An improved method to measure transfer functions using MRI.

PURPOSE: A previously published method for MRI-based transfer function assessment makes use of the so-called transceive phase assumption (TPA). This limits its applicability to shorter leads and/or lower field strengths. A new method is presented where the background electric field is determined from both B 1 + $$ {\mathrm{B}}_1^{+} $$ - and B 1 - $$ {\mathrm{B}}_1^{-} $$ -field distributions, avoiding the TPA and making it more generally applicable. THEORY AND METHODS: These B 1 $$ {\mathrm{B}}_1 $$ -distributions are determined from a spoiled gradient echo multiflip angle acquisition. From the separated B 1 $$ {\mathrm{B}}_1 $$ -components the background electrical field and the induced current are computed. Further improvement is achieved by recasting the B 1 $$ {\mathrm{B}}_1 $$ -field model as a "magnitude squared least squares" problem. The proposed reconstruction method is used to determine transfer functions of various copper wire lengths up to 40 cm inside an elliptical ASTM phantom. The method is first tested on EM-simulated data and subsequently phantom and bench measurements are used to determine transfer functions experimentally. RESULTS: In silica reconstructions demonstrate the validity of the proposed B 1 $$ {\mathrm{B}}_1 $$ -field model resulting in highly accurate reconstructed B 1 $$ {\mathrm{B}}_1 $$ -fields, currents, incident electric fields and transfer functions. The experimental results show slight deviations in the field model, however, resulting transfer functions are accurately determined with high similarity to simulations and comparable to bench measurements. CONCLUSION: A more generally applicable method for MRI-based transfer function assessment is presented. The proposed method circumvents phase assumptions making it applicable for longer objects and/or higher field strengths. Additional improvements are implemented in the B 1 $$ {\mathrm{B}}_1 $$ -mapping method and the solution algorithm.

Modeling and measurement of lead tip heating and resonant length for implanted, insulated wires.

PURPOSE: To study implant lead tip heating because of the RF power deposition by developing mathematical models and comparing them with measurements acquired at 1.5 T and 3 T, especially to predict resonant length. THEORY AND METHODS: A simple exponential model and an adapted transmission line model for the electric field transfer function were developed. A set of wavenumbers, including that calculated from insulated antenna theory (King wavenumber) and that of the embedding medium were considered. Experiments on insulated, capped wires of varying lengths were performed to determine maximum temperature rise under RF exposure. The results are compared with model predictions from analytical expressions derived under the assumption of a constant electric field, and with those numerically calculated from spatially varying, simulated electric fields from body coil transmission. Simple expressions for the resonant length bounded between one-quarter and one-half wavelength are developed based on the roots of transcendental equations. RESULTS: The King wavenumber for both models more closely matched the experimental data with a maximum root mean square error of 9.81°C at 1.5 T and 5.71°C at 3 T compared to other wavenumbers with a maximum root mean square error of 27.52°C at 1.5 T and 22.01°C for 3 T. Resonant length was more accurately predicted compared to values solely based on the embedding medium. CONCLUSION: Analytical expressions were developed for implanted lead heating and resonant lengths under specific assumptions. The value of the wavenumber has a strong effect on the model predictions. Our work could be used to better manage implanted device lead tip heating.

Biological treatment evaluation in thermoradiotherapy: application in cervical cancer patients.

BACKGROUND: Hyperthermia treatment quality is usually evaluated by thermal (dose) parameters, though hyperthermic radiosensitization effects are also influenced by the time interval between the two modalities. This work applies biological modelling for clinical treatment evaluation of cervical cancer patients treated with radiotherapy plus hyperthermia by calculating the equivalent radiation dose (EQDRT, i.e., the dose needed for the same effect with radiation alone). Subsequent analyses evaluate the impact of logistics. METHODS: Biological treatment evaluation was performed for 58 patients treated with 23-28 fractions of 1.8-2 Gy plus 4-5 weekly hyperthermia sessions. Measured temperatures (T50) and recorded time intervals between the radiotherapy and hyperthermia sessions were used to calculate the EQDRT using an extended linear quadratic (LQ) model with hyperthermic LQ parameters based on extensive experimental data. Next, the impact of a 30-min time interval (optimized logistics) as well as a 4­h time interval (suboptimal logistics) was evaluated. RESULTS: Median average measured T50 and recorded time intervals were 41.2 °C (range 39.7-42.5 °C) and 79 min (range 34-125 min), respectively, resulting in a median total dose enhancement (D50) of 5.5 Gy (interquartile range [IQR] 4.0-6.6 Gy). For 30-min time intervals, the enhancement would increase by ~30% to 7.1 Gy (IQR 5.5-8.1 Gy; p < 0.001). In case of 4­h time intervals, an ~ 40% decrease in dose enhancement could be expected: 3.2 Gy (IQR 2.3-3.8 Gy; p < 0.001). Normal tissue enhancement was negligible (< 0.3 Gy), even for short time intervals. CONCLUSION: Biological treatment evaluation is a useful addition to standard thermal (dose) evaluation of hyperthermia treatments. Optimizing logistics to shorten time intervals seems worthwhile to improve treatment efficacy.

[Study on RF Heating of Implants in Different Conductivity Medium].

In MRI examination, RF heating of implants will affect the safety of implant wearers. The conductivity of various tissues in the human body is significantly different, and the medium conductivity will affect the distribution of the RF electric field. Therefore, it is necessary to study the RF heating of different medium conductivity. Based on the analysis of the principle of MRI RF heating, this study build the model of the bird cage coil, ASTM standard phantom and lead, and the conductivity of several typical human tissues is selected as the conductivity in the experiment. Then calculate the power deposition of the lead at 64 MHz. The results show that the medium conductivity has no effect on the distribution of electric field and power deposition, and the hot spot distribution remains unchanged under different conductivity; The smaller the conductivity is, the larger the power deposition of the lead is, and the greater the temperature rise of the lead caused by RF heating is; The change of conductivity and power deposition is approximately linear. At the limit of 2 W/kg whole body specific absorption rate(SAR), the conductivity decreases, and the wire power deposition increases sharply.

Wirelessly interfacing sensor-equipped implants and MR scanners for improved safety and imaging.

PURPOSE: To investigate a novel reduced RF heating method for imaging in the presence of active implanted medical devices (AIMDs) which employs a sensor-equipped implant that provides wireless feedback. METHODS: The implant, consisting of a generator case and a lead, measures RF-induced E $$ E $$ -fields at the implant tip using a simple sensor in the generator case and transmits these values wirelessly to the MR scanner. Based on the sensor signal alone, parallel transmission (pTx) excitation vectors were calculated to suppress tip heating and maintain image quality. A sensor-based imaging metric was introduced to assess the image quality. The methodology was studied at 7T in testbed experiments, and at a 3T scanner in an ASTM phantom containing AIMDs instrumented with six realistic deep brain stimulation (DBS) lead configurations adapted from patients. RESULTS: The implant successfully measured RF-induced E $$ E $$ -fields (Pearson correlation coefficient squared [R2 ] = 0.93) and temperature rises (R2 = 0.95) at the implant tip. The implant acquired the relevant data needed to calculate the pTx excitation vectors and transmitted them wirelessly to the MR scanner within a single shot RF sequence (<60 ms). Temperature rises for six realistic DBS lead configurations were reduced to 0.03-0.14 K for heating suppression modes compared to 0.52-3.33 K for the worst-case heating, while imaging quality remained comparable (five of six lead imaging scores were ≥0.80/1.00) to conventional circular polarization (CP) images. CONCLUSION: Implants with sensors that can communicate with an MR scanner can substantially improve safety for patients in a fast and automated manner, easing the current burden for MR personnel.

Modifying the trajectory of epicardial leads can substantially reduce MRI-induced RF heating in pediatric patients with a cardiac implantable electronic device at 1.5T.

PURPOSE: After epicardial cardiac implantable electronic devices are implanted in pediatric patients, they become ineligible to receive MRI exams due to an elevated risk of RF heating. We investigated whether simple modifications in the trajectories of epicardial leads could substantially and reliably reduce RF heating during MRI at 1.5 T, with benefits extending to abandoned leads. METHODS: Electromagnetic simulations were performed to assess RF heating of two common 35-cm epicardial lead trajectories exhibiting different degrees of coupling with MRI incident electric fields. Experiments in anthropomorphic phantoms implanted with commercial cardiac implantable electronic devices confirmed the findings. Both electromagnetic simulations and experimental measurements were performed using head-first and feet-first positioning and various landmarks. Transfer function approach was used to assess the performance of suggested modifications in realistic body models. RESULTS: Simulations (head-first, chest landmark) of a 35-cm epicardial lead with a trajectory where the excess length of the lead was looped and placed on the inferior surface of the heart showed an 87-fold reduction in the 0.1 g-averaged specific absorption rate compared with the lead where the excess length was looped on the anterior surface. Repeated experiments with a commercial epicardial device confirmed this. For fully implanted systems following low-specific absorption rate trajectories, there was a 16-fold reduction in the average temperature rise and a 28-fold reduction for abandoned leads. The transfer function method predicted a 7-fold reduction in the RF heating in 336 realistic scenarios. CONCLUSION: Surgical modification of epicardial lead trajectory can substantially reduce RF heating at 1.5 T, with benefits extending to abandoned leads.

Developing radio frequency (RF) heating protocol in packed tofu processing by computer simulation.

Packed tofu was produced by reheating the mixture of preheated soymilk and coagulant in a sealed container. This study aimed to replace the conventional heating method with RF heating during the reheating of soymilk for packed tofu production. In this study, dielectric properties (DPs), thermal properties (TPs), and rheological properties of soymilk were determined. A mathematical model was developed to simulate the RF heating process of soymilk to determine the appropriate packaging geometry. Water holding capacity (WHC), texture analysis, color measurement, and microstructure observation were performed to evaluate the quality of RF-heated packed tofu. Results showed that soymilk added with Glucono-Delta-Lactone (GDL) coagulated at the temperature above 60 °C, and the loss factor (ε″) was slightly reduced when soymilk was converted to tofu at coagulation temperature. Based on the simulation results, the cylindrical vessel (φ50 mm × 100 mm) was chosen as the soymilk container for desired heating rate (5.9 °C/min) and uniformity (λ = 0.0065, 0.0069, 0.0016 for top, middle, and bottom layers). The texture analysis revealed that the hardness and chewiness of packed tofu prepared by RF heating were enhanced (maximum 1.36 times and 1.21 times) compared with commercial packed tofu, while the springiness were not significantly changed. Furthermore, the denser network structure was observed inside RF-heated packed tofu by SEM. These results indicated that packed tofu prepared by RF heating was of higher gel strength and sensory quality. RF heating has the potential to be applied in packed tofu production.

Application of Machine learning to predict RF heating of cardiac leads during magnetic resonance imaging at 1.5 T and 3 T: A simulation study.

Predicting magnetic resonance imaging (MRI)-induced heating of elongated conductive implants, such as leads in cardiovascular implantable electronic devices, is essential to assessing patient safety. Phantom experiments have traditionally been used to estimate radio-frequency (RF) heating of implants, but they are time-consuming. Recently, machine learning has shown promise for fast prediction of RF heating of orthopaedic implants when the implant position within the MRI RF coil was predetermined. We explored whether deep learning could be applied to predict RF heating of conductive leads with variable positions and orientations during MRI at 1.5 T and 3 T. Models of 600 cardiac leads with clinically relevant trajectories were generated, and electromagnetic simulations were performed to calculate the maximum of the 1 g-averaged specific absorption rate (SAR) of RF energy at the tips of lead models during MRI at 1.5 T and 3 T. Neural networks were trained to predict the maximum SAR at the lead tip from the knowledge of the coordinates of points along the lead trajectory. Despite the large range of SAR values (∼230 W/kg to âˆ¼ 3200 W/kg and âˆ¼ 10 W/kg to âˆ¼ 3300 W/kg), the root- mean-square error of the predicted vs ground truth SAR remained at 223 W/kg and 206 W/kg, with the R2 scores of 0.89 and 0.85 on the testing set for 1.5 T and 3 T models, respectively. The results suggest that machine learning is a promising approach for fast assessment of RF heating of lead-like implants when only the knowledge of the lead geometry and MRI RF coil features are in hand.

Towards an integrated radiofrequency safety concept for implant carriers in MRI based on sensor-equipped implants and parallel transmission.

To protect implant carriers in MRI from excessive radiofrequency (RF) heating it has previously been suggested to assess that hazard via sensors on the implant. Other work recommended parallel transmission (pTx) to actively mitigate implant-related heating. Here, both ideas are integrated into one comprehensive safety concept where native pTx safety (without implant) is ensured by state-of-the-art field simulations and the implant-specific hazard is quantified in situ using physical sensors. The concept is demonstrated by electromagnetic simulations performed on a human voxel model with a simplified spinal-cord implant in an eight-channel pTx body coil at 3 T . To integrate implant and native safety, the sensor signal must be calibrated in terms of an established safety metric (e.g., specific absorption rate [SAR]). Virtual experiments show that E -field and implant-current sensors are well suited for this purpose, while temperature sensors require some caution, and B 1 probes are inadequate. Based on an implant sensor matrix Q s , constructed in situ from sensor readings, and precomputed native SAR limits, a vector space of safe RF excitations is determined where both global (native) and local (implant-related) safety requirements are satisfied. Within this safe-excitation subspace, the solution with the best image quality in terms of B 1 + magnitude and homogeneity is then found by a straightforward optimization algorithm. In the investigated example, the optimized pTx shim provides a 3-fold higher mean B 1 + magnitude compared with circularly polarized excitation for a maximum implant-related temperature increase ∆ T imp ≤ 1 K . To date, sensor-equipped implants interfaced to a pTx scanner exist as demonstrator items in research labs, but commercial devices are not yet within sight. This paper aims to demonstrate the significant benefits of such an approach and how this could impact implant-related RF safety in MRI. Today, the responsibility for safe implant scanning lies with the implant manufacturer and the MRI operator; within the sensor concept, the MRI manufacturer would assume much of the operator's current responsibility.

Study on RF Heating of Implants in Different Conductivity Medium / 中国医疗器械杂志

In MRI examination, RF heating of implants will affect the safety of implant wearers. The conductivity of various tissues in the human body is significantly different, and the medium conductivity will affect the distribution of the RF electric field. Therefore, it is necessary to study the RF heating of different medium conductivity. Based on the analysis of the principle of MRI RF heating, this study build the model of the bird cage coil, ASTM standard phantom and lead, and the conductivity of several typical human tissues is selected as the conductivity in the experiment. Then calculate the power deposition of the lead at 64 MHz. The results show that the medium conductivity has no effect on the distribution of electric field and power deposition, and the hot spot distribution remains unchanged under different conductivity; The smaller the conductivity is, the larger the power deposition of the lead is, and the greater the temperature rise of the lead caused by RF heating is; The change of conductivity and power deposition is approximately linear. At the limit of 2 W/kg whole body specific absorption rate(SAR), the conductivity decreases, and the wire power deposition increases sharply.

Radio Frequency Heating of Washable Conductive Textiles for Bacteria and Virus Inactivation.

The ongoing COVID-19 pandemic has increased the use of single-use medical fabrics such as surgical masks, respirators, and other personal protective equipment (PPE), which have faced worldwide supply chain shortages. Reusable PPE is desirable in light of such shortages; however, the use of reusable PPE is largely restricted by the difficulty of rapid sterilization. In this work, we demonstrate successful bacterial and viral inactivation through remote and rapid radio frequency (RF) heating of conductive textiles. The RF heating behavior of conductive polymer-coated fabrics was measured for several different fabrics and coating compositions. Next, to determine the robustness and repeatability of this heating response, we investigated the textile's RF heating response after multiple detergent washes. Finally, we show a rapid reduction of bacteria and virus by RF heating our conductive fabric. 99.9% of methicillin-resistant Staphylococcus aureus (MRSA) was removed from our conductive fabrics after only 10 min of RF heating; human cytomegalovirus (HCMV) was completely sterilized after 5 min of RF heating. These results demonstrate that RF heating conductive polymer-coated fabrics offer new opportunities for applications of conductive textiles in the medical and/or electronic fields.

Sterilizing Ready-to-Eat Poached Spicy Pork Slices Using a New Device: Combined Radio Frequency Energy and Superheated Water.

In this study, a new device was used to inactivate G. stearothermophilus spores in ready-to-eat (RTE) poached spicy pork slices (PSPS) applying radio frequency (RF) energy (27.12 MHz, 6 kW) and superheated water (SW) simultaneously. The cold spot in the PSPS sample was determined. The effects of electrode gap and SW temperature on heating rate, spore inactivation, physiochemical properties (water loss, texture, and oxidation), sensory properties, and SEM of samples were investigated. The cold spot lies in the geometric center of the soup. The heating rate increased with increasing electrode gap and hit a peak under 190 mm. Radio frequency combined superheated water (RFSW) sterilization greatly decreased the come-up time (CUT) compared with SW sterilization, and a 5 log reduction in G. stearothermophilus spores was achieved. RFSW sterilization under 170 mm electrode gap reduced the water loss, thermal damage of texture, oxidation, and tissues and cells of the sample, and kept a better sensory evaluation. RFSW sterilization has great potential in solid or semisolid food processing engineering.

Safety of intracranial electroencephalography during functional electromagnetic resonance imaging in humans at 1.5 tesla using a head transmit RF coil: Histopathological and heat-shock immunohistochemistry observations.

OBJECTIVES: Simultaneous intracranial EEG and functional MRI (icEEG-fMRI) recordings in humans, whereby EEG is recorded from electrodes implanted inside the cranium during fMRI scanning, were made possible following safety studies on test phantoms and our specification of a rigorous data acquisition protocol. In parallel with this work, other investigations in our laboratory revealed the damage caused by the EEG electrode implantation procedure at the cellular level. The purpose of this report is to further explore the safety of performing MRI, including simultaneous icEEG-fMRI data acquisitions, in the presence of implanted intra-cranial EEG electrodes, by presenting some histopathological and heat-shock immunopositive labeling observations in surgical tissue samples from patients who underwent the scanning procedure. METHODS: We performed histopathology and heat shock protein expression analyses on surgical tissue samples from nine patients who had been implanted with icEEG electrodes. Three patients underwent icEEG-fMRI and structural MRI (sMRI); three underwent sMRI only, all at similar time points after icEEG implantation; and three who did not undergo functional or sMRI with icEEG electrodes. RESULTS: The histopathological findings from the three patients who underwent icEEG-fMRI were similar to those who did not, in that they showed no evidence of additional damage in the vicinity of the electrodes, compared to cases who had no MRI with implanted icEEG electrodes. This finding was similar to our observations in patients who only underwent sMRI with implanted icEEG electrodes. CONCLUSION: This work provides unique evidence on the safety of functional MRI in the presence of implanted EEG electrodes. In the cases studied, icEEG-fMRI performed in accordance with our protocol based on low-SAR (≤0.1 W/kg) sequences at 1.5T using a head-transmit RF coil, did not result in measurable additional damage to the brain tissue in the vicinity of implanted electrodes. Furthermore, while one cannot generalize the results of this study beyond the specific electrode implantation and scanning conditions described herein, we submit that our approach is a useful framework for the post-hoc safety assessment of MR scanning with brain implants.

Individualized and accurate SAR characterization method based on equivalent circuit model for MRI system.

PURPOSE: To protect patients from RF heating in MRI scan, this work proposes an accurate and patient-specific whole-body specific absorption rate (SAR) characterization method based on an equivalent circuit model. Compared to the standard pulse energy method defined in NEMA MS 8-2016, this method avoids the complexity of integrating flux loops and has the potential to be easily implemented in MRI scanners. THEORY AND METHODS: In this study, we use an equivalent parallel circuit to model the power distribution on the transmit coil and subject. The coil and subject equivalent resistances are fitted by the frequency response functions of reflection coefficient and are thereafter used to calculate the power ratio between them. To assess the accuracy of this method, we measured the subject absorbed power of 2 phantoms and 5 volunteers and compared it with the standard pulse energy method with flux loops. RESULTS: The resistances, resonant frequencies, and quality factors of the transmit coil are fitted with the equivalent circuit model in both unloaded and loaded conditions. Whole-body SAR of 5 volunteers is measured at 2 different landmarks. In addition, the relationship between SAR and the working frequencies of the transmit coil is measured and analyzed. The subject absorbed power measured by the proposed method demonstrates good accuracy (RMS error and maximum error of 3.77% and 9.47%, respectively) relative to the flux loop method. CONCLUSION: The equivalent circuit model-based method enables individualized, accurate, and simplified SAR characterization for clinical applications and research with moderate implementation complexity.

Effect of inter-electrode RF coupling on heating patterns of wire-like conducting implants in MRI.

PURPOSE: In this study, the effects of RF coupling on the magnitude and spatial patterns of RF-induced heating near multiple wire-like conducting implants (such as simultaneous electrical stimulation of stereoelectroencephalography electrodes) during MRI were assessed. METHODS: Simulations and experimental measurements of RF-induced temperature increases near partially immersed wire-like conductors were performed using a phantom with a transmit/receive head coil on a 3T MRI system. The conductors consisted of either a pair of wires or a single simultaneous electrical stimulation of stereoelectroencephalography electrode with multiple contacts, and the locations and lengths of the conductors were varied to study the effect of electromagnetic coupling on RF-induced heating. RESULTS: The temperature increase near a wire within the phantom was dependent not only on its own location and length, but also on the locations and lengths of the other partially immersed wires. In the configurations that were studied, the presence of a second implant could increase the heating near the tip of the conductor by as much as 95%. CONCLUSION: The level of RF-induced heating during an MR scan is affected significantly by RF coupling when more than one wire-like implant is present. In some of the configurations studied, the heating was increased by the presence of a second conductor partially immersed in the phantom. Thus, RF coupling is an important factor to consider in the assessment of safety issues for MRI when multiple implants are present.

Safety of MRI in patients with retained cardiac leads.

PURPOSE: To evaluate the safety of MRI in patients with fragmented retained leads (FRLs) through numerical simulation and phantom experiments. METHODS: Electromagnetic and thermal simulations were performed to determine the worst-case RF heating of 10 patient-derived FRL models during MRI at 1.5 T and 3 T and at imaging landmarks corresponding to head, chest, and abdomen. RF heating measurements were performed in phantoms implanted with reconstructed FRL models that produced highest heating in numerical simulations. The potential for unintended tissue stimulation was assessed through a conservative estimation of the electric field induced in the tissue due to gradient-induced voltages developed along the length of FRLs. RESULTS: In simulations under conservative approach, RF exposure at B1+ ≤ 2 µT generated cumulative equivalent minutes (CEM)43 < 40 at all imaging landmarks at both 1.5 T and 3 T, indicating no thermal damage for acquisition times (TAs) < 10 min. In experiments, the maximum temperature rise when FRLs were positioned at the location of maximum electric field exposure was measured to be 2.4°C at 3 T and 2.1°C at 1.5 T. Electric fields induced in the tissue due to gradient-induced voltages remained below the threshold for cardiac tissue stimulation in all cases. CONCLUSIONS: Simulation and experimental results indicate that patients with FRLs can be scanned safely at both 1.5 T and 3 T with most clinical pulse sequences.

Comparison of SAR distribution of hip and knee implantable devices in 1.5T conventional cylindrical-bore and 1.2T open-bore vertical MRI systems.

PURPOSE: There is increasing use of open-bore vertical MR systems that consist of two planar RF coils. A recent study showed that the RF-induced heating of a neuromodulation device was much lower in the open-bore system at the brain and the chest imaging landmarks. This study focused on the hip and knee implants and compared the specific absorption rate (SAR) distribution in human models in a 1.2T open-bore coil with that of a 1.5T conventional birdcage coil. METHODS: Computational modeling results were compared against the measurement values using a saline phantom. The differences in RF exposure were examined between a 1.2T open-bore coil and a 1.5T conventional birdcage coil using SAR in an anatomical human model. RESULTS: Modeling setups were validated. The body placed closed to the coil elements led to high SAR values in the birdcage system compared with the open-bore system. CONCLUSION: Our computational modeling showed that the 1.2T planar system demonstrated a lower intensity of SAR distribution adjacent to hip and knee implants compared with the 1.5T conventional birdcage system.

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

PURPOSE: Rapid detection and mitigation of radiofrequency (RF)-induced implant heating during MRI based on small and low-cost embedded sensors. THEORY AND METHODS: 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. RESULTS: 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. CONCLUSION: 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.

Synthesis and Electronic Applications of Particle-Templated Ti3C2Tz MXene-Polymer Films via Pickering Emulsion Polymerization.

MXene/polymer composites have gained widespread attention due to their high electrical conductivity and extensive applications, including electromagnetic interference (EMI) shielding, energy storage, and catalysis. However, due to the difficulty of dispersing MXenes in common polymers, the fabrication of MXene/polymer composites with high electrical conductivity and satisfactory EMI shielding properties is challenging, especially at low MXene loadings. Here, we report the fabrication of MXene-armored polymer particles using dispersion polymerization in Pickering emulsions and demonstrate that these composite powders can be used as feedstocks for MXene/polymer composite films with excellent EMI shielding performance. Ti3C2Tz nanosheets are used as the representative MXene, and three different monomers are used to prepare the armored particles. The presence of nanosheets on the particle surface was confirmed by X-ray photoelectron spectroscopy and scanning electron microscopy. Hot pressing the armored particles above Tg of the polymer produced Ti3C2Tz/polymer composite films; the films are electrically conductive because of the network of nanosheets templated by the particle feedstocks. For example, the particle-templated Ti3C2Tz/polystyrene film had an electrical conductivity of 0.011 S/cm with 1.2 wt % of Ti3C2Tz, which resulted in a high radio frequency heating rate of 13-15 °C/s in the range of 135-150 MHz and an EMI shielding effectiveness of ∼21 dB within the X band. This work provides a new approach to fabricate MXene/polymer composite films with a templated electrical network at low MXene loadings.

Radiofrequency-induced heating of broken and abandoned implant leads during magnetic resonance examinations.

PURPOSE: The risks of RF-induced heating of active implantable medical device (AIMD) leads during MR examinations must be well understood and realistically assessed. In this study, we evaluate the potential additional risks of broken and abandoned (cut) leads. METHODS: First, we defined a generic AIMD with a metallic implantable pulse generator (IPG) and a 100-cm long lead containing 1 or 2 wires. Next, we numerically estimated the deposited in vitro lead-tip power for an intact lead, as well as with wire breaks placed at 10 cm intervals. We studied the effect of the break size (wire gap width), as well as the presence of an intact wire parallel to the broken wire, and experimentally validated the numeric results for the configurations with maximum deposited in vitro lead-tip power. Finally, we performed a Tier 3 assessment of the deposited in vivo lead-tip power for the intact and broken lead in 4 high resolution virtual population anatomic models for over 54,000 MR examination scenarios. RESULTS: The enhancement of the deposited lead-tip power for the broken leads, compared to the intact lead, reached 30-fold in isoelectric exposure, and 16-fold in realistic clinical exposures. The presence of a nearby intact wire, or even a nearby broken wire, reduced this enhancement factor to <7-fold over the intact lead. CONCLUSION: Broken and abandoned leads can pose increased risk of RF-induced lead-tip heating to patients undergoing MR examinations. The potential enhancement of deposited in vivo lead-tip power depends on location and type of the wire break, lead design, and clinical routing of the lead, and should be carefully considered when performing risk assessment for MR examinations and MR conditional labeling.