Guidelines for predicting lesion size at common endocardial locations during radio-frequency ablation

We used the finite element method to study the effect of radio-frequency (RF) catheter ablation on tissue heating and lesion formation at different intracardiac sites exposed to different regional blood velocities. We examined the effect of application of RF current in temperature- and power-controlled mode above and beneath the mitral valve annulus where the regional blood velocities are high and low respectively. We found that for temperature-controlled ablation, more power was delivered to maintain the preset tip temperature at sites of high local blood velocity than at sites of low local blood velocity. This induced more tissue heating and larger lesion volumes than ablations at low velocity regions. In contrast, for power-controlled ablation, tissue heating was less at sites of high compared with low local blood velocity for the same RF power setting. This resulted in smaller lesion volumes at sites of low local velocity. Our numerical analyzes showed that during temperature-controlled ablation at 60 degrees C, the lesion volumes at sites above and underneath the mitral valve were comparable when the duration of RF current application was 10 s. When the duration of RF application was extended to 60 s and 120 s, lesion volumes were 33.3% and 49.4% larger above the mitral valve than underneath the mitral valve. Also, with temperature-controlled ablation, tip temperature settings of 70 degrees C or greater were associated with a risk of tissue overheating during long ablations at high local blood velocity sites. In power-controlled ablation (20 W), the lesion volume formed underneath the mitral valve was 165.7% larger than the lesion volume above the mitral valve after 10 s of ablation. We summarized the guidelines for energy application at low and high flow regions.     

FDTD computation of temperature rise in the human head for portabletelephones

Temperature rises in the human head for portable telephones were computed with an anatomically based head model at 900 MHz and 1.5 GHz. The specific absorption rate (SAR) in the human head was determined using the finite-difference time-domain (FDTD) method, while a bioheat equation was numerically solved also using the FDTD method. The portable telephone was modeled by a quarter-wavelength monopole antenna on a dielectric covered metal box. The source geometries considered were the telephone barely touching the ear and the telephone pressing the ear, both having a vertical alignment at the side of the head. The antenna output power was set to be consistent with the portable telephones of today: 0.6 W at 900 MHz and 0.27 W at 1.5 GHz. Computed results show that a phone time of 6-7 min yields a temperature rise of approximately 90% of the steady-state value. Application of the ANSZ/IEEE safety guidelines restricting the 1-g-averaged spatial peak SAR to 1.6 W/kg results in the maximum temperature rise in the brain of 0.06°C, and application of the ICNIRP/Japan safety guidelines restricting the 10-g-averaged spatial peak SAR to 2 W/kg results in the maximum temperature rise in the brain of 0.11°C, both at 900 MHz and 1.5 GHz     

Diode-laser-based absorption spectroscopy diagnostics of a jet-typeO2(1Δ) generator for chemical oxygen-iodinelasers

Using diode-laser-based diagnostics, O₂(¹Δ) yield and water vapor fraction were measured at the exit of a jet-type singlet oxygen generator (JSOG) for a chemical oxygen-iodine laser (COIL). Chlorine utilization and gas temperature at the generator exit were also measured, simultaneously. For conditions corresponding to the maximum chemical efficiency of the supersonic COIL energized by the JSOG, the O₂(¹Δ) yield, water vapor fraction, chlorine utilization, and temperature at the generator exit are 0.65, 0.08 and 0.92, and 30°C, respectively. Increase of the basic hydrogen peroxide temperature results in an increase of the water vapor fraction caused by an increase of the saturated water vapor pressure in the generator. As the pressure in the generator rises from 18 to 60 torr, the yield decreases from 0.65 to O.48. Dependence of the yield on the generator pressure is consistent with a rate constant for the O₂ (¹Δ) energy pooling reaction of 2.7×10^-17 cm³·S^-1. The same rate constant explains the measured variation of the temperature along the flow in the diagnostic cell     

An Optically Coupled System for Quantitative Monitoring of MRI-Induced RF Currents Into Long Conductors

The currents induced in long conductors such as guidewires by the radio-frequency (RF) field in magnetic resonance imaging (MRI) are responsible for potentially dangerous heating of surrounding media, such as tissue. This paper presents an optically coupled system with the potential to quantitatively measure the RF currents induced on these conductors. The system uses a self shielded toroid transducer and active circuitry to modulate a high speed light-emitting-diode transmitter. Plastic fiber guides the light to a photodiode receiver and transimpedance amplifier. System validation included a series of experiments with bare wires that compared wire tip heating by fluoroptic thermometers with the RF current sensor response. Validations were performed on a custom whole body 64 MHz birdcage test platform and on a 1.5 T MRI scanner. With this system, a variety of phenomena were demonstrated including cable trap current attenuation, lossy dielectric Q-spoiling and even transverse electromagnetic wave node patterns. This system should find applications in studies of MRI RF safety for interventional devices such as pacemaker leads, and guidewires. In particular, variations of this device could potentially act as a realtime safety monitor during MRI guided interventions.      

Sequential activation of a segmented ground pad reduces skin heating during radiofrequency tumor ablation: optimization via computational models

Radiofrequency (RF) ablation has become an accepted treatment modality for unresectable tumors. The need for larger ablation zones has resulted in increased RF generator power. Skin burns due to ground pad heating are increasingly limiting further increases in generator power, and thus, ablation zone size. We investigated a method for reducing ground pad heating in which a commercial ground pad is segmented into multiple ground electrodes, with sequential activation of ground electrode subsets. We created finite-element method computer models of a commercial ground pad (14 times 23 cm) and compared normal operation of a standard pad to sequential activation of a segmented pad (two to five separate ground electrode segments). A constant current of 1 A was applied for 12 min in all simulations. Time periods during sequential activation simulations were adjusted to keep the leading edge temperatures at each ground electrode equal. The maximum temperature using standard activation of the commercial pad was 41.7degC. For sequential activation of a segmented pad, the maximum temperature ranged from 39.3degC (five segments) to 40.9degC (two segments). Sequential activation of a segmented ground pad resulted in lower tissue temperatures. This method may reduce the incidence of ground pad burns and enable the use of higher power generators during RF tumor ablation.     

Three-dimensional finite element analysis of current density andtemperature distributions during radio-frequency ablation

This study analyzed the influence of electrode geometry, tissue-electrode angle, and blood flow on current density and temperature distribution, lesion size, and power requirements during radio-frequency ablation. The authors used validated three-dimensional finite element models to perform these analyses. They found that the use of an electrically insulating layer over the junction between electrode and catheter body reduced the chances of charring and coagulation. The use of a thermistor at the tip of the ablation electrodes did not affect the current density distribution. For longer electrodes, the lateral current density decreased more slowly with distance from the electrode surface. The authors analyzed the effects of three tissue-electrode angles: 0, 45, and 90°. More power was needed to reach a maximal tissue temperature of 95°C after 120 s when the electrode-tissue angle was 45°. Consequently, the lesions were larger and deeper for a tissue-electrode angle of 45° than for 0 and 90°. The lesion depth, volume, and required power increased with blood flow rate regardless of the tissue-electrode angle. The significant changes in power with the tissue-electrode angle suggest that it is safer and more efficient to ablate using temperature-controlled RF generators. The maximal temperature was reached at locations within the tissue, a fraction of a millimeter away from the electrode surface. These locations did not always coincide with the local current density maxima. The locations of these hottest spots and the difference between their temperature and the temperature read by a sensor placed at the electrode tip changed with blood flow rate and tissue-electrode angle     

Effect of proton isolation on DC and RF performance of GaAs planardoped barrier diodes

Planar doped barrier (PDB) diodes have been fabricated using both mesa and proton implant isolation. Comparative measurements of DC characteristics, RF noise figure and tangential sensitivity indicate that proton implant isolation gives devices of favourable performance following annealing at 290°C for 20 min, with the advantage of a planar process     

Temperature rises in the human eye exposed to EM waves in thefrequency range 0.6-6 GHz

The temperature rises in the human eye for plane wave exposures are investigated in the frequency range between 600 MHz and 6 GHz, which covers the hot spot frequency range. As a first step, the specific absorption rates (SARs) are calculated with the use of the finite-difference time-domain (FDTD) method and the mechanism of hot-spot formation is discussed. Then the temperature rises in the human eye are calculated by using Pennes' bioheat equation. In addition, the dependence of SARs and temperature rises on the electromagnetic (EM) wave polarization and the eye dimension is discussed. Furthermore, the temperature rises calculated are compared with the values found in the literature pertaining to microwave-induced cataract formation. Numerical results show that hot spots appear in a certain frequency range and that the location and number of hot spots depend on the frequencies of the incident wave. In particular, the averaged SARs and the temperature rise are found to depend obviously on the polarization of the EM wave. Additionally, the deviations in the SAR and the temperature rise caused by the eye size are found to be within 10%. Furthermore, the maximum temperature rise due to the incident EM power density of 5.0 mW/cm² , which is the maximum permissible exposure limit for controlled environments, is found to be 0.30°C at 6.0 GHz. This value is small but not negligible, as compared with the threshold temperature rise 3.0°C for cataract formation     

AlGaAs/InGaAs heterostructure doped-channel FET's exhibiting goodelectrical performance at high temperatures

High-power and high-efficiency GaAs heterostructure field-effect transistors (FETs) are attracting tremendous attention in RF power amplifier applications. However, thermal effects can be an important issue in RF power devices, owing to the huge amount of heat generated during their operation. In this paper, the temperature-dependent characteristics of Al_0.3Ga_0.7As/In_0.15Ga_0.85 As doped-channel FETs (DCFETs) are investigated and compared with conventional pseudomorphic-HEMTs (pHEMTs) devices, in terms of their dc, microwave and RF power performance at temperatures ranging from room temperature to 150°C. Due to conducting carriers being less influenced by temperature and the better Schottky diode characteristics that can be obtained in DCFETs, the intrinsic device parameters and output performance remain almost constant at high temperatures, which also results in better device reliability. The performance variation of DCFETs associated with temperatures from 25°C to 150°C all fall within a single digit, i.e., output power (P_out, 16.2 dBm versus 15.8 dBm), power gain (G_p, 16.6 dB versus 15.1 dB), power added efficiency (PAE, 34.2% versus 31.3%), which is not the case for conventional pHEMTs. Therefore, DC devices are very promising for microwave power device applications operating at high temperature     

Electromagnetic annealing for the 100 nm technology node

Electromagnetic induction heating (EMIH) is a novel rapid thermal processing technique that uses microwave and radio frequency (RF) radiation to directly heat silicon wafers. Heating rates of 125°C/s have been achieved and 75 mm diameter wafers have been heated above 1000°C using only 950 W of power. EMIH has been used to activate shallow implanted dopants with minimal diffusion of the junction depth. It is speculated that the exposure of the wafer to intense electric fields during the anneal may provide an additional driving force for dopant activation, allowing for higher activation at lower temperatures. Post-anneal junction depths less than 25 nm with sheet resistances between 700 and 1000 ohms/square have been achieved without the use of a controlled low oxygen ambient. The EMIH Rs-Xj curve penetrates the SEMATECH 100 nm technology box and with further optimizations may satisfy the 70 nm technology node     

Low-threshold current, high-efficiency 1.3-μm wavelengthaluminum-free InGaAsN-based quantum-well lasers

We demonstrate high-performance Al-free InGaAsN-GaAs-InGaP-based long-wavelength quantum-well (QW) lasers grown on GaAs substrates by gas-source molecular beam epitaxy using a RF plasma nitrogen source. Continuous wave (CW) operation of InGaAsN-GaAs QW lasers is demonstrated at λ=1.3 μm at a threshold current density of only J_TH =1.32 kA/cm². These narrow ridge (W=8.5 μm) lasers also exhibit an internal loss of only 3.1 cm^-1 and an internal efficiency of 60%. Also, a characteristic temperature of T₀=150 K from 10°C to 60°C was measured, representing a significant improvement over conventional λ=1.3 μm InGaAsP-InP lasers. Under pulsed operation, a record high maximum operating temperature of 125°C and output powers greater than 300 mW (pulsed) and 120 mW (CW) were also achieved     

Temperature distribution in semiconductor wafers heated in ahot-wall-type rapid diffusion furnace

Transient temperature distribution was calculated for wafers heated in a new hot-wall-type rapid diffusion furnace. Two-dimensional radiative heat transfer was combined with unsteady conduction in wafers and the furnace. The furnace is composed of parallel plate heaters, and heats wafers to a temperature of about 1000°C. The heaters are divided into four zones and their heating powers are PID-controlled. Two wafers on a holder are inserted vertically from the bottom of the furnace, and heated for three minutes. The calculated results show the wafer temperature approached the desired heating temperature about one minute after insertion, agreeing with experimental results. The average temperature distribution in the wafers during heating is found to be within ±1°C at 1000°C, when the heating power (temperature) of the four zones is properly controlled. The effects of heater temperature, insertion speed, and holder thickness on the temperature distribution in wafers were calculated. The new hot-wall-type rapid diffusion furnace can be used to manufacture future VLSI     

A custom-designed receiver-stimulator chip for an advancedmultiple-channel hearing prosthesis

A receiver-stimulator integrated circuit for an advanced multiple-channel cochlear implant was custom designed and fabricated successfully, using a 3-μm CMOS process with two layers of metal. The chip contains nearly all of the implant electronics, including a data receiver, digital-to-analog converter, three stimulus current generators and timing controllers, 20 output stages, and an outward telemetry subsystem. The measured power consumption of the chip when quiescent, with supply voltage of the receiver stimulator (V_DD) at 12 V and while receiving unmodulated RF carrier, was 6.8 mW. The implant's total power consumption when stimulating under worst-case conditions (three stimuli being generated at the maximum rate with maximum current levels and durations) was 45 mW     

Optimization of fluorine passivation of stainless steel surfaces

The optimum conditions for the fluorine passivation of 316L stainless steel are described. The direct fluoridation products formed at temperatures of 320°C or lower are composed solely of FeF₂ , while those which were formed at the temperatures of 330°C or higher have a compound-phase composition of FeF₂ and FeF ₃. At a critical temperature (400°C for 316L stainless steel) of the thermal modification process, FeF₃ is converted to FeF₂ and disappears completely as the temperature rises. Meanwhile, CrF₃ is formed at a certain temperature (440°C for 316L stainless steel). The two-phase composition gets further crystallized as the thermal modification temperature rises. As the crystal growth induces the cracks on the fluoridated film, it is very difficult to form a satisfactory passivation film from the two-phase composition by thermal modification. It is confirmed that excellent passivation film has been obtained from the single-phase composition by the optimum fluoridation following the optimum thermal modification     

A computer simulation of radio-frequency ablation of theendocardium

A computer simulation of radio-frequency (RF) ablation of the endocardium is performed. The objective is to quantify some of the parameters affecting lesion growth, and to obtain theoretical data which can be used as a guide to maximize the lesions obtained with the procedure. The model under consideration consists of a block of heart tissue with the catheter electrode making contact at a right angle on one side (endocardium) and a large grounded electrode on the other side. An RF electrical current flows between the electrodes, heating the tissue. The simulations provide information on the time evolution of the tissue temperature, lesion dimension and tissue resistance. A first set of calculations is based on an applied RF voltage that maintains the maximum tissue temperature at 100°C. The results reveal that: 1) the lesions achievable by RF ablation are considerably larger than those obtained with a hot-tip catheter of the same size; 2) increasing the electrode radius enlarges the lesion because of an associated increase in contact surface area; 3) an increase in electrode length also enlarges the lesion because of the larger convective losses to the blood flow; 4) a large difference in temperature may exist between the electrode and the tissue because of the cooling effect of the blood flow; and 5) the lesions grow as long as power is applied. Other simulations in which the RF voltage is constant show that the lesions can be enlarged by lowering the applied voltage while increasing the duration. Agreement and discrepancies between the simulations and reported experimental results are identified. Finally, suggestions for improving the procedure are given     

Single transverse mode operation of 1.55-μm buriedheterostructure vertical-cavity surface-emitting lasers

In this letter, we report the single-mode operation of 1.55-μm buried heterostructure vertical-cavity surface-emitting lasers (VCSELs) fabricated on a GaAs-AlAs distributed Bragg reflector using thin-film wafer fusion. A 7-μm VCSEL exhibits a single transverse mode at up to about 0.1-mW maximum optical output power and 75°C maximum operation temperature under continuous-wave operation     

High-temperature, low threshold current, and uniform operation1×12 monolithic AlGaInAs/InP strain-compensated multiple quantumwell laser array in 1.5 μm

In this paper, we describe the fabrication of a monolithically integrated 1×12 array of 1.5-μm AlGaInAs/InP strain-compensated multiple-quantum-well (MQW) lasers, which has high reliability and highly uniform characteristics in low threshold current, slope efficiency, and lasing wavelength. Besides, each diode on the array exhibits a high characteristic temperature of 88 K and a low slope-efficiency drop of less than 1 dB between 20-80°C and a lasing wavelength of 1510 nm at 20°C and 20 mA. Also, the diode on the array has a maximum resonance frequency of above 8 GHz or 3-dB modulation bandwidth of 12 GHz     

Wide temperature (-20°C-95°C) operation of an uncooled2.5-Gb/s 1300-nm DFB laser

This letter describes a 2.5-Gb/s 1300-nm distributed feedback laser that can operate in a wide temperature range of -20°C to 95°C. We present RF and DC characteristics of the device and the statistical distribution of threshold current and slope efficiency at high temperature. Finally, we demonstrate the device performance in a 2.5-Gb/s small-form-factor module up to 85°C     

磁共振射频信号对医疗器械影响的数值和实验研究

广泛研究了在MRI系统中的医疗器械射频致热问题。采用全波电磁仿真和热仿真工具,研究了器械长度与器械尖端周围的最大温升的相关性。测试结果显示,“最坏情况”的温升测量结果与仿真数据一致。该工作提出了一种高效、准确的方法,用来评估多种不同尺寸的医疗器械在磁共振环境下的射频致热情况。     

High-temperature storage and thermal cycling studies of thick filmand wirewound resistors

The operating ambient temperature for underhood automotive and aerospace applications is increasing. This work was undertaken to evaluate the suitability of thick film and wirewound resistors for distributed aircraft control systems in a 200°C-225°C operating environment. High temperature stability testing of power wirewound and thick film resistors is reported. Dale power wirewound 1 Ω, 100 Ω, and 10 kΩ resistors with power ratings of 5 W and 25 W were tested. The TCR of the 100 Ω, and 10 kΩ resistors was very small, however, the 1 Ω resistor varied by 5% over the temperature range from 25°C to 300°C. Stability with long term storage (10000 h) at 300°C was measured for the wirewound resistors unpowered and powered at 20% of rated power. With the exception of the 10 kΩ/25W resistor, the change in resistance was less than 4%. Wirewound resistors were also thermal cycled 1000 times over a temperature range from -55°C to 225°C with only one failure due to a broken internal connection. Three 900 Series thick film resistor pastes from Heraeus-Cermalloy were studied: 100 Ω/sq., 1 kΩ/sq., 10 kΩ/sq. The temperature coefficient of resistance (TCR) was measured from 27°C to 500°C in 50°C increments. The change in resistance was <±6% up to 300°C. A 2 × 2 matrix of variables was included in the 300°C storage test: untrimmed resistors, resistors trimmed up 50% in value, unpowered, and powered at 1/8 W. Palladium/Silver was the initial termination choice for these 300°C studies, but silver migration under electrical bias lead to electrical shorts between conductor traces on the substrates with powered resistors. Gold terminated thick film resistors were used for powered storage testing at 300°C. The change in resistance after 10000 h at 300°C was < 3% for all test combinations