Young's Modulus Reconstruction for Radio-Frequency Ablation Electrode-Induced Displacement Fields: A Feasibility Study

Radio-frequency (RF) ablation is a minimally invasive treatment for tumors in various abdominal organs. It is effective if good tumor localization and intraprocedural monitoring can be done. In this paper, we investigate the feasibility of using an ultrasound-based Young's modulus reconstruction algorithm to image an ablated region whose stiffness is elevated due to tissue coagulation. To obtain controllable tissue deformations for abdominal organs during and/or intermediately after the RF ablation, the proposed modulus imaging method is specifically designed for using tissue deformation fields induced by the RF electrode. We have developed a new scheme under which the reconstruction problem is simplified to a 2-D problem. Based on this scheme, an iterative Young's modulus reconstruction technique with edge-preserving regularization was developed to estimate the Young's modulus distribution. The method was tested in experiments using a tissue-mimicking phantom and on ex vivo bovine liver tissues. Our preliminary results suggest that high contrast modulus images can be successfully reconstructed. In both experiments, the geometries of the reconstructed modulus images of thermal ablation zones match well with the phantom design and the gross pathology image, respectively.      

Design procedure for a Fresnel-zone plate antenna

A design procedure for two types of Fresnel-zone plate antennas is presented: viz. antennas with absorbing/transparent zones and phase-correcting zones. Design objectives are the optimization of the gain of the lens of the antenna and the side-lobe envelope of its radiation pattern. The design rules are derived from numerical simulations.     

Finite-element analysis of hepatic cryoablation around a large blood vessel

Cryoablation is a minimally invasive ablation technique for primary and metastatic hepatic tumors. Inadequate freezing around large blood vessels due to the warm blood flow can lead to local recurrence, and thus, necessitates close application of a cryoprobe to the large blood vessels. In this study, we constructed a perfusion model with an ex vivo bovine liver and ablated the tissue around a large blood vessel with one or two cryoprobes applied to the side of the vessel. The finite-element computer model developed in our previous study was modified to include a blood vessel and its convective heat transfer to the vicinity of the blood vessel. We compared the predicted simulation results to those acquired from this ex vivo perfusion model. The results indicate that blood vessels act as a heat source and generate steep temperature profiles in the area next to the large blood vessel. After validation, the maximum allowable distance between the cryoprobe and the large blood vessel for successful cryoablation was presented. The results of this study should be considered when placing cryoprobes in the vicinity of large blood vessels.     

Cylindrical and three-dimensional corner reflector antennas

Two reflector antennas are proposed. The first is constructed by adding a cylindrical reflecting surface of suitable radius to theV-shaped corner reflector antenna. The feeding element is a half-wavelength dipole. The resulting cylindrical corner reflector provided a 2 dB increase in gain, minimum sidelobe level, low input reactance, and uncritical dependence of performance on frequency. The second antenna is constructed by adding a cylindrical surface to the three-dimensional corner reflector. This extension provided an increase in gain of at least 6.5 dB, an input resistance compatible with the commercially available 50- or75-Omegacoaxial cables, low input reactance, and uncritical dependence of performance on frequency. A grid-type cylindrical corner reflector antenna, and a three-dimensional corner reflector antenna with a cylindrical subsurface of finite reflecting surfaces were designed, and the measured input impedances, gains, and field patterns showed excellent agreement with the theoretical results for both antennas.     

Low-Q, Electrically Small, Efficient Near-Field Resonant Parasitic Antennas

Metamaterial-inspired electrically small Z, stub and canopy antennas are reported. They are near-field, resonant parasitic designs. Different Z and stub antenna configurations and the effect on their Q values are studied. Their behavior led to the canopy antenna design. At the size of ka ~ 0.046, the canopy antenna is an electric-based antenna with high overall efficiency (over 90%) and low Q-ratio value and whose input resistance is almost completely matched to a 50 Omega source. The resonant frequency, ~300 MHz, in the UHF band is selected for the designs. The canopy antenna is studied extensively to explore the lowest achievable Q values. Various coupling configurations, canopy shapes, and metal-air ratios are investigated. Circuit models are also introduced to explain the radiation mechanism. Numerical simulation results are analyzed and compared with previously derived Q value limits for electrically small antennas that are based on the standard circuit models of spherical wave multipoles. The Q value of the canopy antenna for the lowest order, single electric resonance is shown to reach a fundamental limit of approximately 1.75 times the Chu value.     

Three-dimensional electromagnetic power deposition in tumors usinginterstitial antenna arrays

Interstitial arrays of insulated antennas have shown promise for microwave hyperthermia treatment of deep-seated tumors. Available analytical techniques for predicting the electromagnetic (EM) power deposition of these antennas have been limited to the case of a homogeneous conductive medium surrounding the array. Since tumors and host tissue may differ in their electrical characteristics, it is necessary to consider the impact of this variation in electrical properties and the geometry of the tumor in the calculation of the EM field distribution and power deposition pattern when modeling interstitial antennas. In this paper a three-dimensional model of a tumor of arbitrary shape subjected to the fields of an interstitial antenna array is developed to predict the EM power deposition in an inhomogeneous tumor-tissue medium. The volume integral equation for the imbedded tumor is developed and solved by method of moments. The incident fields are calculated based on the available formulation of interstitial antennas in homogeneous media. The accuracy of the developed computer code was checked by comparing the results from the volume integral approach with the Mie solution for the special case of spherical tumors. Good comparison was obtained for tumors with properties approximately 25 percent different from those of the surrounding tissue. Comparisons of results from models of antenna arrays with and without imbedded tumors show significant differences in their predictions of the EM power deposition in the tumor. Hyperthermia protocols generally specify uniform temperature distribution within the tumor. The developed inhomogeneous model was used to examine the feasibility of controlling the uniformity of the power deposition pattern in large tumors by adjusting the amplitude or relative phase between the array elements. Results are presented to show that a phase lead of +90 degrees or relative amplitude of 4.0 on one antenna in a square array of four antennas could be used to shift the power deposition pattern to sequentially heat outer portions of a 2 cm diameter tumor, thereby achieving a more uniform time-averaged temperature distribution in the tumor.     

Electromagnetics for the heart

Previously, the treatment of patients with cardiac arrhythmias was mostly palliative, involving lifelong dependence on medication. Moreover, in a significant portion (10% to 15%) of these patients, available drug therapy has been found unsatisfactory because of a lack of meaningful response or unacceptable, side effects. Surgical intervention has been the principal method of treatment in these cases. However, alternatives to surgery have been sought, in an effort to reduce the cost and morbidity of surgical treatment. During the past decade, minimally invasive microwave and radio-frequency (RF) cardiac ablation - in particular, RF cardiac ablation - has become a widely used procedure for the treatment of cardiac arrhythmias. Minimally invasive intervention offers many benefits: long incisions are replaced with a puncture wound; major cardiac and pulmonary complications are sidestepped; the need for postoperative intensive care is significantly reduced; and, in many cases, minimally invasive intervention offers a "cure" without major surgery. Furthermore, it has important advantages over drugs that are merely palliative, and only alleviate symptoms. Namely, it avoids the side effects and inconvenience of chronic drug therapy.     

High transmission gain inverted-F antenna on low-resistivity Si for wireless interconnect

Inverted-F antennas of 2-mm axial length are designed and fabricated on a low-resistivity silicon substrate (10 /spl Omega//spl middot/cm) using a post back-end-of-line process. For the first time, their performances are measured up to 110 GHz for wireless interconnects. Results show that a sharp resonance can be seen at 61 GHz for the antenna, and a high transmission gain of -46.3 dB at 61 GHz is achieved from the pair of inverted-F antennas at a separation of 10 mm on a standard 10 /spl Omega//spl middot/cm silicon wafer of 750-/spl mu/m thickness.     

Ultra wideband antenna

Ultra wideband antennas are specifically designed to transmit and/or receive very short time durations of electromagnetic energy. It is well known that UWB antenna design remains the major factor in the progress of UWB technology. This article describes a study of conventional antennas and why they are not suitable for a UWB system.     

Radiation Efficiency of Nano-Radius Dipole Antennas in the Microwave and Far-infrared Regimes

At microwave and far-infrared frequencies, the radiation efficiency of a wire antenna with a radius value smaller than a few hundred nanometers is very low, due to large wire impedances and associated high ohmic losses. However, with the continued miniaturization of electronic devices, nano-radius interconnects and antennas are desirable. In this work, the relationships among wire radius, conductivity, frequency, and ohmic loss are examined for dipole antennas. Simple formulas are derived for the distributed resistance, effective conductivity, and radius required to achieve a desired radiation efficiency, and particular emphasis is given to half-wavelength antennas. Several methods to improve antenna efficiency at sub-100-nm radius values are discussed, including the use of superconducting nanowires and multi-wall carbon nanotubes.     

早期乳腺肿瘤的超宽带微波成像

通过采用时域有限差分方法探讨了一种收发天线分离的早期乳腺癌超宽带微波成像检测方案.在收发共用的超宽带天线设计中要求天线的终端反射不超过-120dB,而采用收发天线分离方案则避免了这一苛刻要求.采用共焦成像算法给出的成像结果表明,该方案亦可探测直径10mm以下的早期乳腺肿瘤.     

Low-cost antenna of series-fed printed strip dipoles

A newly developed antenna comprising series-fed printed strip dipoles is described. The effect of substrate permittivity on the radiation properties of the antenna is investigated, using an analysis model based on the general concept of the equivalent radius of cylindrical antennas. An antenna with a gain of >9 dB and VSWR⩽1.5 over the frequency range 1.8-2.2 GHz is presented     

Heating characteristics of thin helical antennas with conductingcores in a lossy medium. I. Noninsulated antennas

We report a combined theoretical and experimental study of the heating characteristics of helical antennas in lossy dielectric media. Proposed biomedical application of such antennas include angioplasty, hyperthermia, and catheter ablation of tissue. The study focuses on helical antennas, operated in the normal mode (wavelength greater than antenna diameter but comparable to antenna length), that are terminated at one end by a short circuit and at the other by a coaxial feedpoint. The analytical model is based on the helical sheath approximation, extended to the case of lossy media. In addition, experimental studies were performed on helical antennas immersed in aqueous electrolyte of various conductivity. The antennas show two distinct modes of propagation: a slow mode similar to that observed in helical antennas in loss-free media, and a faster mode. The analytical/numerical results are in good agreement with experimental data, thus demonstrating the validity of the model     

Cavity-Backed Miniature Wideband UHF Circular Polarized Antenna With Textured Dielectrics

Use of high permittivity dielectrics with antennas is known to reduce their operating frequency but also introduces side effects such as reduced gain and bandwidth. This paper presents a miniaturization scheme for a 15 cm (6 inch) square circularly polarized (CP), cavity-backed antenna using textured dielectric loading and a folded metallic strip feed. The proposed design has an operational frequency f, around 500 MHz implying a linear size of 0.25lambda₀. The challenge in designing such a small size antenna is to increase bandwidth (10-15%) while retaining a gain of 2-4 dBi. By carefully positioning high permittivity textured dielectrics within the aperture in conjunction with shape design, we show that bandwidth and gain are retained as compared to comparable size antennas in the literature. As part of our design, we present parametric studies through computer simulations and a prototype antenna is fabricated and measured for verification     

The Effects of Driving Frequency and Antenna Length on Power Deposition Within a Microwave Antenna Array Used for Hyperthermia

The theory of the linear, insulated antenna embedded in an electrically dense medium is applied to microwave antennas used for hyperthermia cancer therapy. The pattern of power deposition is computed for a square array of four antennas with a side length of 3 cm under the assumption of no coupling among antennas. The driving frequency is set to seven values between 300 and 915 MHz, and the antenna halflength is set to three values: 3 cm, 6 cm, and the resonant value.     

Evaluating near-field radiation patterns of commercial antennas

A source reconstruction technique from the measured near fields is proposed to obtain a set of equivalent currents that will characterize the forward and backward radiation patterns of an antenna. Once the equivalent sources are determined, the electromagnetic field at any aspect angle and distance from the antenna can be calculated. In this paper, the method is applied to the evaluation of the radiation from commercial antennas at any observation point. The electric field patterns of a DCS base station antenna at 1800 MHz and a horn antenna at 2500 MHz have been calculated and plotted at several distances from the antenna. This method can be used in characterizing the "reference volumes" or exclusion zones for transmitting antennas dealing with the maximum levels of electromagnetic radiation safe for human exposure, as stated in many national and international regulations.     

低剖面可承载UHF/L双频共口径圆极化定向天线

机/车载多体制通信、测控、探测等多无线系统并存,天线林立,天线间耦合干扰严重,同时与高速机/车薄壁外壳共形的天线还需具有较强承载能力.本文设计了一种适于车壳共形的低剖面可承载双频共口径定向圆极化缝隙天线.天线主体采用开口缝隙的形式来实现天线的小型化和宽带设计,利用微带线来进行耦合馈电.通过在低频天线中心开槽嵌入高频天线...     

计算曲线天线电流分布的一种新途径─参数形式下的B样条有限元法──Ⅱ：应用实例─多臂螺旋天线的电流分布

多臂螺旋天线由于其宽频和多模特征，它在空中和地面天线系统中得到了广泛的应用。本文详细描述了基于第一部分提出的参数形式下的Ｂ样条有限元法计算等角螺旋天线和阿基米德平面螺旋天线电流分布的实施过程。通过与有关文献比较，计算实践表明本文方法具有实施简便、计算量少等优点。     

SAR Patterns from an Interstitial Microwave Antenna-Array Hyperthermia System

In recent years, a number of groups have been investigating the use of interstitial microwave antenna-array hyperthermia (IMAAH) systems for the treatment of superficial and deep-seated tumors. A critical parameter in any hyperthermia system is the SAR (specific absorption rate) pattern, which gives information about the energy absorption in the tumor and surrounding tissue. In this paper, we compare the theoretical and measured values of the SAR distribution for an array of four 915-MHz antennas implanted on the corners of a 2-cm square array. The overall length of the antennas was assumed to be 7 cm theoretically and was either 6 or 7 cm for the measurements. In general, there was good agreement between the theoretical predictions and the experimental results. In particular, both theory and experiment demonstrated that the maximum SAR occurred in the junction plane of the antenna array and at the center of the square array. Similarly, both showed that the Iongitudinal extent of the heating pattern is about 5 cm for a single antenna, but closer to 3 cm for an array where the Iongitudinal extent is defined by the length in the longitudinal direction where the SAR is greater than 50 percent of the maximum. The experiments showed that the SAR patterns were quite reproducible, and that the use of double stub tuners to minimize the reflected power for each antenna resulted in poorer SAR patterns due to phase variations created by the tuners. However, the use of an attenuator with each antenna to equalize the maximum SAR from each antenna did improve the SAR distributions slightly.