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     

Noninvasive vasectomy using a focused ultrasound clip: thermalmeasurements and simulations

INTRODUCTION: Conventional surgical vasectomy may lead to complications including bleeding, infection, and scrotal pain. Noninvasive transcutaneous delivery of therapeutic focused ultrasound has previously been shown to thermally occlude the vas deferens. However, skin burns and inconsistent vas occlusion have presented complications. This study uses bio-heat transfer simulations and thermocouple measurements to determine the optimal ablation dosimetry for vas occlusion without skin burns. METHODS: A 2-rad ultrasound transducer mounted on a vasectomy-clip-delivered ultrasound energy at 4 MHz to the canine vas deferens co-located at the focus between the clip jaws. Chilled degassed water was circulated through an attached latex balloon, providing efficient ultrasound coupling into the tissue and active skin cooling to prevent skin burns. Thermocouples placed at the vas, intradermal, and skin surface locations recorded temperatures during ablation. Procedures were performed with transducer acoustic powers of 3-7 W and sonication times of 60-120 s on both the left and right vas deferens (n = 2) in a total of four dogs (precooling control, 3 W/120 s, 5 W/90 s, 7 W/60 s). Measurements were compared with bio-heat transfer simulations modeling the effects of variations in power and sonication time on tissue temperatures and coagulation zones. RESULTS: Active skin cooling produces a thermal gradient in the tissue during ablation, allowing sufficient thermal doses to be delivered to the vas without skin burns. However, low-power, long-duration heating produced excessive tissue necrosis due to thermal diffusion, while high power and short heating times reduced the therapeutic window and produced skin burns presumably due to direct ultrasound absorption. CONCLUSIONS: Both simulations and experiments suggest that a therapeutic window exists in which thermal occlusion of the vas may be achieved without the formation of skin burns in the canine model (power = 5-7 W, surface intensity = 1.4-1.9 W/cm2, time = 20-50 s). This range of ablation parameters will help guide future experiments to refine incisionless vasectomy using focused ultrasound.     

High-Power CW Klystron for Fusion Experiments

A 3.7-GHz 700-kW klystron in continuous-wave (CW) operation has been developed to upgrade the lower hybrid RF plasma heating power in a tokamak up to 10 MW. The klystron is equipped with a diode gun, a five-cavity RF structure, two BeO-disk RF windows, and a large-size X-ray-shielded hypervapotron collector. The output power is recombined in a four-port junction which we also developed. The tube is designed to deliver 620-kW CW RF power with a mismatched load (VSWR = 1.4) and 700-kW CW with a matched load. Several prototypes have been built with successive design improvements. The major improvement was to change one single RF output into two RF outputs. The most recently built prototype meets all design specifications at 73.1 kV and 20.7 A, with an efficiency of 47% on a matched load and 40% with a 1.4 : 1 VSWR load, worst case phase. The power losses dissipated in the body have been measured as low as 17 kW, which corresponds to the RF heating and implies low beam interception. The measured temperatures of the output cavity noses and collector wall have been kept below 130degC and 200degC, respectively, which results in large thermal margin.     

Temperature-controlled and constant-power radio-frequency ablation: what affects lesion growth?

Radio-frequency (RF) catheter ablation is the primary interventional therapy for the treatment of many cardiac tachyarrhythmias. Three-dimensional finite element analysis of constant-power (CPRFA) and temperature-controlled RF ablation (TCRFA) of the endocardium is performed. The objectives are to study: 1) the lesion growth with time and 2) the effect of ground electrode location on lesion dimensions and ablation efficiency. The results indicate that: a) for TCRFA: i) lesion growth was fastest during the first 20 s, subsequently the lesion growth slowed reaching a steady state after 100 s, ii) positioning the ground electrode directly opposite the catheter tip (optimal) produced a larger lesion, and iii) a constant tip temperature maintained a constant maximum tissue temperature; b) for CPRFA: i) the lesion growth was fastest during the first 20 s and then the lesion growth slowed; however, the lesion size did not reach steady state even after 600 s suggesting that longer durations of energy delivery may result in wider and deeper lesions, ii) the temperature-dependent electrical conductivity of the tissue is responsible for this continuous lesion growth, and iii) an optimal ground electrode location resulted in a slightly larger lesion and higher ablation efficiency.     

Analysis and Control of the Current Distribution under Circular Dispersive Electrodes

An idealized model for a circular electrosurgical dispersive electrode is analyzed by solving Laplace's equation for the potential distribution. The resulting analytical solution shows the origin of the perimetrical burn problem, and, simultaneously, points the way toward improved electrode designs. One possible improved electrode design, a segmented circular electrode in which the annular segments are connected to the RF generator through a series of current-leveling resistors, is proposed and discussed. Extensions of this concept and future areas for research are also discussed.     

RF heating of implanted spinal fusion stimulator during magneticresonance imaging

Radio frequency (RF) heating of an implanted spinal fusion stimulator (SpF) during magnetic resonance imaging (MRI) was studied on a full-size human phantom. Heating during MRI scans (GE Signa 4X, 1.5 T) was measured with RF-transparent fiberoptic sensors. With the implant correctly connected, the maximum temperature rises were less than 2°C during the 26 min that the scans were at maximum RF power. At the tip of a broken stimulator lead (connecting the SpF generator and its electrodes), the maximum temperature rise was 11-14°C. Regular 4-min scans of the spinal cord produced similar temperature rises at the broken tip. After the generator and the leads were removed, heating at the electrode connector tip was less than 1.5°C. The control temperature rises at the same locations, without the stimulator, were less than 0.5°C. This study shows that spinal fusion stimulator heating is within the Food and Drug Administration safety guideline of 2°C. However, if a lead wire is broken, it is unsafe during MRI scans. Radiological examinations will be necessary to ensure the integrity of the implant     

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.     

RF ablation at low frequencies for targeted tumor heating: in vitro and computational modeling results

RF ablation uses RF current to heat and kill cancer applied via an electrode inserted under image guidance. Tumor has about half the electrical resistivity of normal tissue below 20 kHz, but similar resistivity above 500 kHz. We placed normal porcine liver tissue in contact with agar gel having similar resistivity as tumor within 20-450 kHz. A needle electrode was placed with half of the electrically active tip in each layer. We performed ablation with electric current applied for 12 min at 30 W, either at 20 or 450 kHz (n = 7 each), while measuring temperature via thermocouples 4 and 8 mm from the electrode. Mathematical heat-transfer models were created of an equivalent configuration and temperature profile determined at both frequencies. At 8-mm distance, at 450 kHz, tumor gel phantom and normal tissue obtained similar temperatures (57.5 ± 1.4 versus 58.7 ± 2.5 (°)C); at 20 kHz, tumor phantom obtained significantly higher temperatures than normal tissue (65.6 ± 2.0 versus 57.2 ± 5.6 (°)C, p < 0.01). Computer models confirm these results, and show the ablation zone diameter to be larger within the tumor phantom at 20 kHz compared to 450 kHz. Heating at low RFs may thus allow targeted heating of tumor tissue and reduced heating of normal tissue.     

An Assessment of µ-Czochralski, Single-Grain Silicon Thin-Film Transistor Technology for Large-Area, Sensor and 3-D Electronic Integration

Single-grain (SG) thin-film transistors (TFTs) fabricated inside location-controlled silicon grains using the mu-Czochralski method are benchmarked for analog and RF applications. Each silicon grain is defined by excimer laser recrystallization of polysilicon. Thin-film transistors may be fabricated in this manner on silicon or low-cost flexible plastic substrates as processing temperatures remain below 350degC, making the SG-TFT a potential enabling technology for large-area highly integrated electronic systems or systems-in-package with low manufacturing cost. Operational amplifier and voltage reference circuits of varying complexity were designed and measured in order to evaluate the effects of channel position and processing variation on analog circuits. A two-stage telescopic cascode operational amplifier fabricated in an experimental 1.5 mum SG-TFT technology demonstrates a DC gain of 55 dB (unity-gain bandwidth of 6.3 MHz), while a prototype CMOS voltage reference with a power supply rejection ratio (PSRR) of 50 dB is also demonstrated. With f_T comparable to single-crystal MOSFETs of comparable gate length, the SG-TFT can also enable RF circuits for wireless applications. A 12 dB gain RF cascode amplifier with on-chip inductors and operating in the 433 MHz ISM band is demonstrated. Excellent agreement with simulations is attained using a modified BSIM-SOI model extracted from measurements of experimental SG-TFT devices.     

Electrical characteristics of a linear, nonequilibrium, MHD generator

The purpose of this experimental study was to determine the electrical performance of a linear, segmented electrode, MHD generator. Various noble gases were shock-heated to plasma conditions that allowed MHD generator operation with Hall parameters ωeτeranging from 1 to 20 and ratios of electron to static gas temperatures of 1 to 2.5. For Te/Tg> 1, the discharge structure in the generator was nonuniform. The major results were as follows. 1) By comparing the experiments with an MHD generator theory that included the effect of nonuniform extrathermal ionization, it was found that the Faraday generator operated in the "normal" mode, in which the current vector is approximately perpendicular to the axial flow direction, at all values of ωeτe. The electrical performance of the generator at high ωeτewas much better than that predicted for the "shorted" mode of operation in which the current vector is inclined in the axial Hall direction. 2) It was found that it was possible to obtain good agreement between the experiments and a simple uniform plasma, segmented electrode theory that included the Lorentz force effects, the electrode voltage losses, and the increased dissipation due to the plasma nonuniformities in the analyses. 3) Because of the relatively coarse electrode segmentation, it could not be established conclusively whether the effect of the electrothermal instabilities or the effect of finite electrode segmentation limited the maximum attainable Hall field.     

High-Power High-Brightness Operation of a 2.25- $mu$m (AlGaIn)(AsSb)-Based Barrier-Pumped Vertical-External-Cavity Surface-Emitting Laser

We report on the output power performance and beam profile analysis of an optically pumped GaSb-based vertical-external-cavity surface-emitting laser operating at 2.25 mum. The use of a SiC heatspreader and a precise control of the temperature-dependent modal gain allowed for improved high-power operation with a maximum continuous-wave output power of 3.4W at 10degC heatsink temperature. At 0degC, a maximum output power >2.9 W was observed and still more than 1.6 W were obtained at room temperature. Using second-order moments for the definition of the beam diameter, a beam propagation factor M²~5 of was measured at maximum output power. Optimizing the beam quality of the laser resulted in a beam profile close to the Gaussian TEM₀₀ mode (M² ap 1.5) and still more than 2-W output power at 0degC.     

Evaluation of microwave and radio frequency catheter ablation in amyocardium-equivalent phantom model

A highly localized burst of energy applied to the myocardium via a transvenous catheter-mounted power source can be used to destroy endocardial tissue regions which mediate life-threatening arrhythmias. In the past, high-voltage direct current pulses, radio-frequency (RF) current, and laser light have been used as energy sources. In this paper, the use of 2450 MHz microwave energy applied via a miniature coaxial cable-mounted helical coil antenna designed specifically for this application was investigated as a means to increase the treated volume of cardiac tissue in a controllable and efficient manner during ablation. Using an array of fiber optic temperature probes implanted in a saline-perfused, tissue-equivalent gel phantom model designed to simulate the myocardium during ablation, the heating pattern from the microwave antenna was characterized and compared to that induced by a commercial RF electrode catheter at 550 kHz. Effects of variable contact angle between the heat source and heart wall were assessed in terms of the radial penetration and overall volume of heated tissue. Heating patterns from the RF electrodes dropped off much more abruptly both radially and axially than the microwave antenna such that the volume of effectively heated tissue was more than ten times larger for the microwave antenna when the heat sources were well-coupled to the tissue, and more than four times larger for the microwave antenna when the sources were angled 30 degrees away from the tissue surface.     

A Sub-1-V, 10 ppm/ °C, Nanopower Voltage Reference Generator

An extreme low power voltage reference generator operating with a supply voltage ranging from 0.9 to 4 V has been implemented in AMS 0.35-mum CMOS process. The maximum supply current measured at the maximum supply voltage and at 80degC is 70 nA. A temperature coefficient of 10 ppm/degC is achieved as the combined effect of 1) a perfect suppression of the temperature dependence of mobility; 2) the compensation of the channel length modulation effect on the temperature coefficient; and 3) the absence of the body effect. The power supply rejection ratio without any filtering capacitor at 100 Hz and 10 MHz is lower than -53 and -42 dB, respectively. The occupied chip area is 0.045 mm².     

Analysis of RF MEMS switch packaging Process for yield improvement

Radio frequency microelectro-mechanical systems (RF MEMS) switches offer significant performance advantages in high-frequency RF applications. The switches are actuated by electrostatic force when voltage was applied to the electrodes. Such devices provide high isolation when open and low contact resistance when closed. However, during the packaging process, there are various possible failure modes that may affect the switch yield and performance. The RF MEMS switches were first placed in a package and went through lid seal at 320degC. The assembled packages were then attached to a printed circuit board at 220degC. During the process, some switches failed due to electrical shorting. Interestingly, more failures were observed at the lower temperature of 220degC rather than 320degC. The failure mode was associated with the shorting bar and the cantilever design. Finite element simulations and simplified analytical solutions were used to understand the mechanics driving the behaviors. Simulation results have shown excellent agreement with experimental observations and measurements. Various solutions in package configurations were explored to overcome the hurdles in MEMS packaging and achieve better yield and performance     

Three-dimensional finite-element analyses for radio-frequencyhepatic tumor ablation

Radio-frequency (RF) hepatic ablation, offers an alternative method for the treatment of hepatic malignancies. We employed finite-element method (FEM) analysis to determine tissue temperature distribution during RF hepatic ablation. We constructed three-dimensional (3-D) thermal-electrical FEM models consisting of a four-tine RF probe, hepatic tissue, and a large blood vessel (10-mm diameter) located at different locations. We simulated our FEM analyses under temperature-controlled (90 degrees C) 8-min ablation. We also present a preliminary result from a simplified two-dimensional (2-D) FEM model that includes a bifurcated blood vessel. Lesion shapes created by the four-tine RF probe were mushroom-like, and were limited by the blood vessel. When the distance of the blood vessel was 5 mm from the nearest distal electrode 1) in the 3-D model, the maximum tissue temperature (hot spot) appeared next to electrodes A. The location of the hot spot was adjacent to another electrode 2) on the opposite side when the blood vessel was 1 mm from electrode A. The temperature distribution in the 2-D model was highly nonuniform due to the presence of the bifurcated blood vessel. Underdosed areas might be present next to the blood vessel from which the tumor can regenerate.     

Noncontact radio-frequency ablation for obtaining deeper lesions

Radio-frequency (RF) cardiac catheter ablation has been very successful for treating some cardiac arrhythmias, however, the success rate for ventricular tachycardias is still not satisfactory. Some existing methods for developing deeper lesions include active cooling of the electrode and modifying the electrode shape. We propose a method of noncontact ablation, to solve this problem. We apply 120 W of power through an 8-mm electrode for a 120-s duration, with distances from 0 to 3 mm between electrode and myocardium, to create lesions in myocardium. We apply flow rates of 1, 3, and 5 L/min to determine their effect. Results show that with an optimal distance from 0.5 to 1.5 mm between electrode and myocardium, we increase lesion depth from 7.5 mm for contact ablation to 9.5 mm for noncontact ablation. For different flow rates, the optimal distance various. The effect of flow rate is not obvious. Higher flow rate does not lead to a deeper lesion.     

Flow effect on lesion formation in RF cardiac catheter ablation

This study investigated the flow effect on the lesion formation during radio-frequency cardiac catheter ablation in temperature-controlled mode. The blood flow in heart chambers carries heat away from the endocardium by convection. This cooling effect requires more power from the ablation generator and causes a larger lesion. We set up a flow system to simulate the flow inside the heart chamber. We performed in vitro ablation on bovine myocardium with three different flow rates (0 L/min, 1 L/min and 3 L/min) and two target temperatures (60 degrees C and 80 degrees C). During ablation, we also recorded the temperatures inside the myocardium with a three-thermocouple temperature probe. The results show that lesion dimensions (maximum depth, maximum width and lesion volume) are larger in high flow rates (p<0.01). Also, the temperature recordings show that the tissue temperature rises faster and reaches a higher temperature under higher flow rate.     

Modeling bipolar phase-shifted multielectrode catheter ablation

Atrial fibrillation (AFIB) is a common clinical problem affecting approximately 0.5-1% of the United States population. Radio-frequency (RF) multielectrode catheter (MEC) ablation has successes in curing AFIB. We utilized finite-element method analysis to determine the myocardial temperature distribution after 30 s, 80 degrees C temperature-controlled unipolar ablation using three 7F 12.5-mm electrodes with 2-mm interelectrode spacing MEC. Numerical results demonstrated that cold spots occurred at the edges of the middle electrode and hot spots at the side electrodes. We introduced the bipolar phase-shifted technique for RF energy delivery of MEC ablation. We determined the optimal phase-shift (phi) between the two sinusoidal voltage sources of a simplified two-dimensional finite-element model. At the optimal phi, we can achieve a temperature distribution that minimizes the difference between temperatures at electrode edges. We also studied the effects of myocardial electric conductivity (sigma), thermal conductivity (k), and the electrode spacing on the optimal phi. When we varied sigma and kappa from 50% to 150%, optimal phi ranged from 29.5 degrees to 23.5 degrees, and in the vicinity of 26.5 degrees, respectively. The optimal phi for 3-mm spacing MEC was 30.5 degrees. We show the design of a simplified bipolar phase-shifted MEC ablation system.     

Comparison of power deposition patterns produced by microwave andradio frequency cardiac ablation catheters

Transcatheter microwave and radio frequency (RF) electrodes are used for the ablative treatment of cardiac arrhythmias. The authors compare the power deposition patterns of microwave antennas and RF electrodes in saline phantoms of biological material. The decrease in power deposition as a function of distance away from the electrode is nearly exponential for RF and is considerably steeper than microwave antennas. These results suggest that microwave antennas are capable of heating a larger volume and thus creating a greater lesion than RF     

Design and characterization of a 10-Gb/s dual-drive Z-cut Ti:LiNbO/sub 3/ electrooptical modulator

The design and characterization of a dual-drive (DD) Z-cut Ti:LiNbO/sub 3/ electrooptical modulator are presented. Both the radio frequency (RF) electrode layout and the optical splitters and combiners of the Mach-Zehnder interferometer are investigated: The former is designed to avoid RF line crosstalk, whereas the latter is to obtain a low-loss longitudinally short splitter structure, able to properly separate the interferometer straight optical waveguide sections. First, the RF analysis is addressed, simulating the small-signal behavior of the DD coplanar waveguide electrode structure as a function of the central ground width. The performances of different splitter layouts (circular segment bends, sin-bends, and offset bends) are then investigated using the beam propagation method. Finally, experimental results on the electrical and optical test pattern structures, and of the DD Z-cut complete modulator, are discussed and compared with simulations.