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.     

Ultrahigh-field MRI whole-slice and localized RF field excitations using the same RF transmit array

In this paper, a multiport driving mechanism is numerically implemented at ultra high-field (UHF) magnetic resonance imaging (MRI) to provide 1) homogenous whole-slice (axial, sagittal, or coronal) and 2) highly localized radio frequency (RF) field excitation within the same slices, all with the same RF transmit array (here chosen to be a standard transverse electromagnetic (TEM) resonator/coil). The method is numerically tested using a full-wave model of a TEM coil loaded with a high-resolution/18-tissue/anatomically detailed human head mesh. The proposed approach is solely based on electromagnetic and phased array antenna theories. The results demonstrate that both homogenous whole-slice as well as localized RF excitation can be achieved within any slice of the head at 7 T (298 MHz for proton imaging).     

CMOS high-speed dual-modulus frequency divider for RF frequencysynthesis

The architecture of a high-speed low-power-consumption CMOS dual-modulus frequency divider is presented. Compared to other designs fabricated with comparable CMOS technologies, this architecture has a better potential for high-speed operation. The circuit consumes less power than previously reported CMOS circuits, and it approaches the performance previously achieved only by bipolar or GaAs devices. The proposed circuit uses level-triggered differential logic to create an input-frequency-entrained oscillator performing a dual-modulus frequency division. In addition to high-speed and low-power consumption, the divider has a low-input signal level requirement which facilitates its incorporation into RF applications. Fabricated with a 1.2-μm 5-V CMOS technology, the divider operates up to 1.5 GHz, consuming 13.15 mW, and requiring less than 100 mV rms input amplitude     

High frequency characterization of gate resistance in RF MOSFETs

The gate resistances (R_g) of MOSEETs with various geometries have been characterized at various bias conditions at high frequency (HF). The results show that R_g decreases when either channel length (L_f) or per-finger-width (W_f) increases before reaching a critical L_f or W _f, and then starts to increase as L_f or W_f continues to increase. The irregular geometry dependence of R_g is caused by the combined distributed effects in both the gate and channel at HF. Stronger contribution from the distributed channel to the effective R_g is observed in the saturation region of devices with longer channel length (L_f) at lower gate bias (V_gs). The results show that an optimized design of the per-finger-width is necessary for an rf MOSFET to achieve the lowest effective R_g, which is desirable in rf applications     

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.     

Temperature measurement within myocardium during in vitro RF catheter ablation

While most commercial ablation units and research systems can provide catheter tip temperature during ablation, they do not provide information about the temperature change inside the myocardium, which determines the lesion size. We present the details of a flow simulation and temperature measurement system, which allows the monitoring of the temperature change inside the myocardium during in vitro radio frequency (RF) cardiac catheter ablation at different blood flow rates to which the catheter site may be exposed. We set up a circulation system that simulated different blood flow rates of 0 to 5 L/min at 37 degrees C. We continuously measured the temperature at the catheter tip using the built-in thermistor and inside the myocardium using a three-thermocouple probe. The system provides a means for further study of the temperature inside myocardium during RF catheter ablation under different flow conditions and at different penetration depths.     

Using electrical impedance to predict catheter-endocardial contact during RF cardiac ablation

During radio-frequency (RF) cardiac catheter ablation, there is little information to estimate the contact between the catheter tip electrode and endocardium because only the metal electrode shows up under fluoroscopy. We present a method that utilizes the electrical impedance between the catheter electrode and the dispersive electrode to predict the catheter tip electrode insertion depth into the endocardium. Since the resistivity of blood differs from the resistivity of the endocardium, the impedance increases as the catheter tip lodges deeper in the endocardium. In vitro measurements yielded the impedance-depth relations at 1, 10, 100, and 500 kHz. We predict the depth by spline curve interpolation using the obtained calibration curve. This impedance method gives reasonably accurate predicted depth. We also evaluated alternative methods, such as impedance difference and impedance ratio.     

Influence of silicon doping density on vacuum field emission triode at high frequency range

Silicon FEA will affect the high frequency application of field emission tubes when it works at the microwave frequency range. This article shows that the electron emitting will be influenced by the majority carrier response time in semiconductor silicon. The surface capacitance and delay time of n-type and p-type silicon are calculated by using semiconductor theory. The result shows that the semiconductor conductivity will determine the maximum work frequency of device. The maximum work frequency (no considering other effects such as C_gc, g_m etc.) will be decreased from about 200 GHz to 2 GHz when the resistivity of p-type silicon is increased from 0.1 Ω · cm to 10 Ω cm.     

RF frequency shifting via optically switched dual-channel PZT fiber stretchers

We demonstrate a new technique for RF serrodyne frequency shifting with ultrahigh sideband suppression. This is accomplished using two parallel optically switched piezo fiber stretchers driven with two out-of-phase serrodyne modulations. We achieved sideband suppression of nearly 50 dB, an improvement of almost 30 dB compared to the standard fiber stretching approach. The frequency shifter is easily integrated into photonic and RF systems, is functional in certain broad-band applications, and allows for tunable translations.     

GNSS射频芯片中小数分频技术研究

针对GNSS射频前端PLL频率综合器中的低杂散小数分频问题,提出了分别基于累加器结构和MASH1-1-1Δ-∑结构的两种小数分频调制器实现方案。进而选取3.996 MHz为GNSS射频前端模拟中频频率,16.368 MHz为PLL频率综合器参考频率,在GPS L1和BD-2 B1频点上对30级累加器级联结构和MASH1-1-1Δ-∑结构的输出功率谱进行分析,并在此基础上对它们的小数杂散特性进行了对比研究。结果表明,MASH1-1-1Δ-∑结构具有噪声整形功能,可将小数杂散由低频段推至高频段,从而在低频段获得更优的杂散特性。由于高频段的杂散可被PLL环路滤波器滤除,故MASH1-1-1Δ-∑结构更适合用在基于PLL的频率综合器中。     

Polarization and frequency effects on whole animal absorption of RF energy

A parallel-plate waveguide is used to determine wide-band RF (285 to 4000 MHz) absorption characteristics of 96- to 390-g rats and biological-phantom prolate spheroidal bodies. The results compare well to those of free space irradiation. At resonance, for E along the long dimension (a) power deposition of 9 times higher then that for H||a orientation is observed.     

RF and microwave frequency properties of a reverse-biased thickswitching p-i-n diode

The frequency response of the dc reverse-bias voltage keeping high-impedance p-i-n diode in low-distortion state is studied using different mathematical and theoretical models. This paper discusses the comparison between the theoretical and experimental results     

FEM analysis of predicting electrode-myocardium contact from RF cardiac catheter ablation system impedance

We used the finite-element method (FEM) to model and analyze the resistance between the catheter tip electrode and the dispersive electrode during radio-frequency cardiac catheter ablation for the prediction of myocardium-electrode contact. We included deformation of the myocardial surface to achieve accurate modeling. For perpendicular catheter contact, we measured the side view of myocardial deformation using X-ray projection imaging. We averaged the deformation contour from nine samples, and then incorporated the contour information into our FEM model. We measured the resistivity of the bovine myocardium using the four-electrode method, and then calculated the resistance change as the catheter penetrated into the myocardium. The FEM result of resistance versus catheter penetration depth matches well with our experimental data.     

Discharge-induced frequency modulation of RF excited CO2waveguide lasers

The mechanisms causing a shift in the oscillation frequency of an RF-excited CO₂ waveguide laser through changes in the discharge excitation power are investigated. Frequency shifts in the range of ±0.5 to 1 MHz per watt of RF input power were measured. These are shown to be consistent with the effects of thermal expansion of the laser gas caused by an increase of the gas temperature due to RF power absorption. It is also shown that the effects of gas dissociation are small but significant, whereas, contrary to earlier suggestions, the effects of electron density fluctuations are negligible. The discharge-induced frequency shift may be used as a simple frequency modulator with a frequency deviation of ±30 MHz, although the usefulness of this effect is limited to a bandwidth of about 500 Hz, due to the acoustic resonance of the waveguide channel     

Performance of RF MEMS switches at low temperatures

The actuation voltage of microelectromechanical system (MEMS) metal switches was investigated at temperatures ranging from 10 to 290 K. The investigation shows a 50% increase in the actuation voltage at low temperature. A comparison has been made using a published model and showed similar increment of actuation voltage at low temperature     

基于MATLAB的超声换能器声场可视化软件开发

对超声换能器的声场进行数值计算是当前研究的热点,在分析超声换能器声场模型的基础上,用MATLAB的GUI工具箱对声场进行可视化设计。通过输入换能器参数和介质参数,可以可视化地得到不同情况下的声场分布图,该软件可用于优化检测工艺,也可以指导探头设计。     

真实火箭燃气射流的实时显示及定量测试技术研究

提出了一种实时显示真实火箭燃气射流和定量测试其参数分布的技术。它是由激光光源、大口径莫尔偏折仪、CCD、监视器、图象存储和处理系统以及486计算机等组成。利用这种技术,实时显示了真实火箭燃气射流场,定量计算了流场的参量分布.