltage vectors obtained with D-Matrix.
Results
The D-matrix for the 8-channel pTx setting is acquired in 25.6ms. Induced voltages at the guidewire tip correlate well with induced temperatures under varying pTx driving conditions(Fig.2). Small differences after stronger heating, i.e. the third measurement point, deviates possibly cooling disturbance to the measurement(Fig.2). OP mode which was instantly calculated by applying the D-matrix formalism has a significantly reduced induced voltage compared to the circular polarized (CP) reference mode(Fig.3). At the given implant position, the induced steady-state voltages are 3mV, 378mV, 475mV for OP, CP and WC mode, respectively. The temperatures correspond well to the induced voltages (Fig.4). A temperature increase for OP was not detectable (<1mK). For CP and WC, it was 16mK and 25mK, respectively(Fig.4).Discussion and Conclusions
The unique possibility of a complete implant-safety assessment in 25.6 ms for an 8-channel coil makes the D-matrix extremely attractive for potential real-time applications. D-matrices for 16-channel even 32-channel pTx coils could be acquired in 0.1s and 0.4s, respectively, which is still real-time compatible. The presented data on D-matrix based pTx mitigation reproduce, e.g., the known hierarchy of WC, CP, and OP mode2,7, thus confirming the suitability of the D-matrix approach. The excellent correlation of D-matrix with the thermistor measurements performed under identical conditions, except for 2000-fold longer acquisition times, as part of the present investigation, again confirms the suitability of the D-matrix as a safety watchdog and mitigation tool. Temperature and phase sensitive E-field probes clearly provide a more comprehensive, better interpretable and in that sense more valuable information. But when speed is required, e.g. for MR-guided interventions or emergency-shutdowns, the D-matrix has a hitherto unmatched advantage. These results are encouraging, in particular since the presented sensor configuration is cheap, has a small footprint and offers two independent readings (field and temperature based) that can be attributed to assess safety in an in-vivo situation. Further experiments are needed to assess the robustness of the method with respect to locations, implant types and MR compatibility.Acknowledgements
This work was funded by the EMPIR grant 17IND01 MIMAS. The EMPIR initiative is co-funded by the European Union’s Horizon 2020 researchand innovation program and the EMPIR participating states.References
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