RF Heating Research Articles

Transfer Functions of a Spinal Cord Stimulation System in Mixed Media and Homogeneous Media for Estimation of RF Heating during MRI Scans.

There is a risk for patients with spinal cord stimulation (SCS) implants that MRI-RF induced heating may cause harm to spinal cord (SC) or spinal nerves. The ISO/TS 10974 Clause 8, Tier 3 based evaluation of RF heating has been commonly used for active implantable manufactures through regulatory body world-wide. Generating TFs for the lead and SCS system is an essential component in Tier 3. The objective of this paper is to investigate the effects of media on transfer functions (TFs) and dissipated power near the distal electrode along clinical pathways of SCS implants. The simulation results indicate that SCS TF measurement and validation should be performed in HCM (0.47 S/m) since it has the closest match with SC mixed media. With the SC mixed media model, the compound tissue effect from tissues around the system was seen and TF follows that of the HCM TFs closely. These trends were confirmed through power calculation of TF equation by integrals of electrical fields along clinical SCS implant pathways in an adult male model, with a 2%-4% underestimation from HCM while LCM overestimates by 72%- 74%. This study indicates appropriate section of media for deriving TFs could lead to more clinically relevant RF heating predictions during MRI scans.

A New Model for RF Ablation Planning in Clinic.

Numerical simulations provide effective way to acquire detailed temperature field during radiofrequency ablation (RF). Based on the patient's real electrical resistance and RF power, we have designed a theoretical model suitable for clinical treatment planning. The human body is assumed to be two cylinders with the inner cylinder simulating the liver with real liver electrical conductivity, while the electrical conductivity of the out cylinder adjusted to match the real resistance recorded when treated with RFA. The orthogonal-array method has been applied to analyze the impact of the main geometric parameters. Results show that a limited range of model parameters with the same resistance and power condition results in similar prediction of ablation range. In addition, RF heating experiments have been performed in the liver of a live pig to validate this model. The simulated temperature fits well with the real temperature. The comparison of the results predicted using the proposed model and previous models finds that the previous uniform-electrical-conductivity model would significantly underestimates or overestimates the ablation range based on the magnitude of the electrical resistance recorded.

Imaging performance of a dedicated radiation transparent RF coil on a 1.0 Tesla inline MRI-linac

This work describes the first imaging studies on a 1.0 Tesla inline MRI-Linac using a dedicated transmit/receive RF body coil that has been designed to be completely radio transparent and provide optimum imaging performance over a large patient opening.A series of experiments was performed on the MRI-Linac to investigate the performance and imaging characteristics of a new dedicated volumetric RF coil: (1) numerical electromagnetic simulations were used to measure transmit efficiency in two patient positions; (2) image quality metrics of signal-to-noise ratio (SNR), ghosting and uniformity were assessed in a large diameter phantom with no radiation beam; (3) radiation induced effects were investigated in both the raw data (k-space) and image sequences acquired with simultaneous irradiation; (4) radiation dose was measured with and without image acquisition; (5) RF heating was studied using an MR-compatible fluoroptic thermometer and; (6) the in vivo image quality and versatility of the coil was demonstrated in normal healthy subjects for both supine and standing positions.Daily phantom measurements demonstrated excellent imaging performance with stable SNR over a period of 3 months (42.6 ± 0.9). Simultaneous irradiation produced no statistical change in image quality (p > 0.74) and no interference in raw data for a 20 × 20 cm radiation field. The coil was found to be efficient over large volumes and negligible RF heating was observed. Volunteer scans acquired in both supine and standing positions provided artefact free images with good anatomical visualisation.The first completely radio transparent RF coil for use on a 1.0 Tesla MRI-Linac has been described. There is no impact on either the imaging or dosimetry performance with a simultaneous radiation beam. The open design enables imaging and radiotherapy guidance in a variety of positons.

Resonance frequency tuning of an RF cavity through sliding mode extremum seeking

Tuning of the natural resonance frequency of an RF cavity is crucial to achieve maximum power transfer from the cavity to the beam, to reduce power requirements, and to guarantee a predictable particle speed gain within each accelerator structure. So far, most operational cavities are frequency tuned through a phase comparison technique, which is sensitive to temperature variations if the facility or signal transportation cables are not temperature controlled. Temperature induced phase errors render this technique labor and time intensive. This paper presents an RF cavity tuning scheme, based on reflected power measurements and sliding mode extremum seeking control, to eliminate the phase measurement related difficulties. A stability analysis provides conditions on how to choose the controller parameters to guarantee stable system operation up to twice the cavity bandwidth, which is a controllable bandwidth improvement of a factor of two. The system has been implemented in two of TRIUMF’s (Canada’s particle accelerator centre) room temperature resonators and long term measurements show that the system effectively counteracts the RF heating effect as well as diurnal temperature variations.

Development and Validation of a Simulation Model for the Temperature Field during High-Frequency Heating of Wood

In the process of applying high-frequency heating technology to wood drying, controlling the material temperature affects both drying speed and drying quality. Therefore, research on the heat transfer mechanism of high-frequency heating of wood is of great significance. To study the heat transfer mechanism of high-frequency heating, the finite element method was used to establish and solve the wood high-frequency heating model, and experimental verification was carried out. With a decrease in moisture content, the heating rate decreased, then increased, and then decreased again. There was no obvious linear relationship between the moisture content and heating rate; the simulation accuracy of the heating rate was higher in the early and later drying stages and slightly lower near the fiber saturation point. For the central section temperature distribution, the simulation and actual measurement results matched poorly in the early drying stage because the model did not fully consider the differences in the moisture content distribution of the actual test materials. In the later drying stage, the moisture content distribution of the test materials became uniform, which was consistent with the model assumptions. Considering the changes in heating rate and temperature distribution, the accuracy of the model is good under the fiber saturation point, and it can be used to predict the high-frequency heating process of wood.

Utility of multi-functional two channel off-axis ion funnel (TCOAIF) in FTICR-MS

We developed and comprehensively characterized an ion guide two channel off-axis ion funnel (TCOAIF) working at medium pressure with multiple ion manipulation capabilities in Fourier transform ion cyclotron resonance mass spectrometer (FTICR-MS). The ion transmission performance was evaluated by running several peptides and proteins, and the operation condition was optimized by tuning the RF parameters and DC electric fields in both drift and ion funnel channels. The results present significant enhancement in ion transmission efficiency using TCOAIF, an order of magnitude improvement in sensitivity on upgraded FTICR MS using TCOAIF was achieved for different samples covering a wide m/z range. The elimination of neutral molecules and contaminants as well as gas from ion source in TCOAIF extended the overall MS system performance with high sensitivity. RF heating was applied to conduct CID with high efficiency at medium pressure for low and high m/z ions providing an alternative approach to investigate structural information for biomolecules. The versatile TCOAIF is compatible with different MS systems and other analytical techniques, the RF-driven CID could be integrated for biologically relevant macromolecules analysis in FTICR-MS.

Dielectric properties of soy protein isolate dispersion and its temperature profile during radio frequency heating

Dielectric properties of soy protein isolate (SPI) dispersions, which were influenced by pH (4–10), concentration (5–15%, w/v) and temperature (20–90 °C), were investigated by a response surface methodology. Results showed that the second polynomial model of dielectric properties effected by pH, concentration, and temperature had good correlations (R2adj ≥ 0.850). All the pH, concentration, and temperature significantly contributed to dielectric properties of SPI dispersions at radio frequencies (13.56, 27.12, and 40.68 MHz). The penetration depth (dp) value decreased (13,488 to 8,789 cm, 27.12 MHz, 20 °C) with the increasing of concentration (5–15%) but decreased (9,558 to 6,027 cm, 27.12 MHz, 15%) with the increasing of temperature (20 to 90 °C). The dp value had an irregular change near the denaturation temperature of SPI. Radio frequency (27.12 MHz) heating rates of SPI dispersions were decreased with the increasing of protein concentration. Practical applications RF heating treatment has been confirmed as an attractive modification method for protein and sterilization method for liquid food. Results from present work could be useful in the development of the protein modification and soy milk sterilization applications.

Retrospective analysis of RF heating measurements of passive medical implants.

The test reports for the RF-induced heating of metallic devices of hundreds of medical implants have been provided to the U.S. Food and Drug Administration as a part of premarket submissions. The main purpose of this study is to perform a retrospective analysis of the RF-induced heating data provided in the reports to analyze the trends and correlate them with implant geometric characteristics. The ASTM-based RF heating test reports from 86 premarket U.S. Food and Drug Administration submissions were reviewed by three U.S. Food and Drug Administration reviewers. From each test report, the dimensions and RF-induced heating values for a given whole-body (WB) specific absorption rate (SAR) and local background (LB) SAR were extracted and analyzed. The data from 56 stents were analyzed as a subset to further understand heating trends and length dependence. For a given WB SAR, the LB/WB SAR ratio varied significantly across the test labs, from 2.3 to 11.3. There was an increasing trend on the temperature change per LB SAR with device length. The maximum heating for stents occurred at lengths of approximately 100 mm at 3 T, and beyond 150 mm at 1.5 T. Differences in the LB/WB SAR ratios across testing labs and various MRI scanners could lead to inconsistent WB SAR labeling. Magnetic resonance (MR) conditional labeling based on WB SAR should be derived from a conservative estimate of global LB/WB ratios.

Evaluation of ultrasound pretreatment and drying methods on selected quality attributes of bitter melon (Momordica charantia L.)

ABSTRACTRecently, numerous attentions have been drawn to bitter melon (Momordica charantia L.) processing due to its outstanding pharmacological properties. Fresh and ultrasound pretreated bitter melon slices were subjected to hot air drying (HAD) and radio frequency assisted hot air drying (RFHAD) in this study. Quality attributes of the dried samples were compared in terms of moisture content, water activity, glass transition temperature, color, rehydration ratio, and content of chlorophyll, ascorbic acid, and total phenolic. Compared with HAD process, results showed that the use of ultrasound pretreatment and RF heating accelerate water loss during drying processes, and then contribute to a shorter drying time, a lower water activity and higher glass transition temperature of dried samples. It also showed RFHAD had better color exhibition, higher retention of chlorophyll, ascorbic acid, and total phenolic due to uniform heating and shorter drying time. Additionally, ultrasound pretreatment greatly improved the rehydration capacity and total phenolic retention. Therefore, considering the quality attributes and drying time, ultrasound pretreatment and radio frequency drying would be very promising alternatives for food processing.

Role of MRI coils in radio frequency heating of implantable medical devices

MRI for patients with implanted medical devices is usually restricted due to the heating hazard arising from the interaction between the device and the electromagnetic field generated by the RF coil. RF coils used clinically have diverse dimensions and their heating effect remains unexamined. By numerical simulation, the similarity of the phantom magnetic field distribution of various coils was characterised by regression, and the heating around a standard implant was examined by correlation analysis. It was shown that the magnetic field distributions in the phantom were similar (>0.95) in the central area. This similarity was more significantly affected by the coil length rather than the diameter. As for the RF heating, the temperature distributions at the implant tips were much more similar (>0.9999) across all the coils. Although there were some differences in the temperature rise per unit RF magnetic field, these differences were within 20%, especially in the central region. These data reveal the influence of coil dimensions on the RF heating.

Computer Simulation of High-Frequency Heating of a Protonated Poly(ethylene oxide) Chain in a Vacuum

Heating of an isolated protonated poly(ethylene oxide) chain by high-frequency electric field was simulated using molecular dynamics. Simulations were performed at the field strength of about 108 V/m and frequencies from 1 to 100 GHz. At initial temperature (500 K), the chain is a globule. Absorption of the field energy heats the chain and leads to a growth in its size. In some frequency ranges, the field causes the chain to rotate as a whole. The rotation leads to a sharp increase in the heating rate and the rate of stretching the chain. The rotating chain has larger size than its non rotating counterpart at the same temperature. Contrary to the non rotating chain, this chain can be in a two-phase state in which the globular and the stretched microphases coexist. The absorption spectrum of the chain is temperature dependent and, at high temperatures, contains two distinctly expressed peaks. In the vicinity of these peaks, the chain rotates with a frequency equal to the field frequency or about half the field frequency. The results obtained are explained using the model of a spatial damped oscillator in which, together with the usual resonance, the subharmonic resonances typical for nonlinear systems arise.

Radiofrequency ablation (RFA)-induced systemic tumor growth can be reduced by suppression of resultant heat shock proteins

Purpose: To determine the role of hepatic radiofrequency ablation (RFA) heating parameters and their activation of heat shock proteins (HSPs) in modulating distant tumor growth.Methods and materials: First, to study the effects of RFA dose on distant tumor growth, rats with subcutaneous R3230 adenocarcinoma (10 ± 1 mm) were assigned to 3 different hepatic RF doses (60 °C × 10 min, 70 °C × 5 min or 90 °C × 2 min) that induced identical sized ablation or sham (n = 6/arm). Post-RFA tumor growth rates, cellular proliferation (Ki-67) and microvascular density (MVD) were compared at 7d. Next, the effect of low and high power doses on local HSP70 expression and cellular infiltration (α-SMA + myofibroblasts and CD68 + macrophages), cytokine (IL-6) and growth factor (HGF and VEGF) expression was assessed. Finally, 60 °C × 10 min and 90 °C × 2 min RFA were combined with anti-HSP micellar quercetin (MicQ, 2 mg/ml). A total of 150 animals were used.Results: Lower RF heating (70 °C × 5 min and 60 °C × 10 min) resulted in larger distant tumors at 7d (19.2 ± 0.8 mm for both) while higher RF heating (90 °C × 2) led to less distant tumor growth (16.7 ± 1.5 mm, p < .01 for both), though increased over sham (13.5 ± 0.5 mm, p < .01). Ki-67 and MVD correlated with tumor growth (p < .01 for all). Additionally, lower dose 60 °C × 10 min hepatic RFA had more periablational HSP70 compared to 90 °C × 2 min (rim: 1.106 ± 163 µm vs. 360 ± 18 µm, p < .001), with similar trends for periablational α-SMA, CD68 and CDC47 (p < .01 for all). Anti-HSP70 MicQ blocked distant tumor growth for lower dose (60 °C × 10: RF/MicQ 14.6 ± 0.4 mm vs. RF alone: 18.1 ± 0.4 mm, p < .01) and higher dose RFA (90 °C × 2 min: RF/MicQ 14.6 ± 0.5 mm vs. RF alone: 16.4 ± 0.7 mm, p < .01).Conclusion: Hepatic RF heating parameters alter periablational HSP70, which can influence and stimulate distant tumor growth. Modulation of RF heating parameters alone or in combination with adjuvant HSP inhibition can reduce unwanted, off-target systemic tumorigenic effects.

Toward in vivo quantification of induced RF currents on long thin conductors.

Most MR-guided catheter-based procedures, and imaging of patients with implanted medical devices, are currently contraindicated due to a significant risk of heating associated with induced RF currents. The induced RF current produces a corresponding artifact which can be used to remotely characterize current and safely predict RF heating. Application of this remote technique in vivo to safely quantify RF heating risk may allow for execution of many scans currently contraindicated. Sources of phase other than induced RF current may present difficulty in practical in vivo. A custom ultra-short echo time (UTE) sequence was developed to minimize unwanted phase contributions. A phantom experiment was performed to compare current characterization using a stock gradient-echo (GRE) sequence and the custom UTE sequence following calibration of the temperature measurement apparatus using a previously published heating prediction technique. Animal experiments were used to investigate the feasibility of using the UTE sequence to quantify RF heating. Current characterization and heating prediction with a stock GRE sequence was equivalent to that with the custom UTE sequence. Heating measurements and image-based predictions in animal experiments agreed within error in all experiments. Through comparison of measured heating and image-based prediction, feasibility of using a custom UTE sequence to quantify RF heating risk in vivo was demonstrated.

Recent developments in radio frequency drying of food and agricultural products: A review

ABSTRACTRadio frequency (RF) drying is an effective and practical dielectric drying method for food and agricultural products due to rapid and volumetric heating, deep penetration and moisture self-balance effects. However, non-uniform heating and sometimes runaway heating are still major problems with implementation of RF drying. RF-related combination drying takes advantages of both conventional drying methods and RF heating, leading to improved drying uniformity, better product quality and higher energy efficiency. This paper provides a brief introduction on the basic principle of RF drying, analyzes the RF heating and drying characteristics and examines recent literature on RF drying applications and possible methods for improving RF heating and drying uniformity. Recommendations for future research have been proposed to achieve practical and effective RF drying requirements and bridge the gap between academia and industry.

High peak and high average radiofrequency power transmit/receive switch for thermal magnetic resonance.

To study the role of temperature in biological systems, diagnostic contrasts and thermal therapies, RF pulses for MR spin excitation can be deliberately used to apply a thermal stimulus. This application requires dedicated transmit/receive (Tx/Rx) switches that support high peak powers for MRI and high average powers for RF heating. To meet this goal, we propose a high-performance Tx/Rx switch based on positive-intrinsic-negative diodes and quarter-wavelength (λ/4) stubs. The λ/4 stubs in the proposed Tx/Rx switch design route the transmitted RF signal directly to the RF coil/antenna without passing through any electronic components (e.g., positive-intrinsic-negative diodes). Bench measurements, MRI, MR thermometry, and RF heating experiments were performed at f = 297 MHz (B0 = 7 T) to examine the characteristics and applicability of the switch. The proposed design provided an isolation of -35.7dB/-41.5dB during transmission/reception. The insertion loss was -0.41dB/-0.27dB during transmission/reception. The switch supports high peak (3.9 kW) and high average (120 W) RF powers for MRI and RF heating at f = 297 MHz. High-resolution MRI of the wrist yielded image quality competitive with that obtained with a conventional Tx/Rx switch. Radiofrequency heating in phantom monitored by MR thermometry demonstrated the switch applicability for thermal modulation. Upon these findings, thermally activated release of a model drug attached to thermoresponsive polymers was demonstrated. The high-power Tx/Rx switch enables thermal MR applications at 7 T, contributing to the study of the role of temperature in biological systems and diseases. All design files of the switch will be made available open source at www.opensourceimaging.org.

Inactivation of Escherichia coli O157:H7 and Staphylococcus aureus in red pepper powder using a combination of radio frequency thermal and indirect dielectric barrier discharge plasma non-thermal treatments

This investigation evaluated the inactivation of foodborne pathogens and quality of red pepper (Capsicum annuum L.) powders treated via intermittent intervention with a combination of radio frequency thermal treatment (RF-TT) and dielectric barrier discharge plasma treatment (DBD-PT). Microbiological analysis of powders inoculated with Escherichia coli O157:H7 and Staphylococcus aureus following RF-TT and DBD-PT combination indicated reductions in the microbial counts proportional with increasing treatment cycles. The combination of 1500 W RF heating and 1000 W DBD-PT treatment reduced microbial counts to below the detection limit (1 log CFU/g) after 2 repetitions of the treatment cycle. RF-TT and DBD-PT combination (5 cycles) did not affect powder extractable colour or free radical scavenging activity. The combined treatment caused negligible changes in CIE L∗, a∗, b∗, and ΔE∗ and extractable colour values. The moisture content and water activity showed significant (p < 0.05) differences compared to the control, likely owing to the drying of samples during intermittent RF-TT and DBD-PT. Thus, the hurdle technology combining RF heating and non-thermal plasma treatment could be useful in improving the microbiological quality of red pepper powder.

Rapid thermal process by RF heating of nano-graphene layer/silicon substrate structure: Heat explosion theory approach

RF heating kinetics of a nano-graphene layer/silicon substrate structure is analyzed theoretically as a function of the thickness and sheet resistance of the graphene layer, the dimensions and thermal parameters of the structure, as well as of cooling conditions and of the amplitude and frequency of the applied RF magnetic field. It is shown that two regimes of the heating can be realized. The first one is characterized by heating of the structure up to a finite temperature determined by equilibrium between the dissipated loss power caused by induced eddy-currents and the heat transfer to environment. The second regime corresponds to a fast unlimited temperature increase (heat explosion). The criterions of realization of these regimes are presented in the analytical form. Using the criterions and literature data, it is shown the possibility of the heat explosion regime for a graphene layer/silicon substrate structure at RF heating.

SINIS bolometer with a suspended absorber

We have developed a Superconductor-Insulator-Normal Metal-Insulator-Superconductor (SINIS) bolometer with a suspended normal metal bridge. The suspended bridge acts as a bolometric absorber with reduced heat losses to the substrate. Such bolometers were characterized at 100-350 mK bath temperatures and electrical responsivity of over 109 V/W was measured by dc heating the absorber through additional contacts. Suspended bolometers were also integrated in planar twin-slot and log-periodic antennas for operation in the submillimetre-band of radiation. The measured voltage response to radiation at 300 GHz and at 100 mK bath temperature is 3*108 V/W and a current response is 1.1*104 A/W which corresponds to a quantum efficiency of ~15 electrons per photon. An important feature of such suspended bolometers is the thermalization of electrons in the absorber heated by optical radiation, which in turn provides better quantum efficiency. This has been confirmed by comparison of bolometric response to dc and rf heating. We investigate the performance of direct SN traps and NIS traps with a tunnel barrier between the superconductor and normal metal trap. Increasing the volume of superconducting electrode helps to reduce overheating of superconductor. Influence of Andreev reflection and Kapitza resistance, as well as electron-phonon heat conductivity and thermal conductivity of N-wiring are estimated for such SINIS devices.

Formation of Hardening Coatings for Working Elements of Agricultural Machines When Combining the High-Frequency Heating and High-Temperature Synthesis of the Materials

A method of surfacing of a wide range of cast, hard, wear-resistant materials on a steel base was created based on the performed investigations. The surfacing was carried out in the conditions of heating with high-frequency currents (HFCs). The exothermic thermite mixtures were used, including the mixtures of metal oxides, aluminum, or silicon. In the course of synthesis, the coatings with the weld pad of 100–900 μm were obtained. The surfacing process was carried out in two main synthesis modes—high and low-temperature using the exothermic thermite mixtures—and allowed extending significantly the raw material base of the surfacing materials, including refusing the use of the traditional surfacing eutectic alloys and charge mixtures with the boron carbide for deep borating. It is shown that the synthesis of the coating of the exothermic thermite mixture FeO/B2O3/Si is identical to the diffusion saturation of the surface layer with the classical mode of borating during the HFC-heating.

Measurements and simulation of RF heating of implanted stereo-electroencephalography electrodes during MR scans.

To assess RF-induced heating during MRI of patients with implanted stereo-electroencephalography electrodes. Simulations and experimental measurements using phantom and a head-only transmit/receive coil on a 3T MR system were performed to evaluate temperature increases at the tip of an 8-contact stereo-electroencephalography electrode and an insulated wire partially immersed into the phantom. The lengths of wire producing maximum (resonant condition) and minimum (anti-resonant condition) heating were evaluated for different entry modes and penetration depths. For both wire and stereo-electroencephalography electrode, resonant lengths were close to odd integral multiples of RF quarter wavelength in air and antiresonant length close to even integral multiples of RF quarter wavelength, both being unaffected by the entry mode. In the resonant condition, temperature increased by as much as a factor of 10 higher than that at antiresonant condition. Larger penetration depths did not change resonant length, but did lead to increased RF heating. For the partially immersed implants like stereo-electroencephalography electrode, the resonant lengths were found to be independent of the penetration depths and entry modes, although the temperature increases may vary. Avoiding such lengths of cables can reduce the risk of tissue heating during in vivo MRI.