RF Heating Research Articles

RF heating in cavity-bellows of CW SRF cryomodule

In a cryomodule, bellows are commonly used to connect two adjacent superconducting cavities. It provides cavities a mechanical flexibility facilitating for thermal contraction or expansion during temperature transitions. While heat from superconducting cavities are removed from helium flowing through their jackets, non-superconducting bellows are cooled down through a relatively slow process of conduction cooling by adjacent superconducting cavities. An excessive heat deposition to the bellows could results in unsustainable rise in temperature that could lead to a thermal runaway. Because of the bellows proximity to the superconducting cavities, resulting thermal runaway could disrupt performance of the cavities and therefore, overall operation. For this reason, the paper presents a comprehensive assessment of RF heating resulting from most potent dynamic heating mechanisms in the interconnecting cavity bellows of 1.3 GHz, continuous wave, cryomodule of the Linac Coherent Light Source-II linear accelerator. Studies are performed for two types of the bellows surfaces i.e. stainless-steel and copper. The results show that RF heating at stainless-steel surface is about four times higher than at copper surface. In this work we also develop a simplified analytical model to analyse the bellows surface temperature growth resulting from RF heating. The model shows a good agreement with numerical analysis and indicates a temperature growth of up to 70 K in stainless-steel bellows in the steady state. Calculations also show that a thin copper coating of 15 μm on stainless-steel surface not only reduces RF heating but also confines the temperature growth well within 10 K. The studies presented here attribute practical importance guiding to characterize copper coating requirements for non-superconducting components in a cryomodule and allowing to model a more relevant cryogenic heat load budget accounting for bellows heating.

High-Frequency Heat Treatment of AISI 1045 Specimens and Current Calculations of the Induction Heating Coil Using Metal Phase Transformation Simulations

AISI 1045 specimen was compared through a high-frequency heat treatment simulation and experiment considering metal phase transformation. Hardening zone predictions were confirmed through cooling and metal phase transformation simulations after obtaining the results from electromagnetic heat transfer simulations. The cooling process was modeled by applying the cooling coefficient of the cooling water in the same way as the actual heat-treatment process. To obtain the current flowing through the coil during high-frequency induction heating, the voltage was measured and applied using the resistance–inductance–capacitance circuit calculation method. Experimental and simulated results of the heating temperature and curing depth of an AISI 1045 specimen with a carbon content of 0.45% were compared; the comparison indicated good agreement between the two. Using the simulation results, we established a method for obtaining the current flowing through the induction coil for predicting the extent and depth of the hardening zone during high-frequency induction heat treatment.

Effects of radio frequency heating on water distribution and structural properties of grass carp myofibrillar protein gel

This study explored effects of radio frequency (RF, 27.12 MHz, 3 kW) heating replacing the first stage (electrode gaps of 120, 140 and 160 mm) or/and the second stage (95 mm) of the water bath heating on water distribution and structural characteristics of grass carp myofibrillar proteins gels. Compared with control, RF heating (140) during the first stage significantly reduced the total time to prepare gels from 70 min to 45.3 min and increased springiness and water holding capacity from 62.9% to 68.3%. It may be attributed to the appropriate RF heating contributing to α-helix turning into random coil and cross-linking via hydrophobic interactions and disulfide bonds, thus forming smooth gels with clear network structures. Structural changes further affected water distribution (immobilized water increasing from 97.8% to 98.7%). Namely, RF (140 mm) heating improved water distribution and structural characteristics of gels, which provided basic information for RF heating surimi gels.

Sub-kelvin temperature management in ion traps for optical clocks.

The uncertainty of the ac Stark shift due to thermal radiation represents a major contribution to the systematic uncertainty budget of state-of-the-art optical atomic clocks. In the case of optical clocks based on trapped ions, the thermal behavior of the rf-driven ion trap must be precisely known. This determination is even more difficult when scalable linear ion traps are used. Such traps enable a more advanced control of multiple ions and have become a platform for new applications in quantum metrology, simulation, and computation. Nevertheless, their complex structure makes it more difficult to precisely determine its temperature in operation and thus the related systematic uncertainty. We present here scalable linear ion traps for optical clocks, which exhibit very low temperature rise under operation. We use a finite-element model refined with experimental measurements to determine the thermal distribution in the ion trap and the temperature at the position of the ions. The trap temperature is investigated at different rf-drive frequencies and amplitudes with an infrared camera and integrated temperature sensors. We show that for typical trapping parameters for In+, Al+, Lu+, Ca+, Sr+, or Yb+ ions, the temperature rise at the position of the ions resulting from rf heating of the trap stays below 700 mK and can be controlled with an uncertainty on the order of a few 100 mK maximum. The corresponding uncertainty of the trap-related blackbody radiation shift is in the low 10-19 and even 10-20 regime for 171Yb+(E3) and 115In+, respectively.

Design of the input power coupler for Boron Neutron Capture Therapy Drift Tube Linac

An accelerator-based Boron Neutron Capture Therapy (BNCT) facility has been built at the campus of China Spallation Neutron Source (CSNS). To upgrade it, a Drift Tube Linac (DTL) is designed to provide a 40 mA, 25% duty factor, 10.1 MeV beam for the target. Two opposing power couplers are employed to deliver forward peak power up to 640 kW for the beam and DTL cavity. A new coaxial power coupler with a single Tristan type window is designed to meet the power and coupling coefficient requirements. In this paper, the detailed RF optimization process of the coupler is described. According to the RF simulation results, the multipacting (MP) phenomenon at critical locations is simulated by the CST PS (Particle Studio). Simulations of thermal and mechanical are also carried out, the cooling design to provide a large margin for RF heating is performed based on the simulation results.

Skin temperature increase after whole body postmortem magnetic resonance imaging

• 1.5 T whole body postmortem magnetic resonance (PMMR) imaging was taken for 1 h. • The causes of skin temperature increase ( T inc ) after PMMR imaging was investigated. • Average specific absorption rates of PMMR imaging were within the safe range. • T inc is hardly influenced by radiofrequency heating but rather by room temperature. • 1.5 T whole body PMMR imaging for 1 h does not significantly warm up a cadaver. To investigate causes for skin temperature increase after whole body postmortem magnetic resonance (PMMR) imaging. We performed PMMR imaging using a 1.5 T clinical scanner on 28 deceased human adults. The corpses were kept in cold storage at 4 ℃ before MRI and the scan room was maintained at 23 ℃. Skin temperatures of corpses before and after MRI were measured and average specific absorption rates (SAR) of the measured points were calculated. Average skin temperature before and after PMMR imaging was 11.8 ± 5.8 ℃ and 15.0 ± 5.1 ℃, respectively. Average skin temperature increase was 3.2 ± 1.4 ℃, and whole-body average SAR in all sequences was 0.97 ± 0.94 W/kg. The skin temperature increase correlated significantly with the difference between the average initial skin temperature before PMMR imaging and the room temperature ( R 2 = 0.388, P = 0.002), but did not correlate significantly with whole-body average SAR in all sequences ( R 2 = 0.032, P = 0.662 > 0.05). The skin temperature increase during whole body PMMR imaging is more likely influenced by room temperature than by RF energy-induced heating effect. RF heating effect in PMMR imaging is minimal, and it appears to be no significant issue to conduct 1.5 T PMMR imaging prior to autopsy.

Thermal conductivity of cryoprotective agents loaded with nanoparticles, with application to recovery of preserved tissues and organs from cryogenic storage.

The objective of this study is to provide thermal conductivity data for CPA-based nanofluids for the benefit of the analyses of cryopreservation by vitrification. Thermal conductivity measurements were conducted using a hot-wire technique on an experimentation platform of the cryomacroscope, to correlate measurements with observed physical effects such as crystallization and fracturing. Tested materials in this study include the CPA cocktails M22, VS55, DP6, and DP6+sucrose. Nanofluids in this study include the above CPA cocktails as base solutions, when mixed with either iron-oxide nanoparticles (IONP) or silica-coated iron-oxide nanoparticles (sIONP). Results of this study demonstrated the addition of sIONP to any of the CPA cocktails tested did not significantly affect its thermal conductivity, its tendency to vitrify or, conversely, its tendency to form rewarming phase crystallization (RPC). Fractures were observed with cryomacroscopy at the onset of rewarming for DP6+sIONP under carefully controlled rewarming conditions without RF activation, despite the inherent opacity of the sIONP solutions. It is likely that using RF heating in order to accelerate rewarming while unifying the temperature distribution would prevent fracture and RPC. However, sIONP were not activated in this study, as the RF heating mechanism would interfere with thermal conductivity measurements. The addition of IONP to DP6 appears to hinder the tendency of the CPA to vitrify, which is a detrimental effect. But it is unlikely that uncoated nanoparticle solutions will be used in practical applications.

RF heating measurement using MR thermometry and field monitoring: Methodological considerations and first in vivo results.

A MR thermometry (MRT) method with field monitoring is proposed to improve the measurement of small temperature variations induced in brain MRI exams. MR thermometry experiments were performed at 7 Tesla with concurrent field monitoring and RF heating. Images were reconstructed with nominal k-space trajectories and with first-order spherical harmonics correction. Experiments were performed in vitro with deliberate field disturbances and on an anesthetized macaque in 2 different specific absorption rate regimes, that is, at 50% and 100% of the maximal specific absorption rate level allowed in the International Electrotechnical Commission normal mode of operation. Repeatability was assessed by running a second separate session on the same animal. Inclusion of magnetic field fluctuations in the reconstruction improved temperature measurement accuracy in vitro down to 0.02°C. Measurement precision in vivo was on the order of 0.15°C in areas little affected by motion. In the same region, temperature increase reached 0.5 to 0.8°C after 20 min of heating at 100% specific absorption rates and followed a rough factor of 2 with the 50% specific absorption rate scans. A horizontal temperature plateau, as predicted by Pennes bioheat model with thermal constants from the literature and constant blood temperature assumption, was not observed. Inclusion of field fluctuations in image reconstruction was beneficial for the measurement of small temperature rises encountered in standard brain exams. More work is needed to correct for motion-induced field disturbances to extract reliable temperature maps.

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

Heating of a multiply protonated poly(ethylene oxide) chain in an intense high-frequency electric field in a vacuum is studied using molecular dynamics simulations. The rate of absorption of the field energy by the chain is determined at the field frequencies from 0.5 to 150 GHz. The absorption spectrum depends on the number of attached protons n, and also on the temperature of the chain. At low n and high temperatures, the spectrum consists of several distinct peaks with the most intense peak at a frequency of about 10 GHz. When n is large, the peaks are less pronounced and the energy absorption rate reaches a maximum at the lowest studied frequency.The effect of n on the energy absorption rate depends on the field frequency. While at low and high frequencies the energy absorption rate increases almost in proportion to n, near the main absorption peak it decreases.This indicates that there are two different mechanisms of the energy absorption. Firstly, the chain absorbs energy during resonant vibrations that occur at several frequencies. Secondly, the chain absorbs energy through a process that occurs over a wide frequency range and becomes more intense with decreasing frequency.

Effect of Device Configuration and Patient's Body Composition on the RF Heating and Nonsusceptibility Artifact of Deep Brain Stimulation Implants During MRI at 1.5T and 3T.

Patients with deep brain stimulation (DBS) implants have limited access to MRI due to safety concerns associated with RF-induced heating. Currently, MRI in these patients is allowed in 1.5T horizontal bore scanners utilizing pulse sequences with reduced power. However, the use of 3T MRI in such patients is increasingly reported based on limited safety assessments. Here we present the results of comprehensive RF heating measurements for two commercially available DBS systems during MRI at 1.5T and 3T. To assess the effect of imaging landmark, DBS lead configuration, and patient's body composition on RF heating of DBS leads during MRI at 1.5T and 3T. Phantom and ex vivo study. Gel phantoms and cadaver brain. 1.5T and 3T, T1 -weighted turbo spin echo. RF heating was measured at the tips of DBS leads implanted in brain-mimicking gel. Image artifact was assessed in a cadaver brain implanted with an isolated DBS lead. Descriptive. We observed substantial fluctuation in RF heating, mainly affected by phantom composition and DBS lead configuration, ranging from 0.14°C to 23.73°C at 1.5T, and from 0.10°C to 7.39°C at 3T. The presence of subcutaneous fat substantially altered RF heating at the electrode tips (3.06°C < ∆T < 19.05° C). Introducing concentric loops in the extracranial portion of the lead at the surgical burr hole reduced RF heating by up to 89% at 1.5T and up to 98% at 3T compared to worst-case heating scenarios. Device configuration and patient's body composition substantially altered the RF heating of DBS leads during MRI. Interestingly, certain lead trajectories consistently reduced RF heating and image artifact. Level of Evidence 1 Technical Efficacy Stage 1 J. MAGN. RESON. IMAGING 2021;53:599-610.

A miniaturized wireless micro-heating strip integrated with an acrylate-composite temperature regulator for restenosis prevention

• This work presents aminiaturized wireless heating strip with novel temperature regulator forwireless hyperthermia treatment. • The proposed device employs an acrylate-based composite which is critical for achieving smooth regulation of temperature. • The resonant methodoffers high frequency selectivity and high efficiency in wireless heating. • The temperature regulator enables the capability of heating regulation in a passive manner, reducing the complexity of the device, and thus further facilitates the miniaturization of the device. This work proposes a miniaturized wireless heating strip designed for safe wireless hyperthermia treatment against restenosis. The proposed heating strip was designed to be inserted into a stent that can be deployed in a blood vessel using a typical catheter-based delivery system. The device consists of a micromachined LC-based resonator and a microheater with a temperature regulator (MHTR). The LC-based resonator was employed to achieve wireless resonant RF heating, with the embedded MHTR serving as an electric current breaker in the resonator circuit to maintain the strip’s temperature within a desired range. An acrylate-based composite was used as the key material of the MHTR for temperature sensing and electric-current breaking. The MHTR allows smooth heating regulation in a passive manner, reducing the complexity of the device, and thus further facilitates the miniaturization of the device. The frequency responses to temperature of devices with and without an MHTR were studied. The performance of devices immersed in physiological saline solution was also examined in detail. Wireless heating tests demonstrated that the proposed MHTR effectively maintained the device temperature at around 45 °C with an output RF power of 500 mW. The results indicate that the proposed device potentially offers safe and reliable hyperthermia treatment.

Ultrathin Silica‐Coated Iron Oxide Nanoparticles: Size‐Property Correlation

AbstractMagnetic nanostructures (MNS) in their superparamagnetic form are promising materials systems for contrast enhancement in magnetic resonance imaging (MRI) and thermal‐activation therapeutics via RF heating. These biomedical applications and associated properties are very much dependent on the structure, composition and size/shape of MNS. Although there are many speculations on the effect of various parameters on MNS, the size dependent properties are not well understood, largely due to challenges in size‐selective large‐scale synthesis for precise measurements and interference from typical large surface coating. Herein we report the size effect of iron oxide MNS on hydrodynamic radius, zeta potential, magnetic property and relaxivity. For this study, we have synthesized a wide range of monodispersed iron oxide MNS, from 7 nm to 20 nm nominal size and phase transferred to aqueous media with very thin layer of silica coating using Polyhedral Oligomeric Selesquioxanes (POSS). The dynamic light scattering (DLS) study and zeta potential measurement were carried out to determine the hydrodynamic radius and surface charges. The hydrodynamic radius follow the linear correlation but the zeta potential does not maintain it due to nonlinear mass/charge ratio relations with varying size. We have also conducted correlative studies of magnetic saturation (Ms) and relaxivities. This work will highlight the size dependent correlation and tailoring of MNS properties for desired biomedical applications.

Analysis of the RF Excitation of Endovascular Stents in Small Gap and Overlap Scenarios Using an Electro-Optical E-field Sensor

To assess the effect of the electro-magnetic coupling of endovascular stents on their RF heating potential in MRI. A custom-built electro-optic E-field probe is used to perform measurements of the scattered E-field at a distance of 2 mm to stent samples with submillimeter resolution. Various combinations of stent lengths are measured at 124 MHz (3T MRI Larmor frequency) with varying gap and overlap between the stents, with and without stent coating, and with distilled water and saline solution as surrounding media. The results are compared to theoretically derived E-field distributions. At an overlap of 10 mm the E-field pattern of two stents collapses to a single dipole indicating excellent coupling between the stents. E-field intensities substantially increase/decrease up to 5-fold/2.5-fold if the total length of the two combined stents is closer/further away from the resonance length of the single stents. Stent coating and conductivity of the surrounding medium strongly influence the E-field patterns of overlapping stents. Measured and calculated E-field patterns are in good agreement. Electro-optic E-field measurements are a valuable tool for RF safety assessments in both single as well as coupled stents. RF induced heating of single stents during MRI has been extensively studied. However, in clinical practice often two or more stents are implanted in close proximity which can substantially change the pattern of the scattered electric fields and the localization and intensity of hot spots. In this study a detailed assessment of the coupling of stents during RF excitation is given.

Device Configuration and Patient's Body Composition Significantly Affect RF Heating of Deep Brain Stimulation Implants During MRI: An Experimental Study at 1.5T and 3T.

Patients with deep brain stimulation (DBS) devices have limited access to magnetic resonance imaging (MRI) due to safety concerns associated with RF heating generated around the implant. The problem of predicting RF heating of conductive leads is complex with a large parameter space and several interplaying factors. Recently however, off-label use of MRI in patients with DBS devices has been reported based on limited safety assessments, raising the concern that potentially dangerous scenarios may have been overlooked. In this work, we present results of a systematic assessment of RF heating of a commercial DBS device during MRI at 1.5T and 3T, taking into account the effect of device configuration, imaging landmark, and patient's body composition. Ninety-six (96) RF heating measurements were performed using anthropomorphic phantoms implanted with a full DBS system. We evaluated eight clinically relevant device configurations, implanted in phantoms with different material compositions, and imaged at three different landmarks (head, shoulder, and lower chest) in 1.5 T and 3T scanners. We observed a substantial fluctuation in the RF heating depending on phantom's composition and device configuration. RF heating in the brain-mimicking gel varied from 0.1°C to 12°C during 1.5 T MRI and from <0.1°C to 4.5°C during 3T MRI. We also observed that certain device configurations consistently reduced RF heating across different phantom compositions, imaging landmarks, and MRI transmit frequencies.

RF Heating Analysis of Implanted lead wires under 3.0 Tesla Magnetic Resonance Imaging System.

RF heating has severely limited the application of high-field magnetic resonance imaging (MRI) in the patients with active implantable medical devices (AIMDs). Based on the finite element methods (FEMs), we study the RF heating of a lead implanted in the human body under 3.0 T MRI. The coupled simulation of electromagnetic and heat transfer is used to analyze the relationship between several factors, such as the resonance length, the implantation position, the implantation configuration, and the thermal conductivity, and RF heating. Results show that the resonance length of the lead is half of the RF wavelength, and the temperature rise exceeds the acceptable range of human body when the resonance occurs. The electromagnetic field distribution in the phantom is not uniform, so the field density around the wire may vary with different implantation positions and configurations. Temperature rise reduces with the decrease of the field density. In addition, RF heating can be reduced by increasing the thermal conductivity.

RF heating of deep brain stimulation implants during MRI in 1.2 T vertical scanners versus 1.5 T horizontal systems: A simulation study with realistic lead configurations.

Patients with deep brain stimulation (DBS) implants are often denied access to magnetic resonance imaging (MRI) due to safety concerns associated with RF heating of implants. Although MR-conditional DBS devices are available, complying with manufacturer guidelines has proved to be difficult as pulse sequences that optimally visualize DBS target structures tend to have much higher specific absorption rate (SAR) of radiofrequency energy than current guidelines allow. The MR-labeling of DBS devices, as well as the majority of studies on RF heating of conductive implants have been limited to horizontal close-bore MRI scanners. Vertical MRI scanners, originally introduced as open low-field MRI systems, are now available at 1.2 T field strength, capable of high-resolution structural and functional imaging. No literature exists on DBS SAR in this class of scanners which have a 90° rotated transmit coil and thus, generate a fundamentally different electric and magnetic field distributions. Here we present a simulation study of RF heating in a cohort of forty patient-derived DBS lead models during MRI in a commercially available vertical openbore MRI system (1.2 T OASIS, Hitachi) and a standard horizontal 1.5 T birdcage coil. Simulations were performed at two major imaging landmarks representing head and chest imaging. We calculated the maximum of 0.1g-averaged SAR (0.1g-SARMax) around DBS lead tips when a B1+ = 4 µT was generated on an axial plane passing through patients body. For head landmark, 0.1g-SARMax reached 220±188 W/kg in the 1.5 T birdcage coil, but only 14±11 W/kg in the OASIS coil. For chest landmark, 0.1g-SARMax was 24±17 W/kg in the 1.5 T birdcage coil and 3±2 W/kg in the OASIS coil. A paired two-tail t-test revealed a significant reduction in SAR with a large effect-size during head MRI (p < 1.5×10-8, Cohen's d = 1.5) as well as chest MRI (p < 6.5×10-10, Cohen's d = 1.7) in 1.2 T Hitachi OASIS coil compared to a standard 1.5 T birdcage transmitter. Our findings suggest that open-bore vertical scanners may offer an untapped opportunity for MRI of patients with DBS implants.

Highly effective inactivation of anti‐nutritional factors (lipoxygenase, urease and trypsin inhibitor) in soybean by radio frequency treatment

SummaryRadio frequency (RF) treatment at 27.12 MHz was employed to inactivate the anti‐nutritional factors (ANFs), including lipoxygenase (LOX), urease and trypsin inhibitor to minimise deleterious effects caused by conventional hot‐air heating. The effects of RF heating on the ANF activities, physicochemical properties and processing quality of intact soybean were investigated. The results revealed that ANF activities were effectively inactivated within 300 s by RF heating and inactivation rates of LOX, urease and trypsin inhibitor were 95.2% (270 s), 93.4% (285 s) and 89.4% (300 s), respectively. In addition, RF heating improved physicochemical properties and processing quality of soybean products when the trypsin inhibitor was inactivated. On the contrast, conventional thermal treatment significantly decreased functional properties and led to the formation of greater aggregates. Our results may provide a novel method to inactivate endogenous enzyme in crops without negative impact on processing quality.

Correction of motion-induced susceptibility artifacts and B0 drift during proton resonance frequency shift-based MR thermometry in the pelvis with background field removal methods.

PurposeThe linear change of the water proton resonance frequency shift (PRFS) with temperature is used to monitor temperature change based on the temporal difference of image phase. Here, the effect of motion‐induced susceptibility artifacts on the phase difference was studied in the context of mild radio frequency hyperthermia in the pelvis.MethodsFirst, the respiratory‐induced field variations were disentangled from digestive gas motion in the pelvis. The projection onto dipole fields (PDF) as well as the Laplacian boundary value (LBV) algorithm were applied on the phase difference data to eliminate motion‐induced susceptibility artifacts. Both background field removal (BFR) algorithms were studied using simulations of susceptibility artifacts, a phantom heating experiment, and volunteer and patient heating data.ResultsRespiratory‐induced field variations were negligible in the presence of the filled water bolus. Even though LBV and PDF showed comparable results for most data, LBV seemed more robust in our data sets. Some data sets suggested that PDF tends to overestimate the background field, thus removing phase attributed to temperature. The BFR methods even corrected for susceptibility variations induced by a subvoxel displacement of the phantom. The method yielded successful artifact correction in 2 out of 4 patient treatment data sets during the entire treatment duration of mild RF heating of cervical cancer. The heating pattern corresponded well with temperature probe data.ConclusionThe application of background field removal methods in PRFS‐based MR thermometry has great potential in various heating applications and body regions to reduce motion‐induced susceptibility artifacts that originate outside the region of interest, while conserving temperature‐induced PRFS. In addition, BFR automatically removes up to a first‐order spatial B0 drift.

Three-Dimensional Compensation for Minimizing Heating of the Ion in Surface-Electrode Trap**Supported by the National Natural Science Foundation of China (Grant Nos. 11734018 and 11674360).

The trapped ions confined in a surface-electrode trap (SET) could be free from rf heating if they stay at the rf potential null of the potential well. We report our effort to compensate three-dimensionally for the micromotion of a single 40Ca+ ion near the rf potential null, which largely suppresses the ion’s heating and thus helps to achieve the cooling of the ion down to 3.4 mK, which is very close to the Doppler limit. This is the prerequisite of the sideband cooling in our SET.

Effect of High Frequency Heat Treatment on the Microstructure and Macroscopic Properties of WC-50Ni+Stellite 1 Coating Layer Fabricated by HVOF Spray Process

Effect of High Frequency Heat Treatment on the Microstructure and Macroscopic Properties of WC-50Ni+Stellite 1 Coating Layer Fabricated by HVOF Spray Process