Lira Hamada

Development and characteristics of a biological tissue-equivalent phantom for microwaves

Various phantoms (simulated biological bodies) have been proposed as a biological model for studies of electromagnetic effects on the human body. This paper reports the characteristics of the phantom developed by the authors that realized electrical characteristics equivalent to the biological body. Examples of its communication and clinical applications are presented. The present phantom is made of agar, polyethylene powder, sodium chloride, TX-151, preservative, and deionized water and simulates the relative permittivity and conductivity of a high-water-content tissue. In the present phantom, electrical characteristics almost equal to those in the biological tissue are realized with a single composition ratio over the frequency range of 300 MHz to 2.5 GHz. It is also possible to simulate the electrical characteristics of an arbitrary high-water-content tissue by adjustment of the composition. No special equipment is needed for fabrication and the preservation is easy. Further, as examples of applications of this phantom to the human body, the SAR measurement examples are presented in the COST 244 human head model and coaxial-slot antenna for hyperthermia. The present phantom is useful as a human model to study the mutual effects of the human body and electromagnetic waves.

Dielectric-loaded coaxial-slot antenna for interstitial microwave hyperthermia: longitudinal control of heating patterns

In this paper, the microwave interstitial antenna with the dielectric load in part near the tip is introduced to realize the tip-heating and to improve the dependence of the heating patterns on the insertion depth. Numerical simulations using the Finite Difference Time Domain (FDTD) method have been conducted at the frequency of 915MHz for four different configurations of the coaxial-slot antenna inserted into a catheter: the media between the antenna and the catheter are (a) no, (b) a thin air layer, (c) a thin dielectric layer, and (d) a thin air layer and a dielectric load in part near the tip. The diameter of the antenna including the catheter is sufficiently small for minimally invasive therapy. Comparison of the SARs for the four configurations makes it clear that the dielectric-loaded antenna can realize the best tip-heating and suppress the hot spot near the surface of the human body. Dependence of the SAR distributions on the insertion depth of the antenna has also been examined. It is found from the investigation that the dielectric-loaded antenna has little dependence on the insertion depth.

A Method of Measuring Gain in Liquids Based on the Friis Transmission Formula in the Near-Field Region

In the 300 MHz to 3 GHz range, probes used to measure specific absorption rate (SAR) of mobile communication devices are usually calibrated using a rectangular waveguide filled with tissue-equivalent liquid. Above 3 GHz, however, this conventional calibration can be inaccurate because the diameter of the probe is comparable to the cross-sectional dimension of the waveguide. Therefore, an alternative method of SAR probe calibration based on another principle was needed and has been developed by the authors. In the proposed calibration method, the gain of the reference antenna in the liquid is first evaluated using the two-antenna method based on the Friis transmission formula in the conducting medium. Then the electric field intensity radiated by the reference antenna is related to the output voltage of the SAR probe at a given point in the liquid. However, the fields are significantly reduced in the liquid, and the gain is impossible to calibrate in the far-field region. To overcome this difficulty, the Friis transmission formula in the conducting medium must be extended to the near-field region. Here, we report results of simulations and experiments on estimated gain based on the extended Friis transmission formula, which holds in the near-field region, and test the validity of the new formula.

Measurement procedure to determine SAR of multiple antenna transmitters using scalar electric field probes

This paper presents a fast estimation technique and a measurement procedure for measurement systems using scalar electric field probes to determine the Specific Absorption Rate (SAR) of multiple antenna transmitting devices. For devices with N-element antenna, measurements for N(N - 1) + 1 pre-known relative phase combinations of the antennas are required in order to evaluate SAR for any other phase combinations. The technique provides accurate estimated SAR, and is able to identify the maximum SAR corresponding to particular phase combinations of the sources.

A method in determination of the specific absorption rate of multi-antenna devices

This paper presents a simple estimation method to evaluate the Specific Absorption Rate (SAR) of multiple antenna transmitting systems with conventional scalar SAR/E-field probes. For systems with two-element antenna, only three measurements with known relative phase combinations of the antennas are required in order to evaluate SAR for any other relative phase combinations.

Basic study of the coaxial antennas for minimally invasive microwave thermal therapy

The number of studies of minimally invasive microwave antennas in medicine is increasing. They are hyperthermia for medical treatment for cancer, cardiac catheter ablation for ventricular arrhythmias treatment, microwave treatment of benign prostatic hypertrophy, and so on. In this paper we describe the characteristics of the minimally invasive microwave antennas by FDTD analysis for hyperthermia and cardiac catheter ablation antennas.

Averaging time required for measuring the specific absorption rate of a MIMO transmitter

This paper presents a new method to measure electric field (E-field) for human-body exposure to electromagnetic fields from Multi-Input Multi-Output (MIMO) wireless communication transmitters, and an estimation of averaging time required in the E-field measurement. The proposed method is to measure time-averaged E-field, and the averaging time is the duration required for a measurement system to make averaged E-field converged. The estimation is based on averaging the square of amplitude of received signals or measured E-field strength. Several simulation results with different measurement parameters for MIMO wireless communication transmitters are presented and discussed. It is pointed out that the averaging time in E-field measurements for multi-antenna transmitters is considerably longer than that for single antenna transmitters.

A SAR-probe calibration system using reference dipole antenna in tissue-equivalent liquid

The SAR-probe is usually calibrated using a rectangular waveguide with a matching dielectric spacer below 3 GHz. However, the probe diameter is comparable with the cross-section area of the waveguide above 3 GHz and it can deteriorate the accuracy of the calibration. Therefore, there is an alternative method for calibrating the SAR-probe; relating the output voltage of the probe to the field intensity produced by a reference antenna in the tissue-equivalent liquid by two-antenna method. In this report, we have built a prototype of a calibration system and presented some calibration results of SAR-probe sensitivity using this method in conducting head-equivalent liquid at 2.45 GHz.

A gain calibration in the liquid including the effects of fresnel field and systematic uncertainty

One of calibration techniques for electric field probes used in the standardized SAR (Specific Absorption Rate) assessment is based on the Friis transmission formula in the far-field region for the conductive medium. In practice, it is difficult to measure power transmission between transmitting and receiving antennas in the far-field region for the liquid used in the SAR assessment because of its large attenuation. To overcome the difficulty, the authors extended the formula taking into account the Fresnel approximation. The extended formula is valid in the Fresnel region of the antennas for the conductive medium. In this paper, the far-field gain of the antennas can be estimated by not only using the extended formula but also considering systematic uncertainty of measuring power transmission. Adding the effect of the measurement uncertainties can make the resulting uncertainty of the gain small, because the measurement uncertainty of the power transmission largely depends on its level so that this behavior can not be ignored in the liquid.

Simulated near-field gain of dielectric-coated circular loop antennas operated in liquid and in HF band

To evaluate specific absorption rate (SAR) in HF band using reference antenna method, dielectric-coated and shielded loop antenna operated in the liquid can be used. It is found that the electric and magnetic field intensity in the near-field region of the reference antenna behave like corresponding far-field if the near-field gain have a plateau as a function of the distance from the antenna. To simulate the perimeter of the loop with a plateau in its near-field gain using Richmonds method of moments, the dielectric property of the cylindrically dielectric shell should be well given by using Lichteneckers logarithmic mixture law if the dielectric shell is made of heat-shrinkable tube.