Recently, wearable wireless devices or terminals have become hot a topic not only in research but also in business. Implantable wireless devices can temporarily be utilized to monitor a patient's condition in an emergency situation or to identify people in highly secured places. Unlike conventional wireless devices, wearable or implantable devices are used on or in the human body. In this sense, body-centric wireless communications (BCWCs) have become a very active area of research. Radio-frequency or microwave medical devices used for cancer treatment systems and surgical operation have completely different functions, but they are used on or in the human body. In terms of research techniques, such medical devices have a lot of similarities to BCWCs. The antennas to be used in the vicinity of the human body should be safe, small and robust. Also, their interaction with the human body should be well considered. This review paper describes some of the wearable antennas as well as implantable antennas that have been studied in our laboratory.

Hyperthermia is one of the modalities for cancer treatment, utilizing the difference of thermal sensitivity between tumor and normal tissue. Interstitial microwave hyperthermia is one of the heating schemes and it is applied to a localized tumor. In the treatments, heating pattern control around antennas are important, especially for the treatment in and around critical organs. This paper introduces a coaxial-dipole antenna, which is one of the thin microwave antennas and can generate a controllable heating pattern. Moreover, generations of an arbitrary shape heating patterns by an array applicator composed of four coaxial-dipole antennas are described.

The body area network (BAN) has attracted attention because of its potential for high-grade wireless communication technology and its safety and high durability. Also, human area transmission of a BAN propagating at an ultra-wide band (UWB) has been demonstrated recently. When considering the efficiency of electromagnetic (EM) propagation inside the human body for BAN and hyperthermia treatment using RF, it is important to determine the mechanism of EM dissipation in the human body. A body heating system for hyperthermia must deposit EM energy deep inside the body. Also, it is important that the EM field generated by the implant system is sufficiently strong. In this study, the specific absorption rate (SAR) distribution is simulated using an EM simulator to consider the biological transmission mechanism and its effects. To utilize the EM field distribution using an implant system for hyperthermia treatment, the SAR distribution inside the human body is simulated. As a result, the SAR distribution is concentrated on the surface of human tissue, the muscle-bolus interface, the pancreas, the stomach, the spleen and the regions around bones. It can also be concentrated in bone marrow and cartilage. From these results, the appropriate location for the implant system is revealed on the basis of the current distribution and differences in the wave impedance of interfacing tissues. The possibility of accurate data transmission and suitable treatment planning is confirmed.

Hyperthermia is one of the modalities for cancer treatment, utilizing the difference of thermal sensitivity between tumor and normal tissue. In this treatment, the tumor or target cancer cell is heated up to the therapeutic temperature between 42 and 45 without overheating the surrounding normal tissues. Particularly, the authors have been studying the coaxial-slot antenna for interstitial microwave hyperthermia. At that time, we analyzed the heating characteristics of the coaxial-slot antenna under the assumption that the human body is a homogeneous medium. In this paper, we analyzed the heating characteristics of the coaxial-slot antenna inside an actual neck tumor by using numerical calculations. The models of calculations consist of MRI tomograms of an actual patient. As a result of the calculations, we observed almost uniform temperature distributions inside the human body including the actual neck tumor, which are similar to the results obtained for a homogeneous medium.

In microwave hyperthermia for cancer therapy, two power feeding techniques can be utilized: incoherent and coherent operations. In the incoherent operation, not-synchronized microwave power is fed into each array element, whereas the coherent operation is achieved by feeding synchronized microwave to the array elements. The authors have been studying the coaxial-slot antenna for interstitial microwave hyperthermia. The antenna is usually employed as an array applicator inserting several antennas into the tissue to generate large heating area. So far we have examined the control of the heating pattern by feeding techniques in order to obtain more uniform and enlarged heating region. Particularly, `tip-heating,' which means sufficient heating at the area near the tip of the applicator, is significant not to damage surrounding normal tissue in interstitial hyperthermia. In this paper, two feeding techniques are combined and calculated temperature distributions in a hexagonal array applicator are examined by solving Pennes bioheat transfer equation by finite difference method. As a result, in the coherent feeding, large heating area was obtained, while better tip-heating was achieved in the incoherent feeding. Moreover, an instance of sequential combination of two feeding techniques is depicted. In this case, temperature distribution had both characteristics of large heating area and tip-heating, therefore the ability of the control of heating characteristics by sequential combination of the coherent and the incoherent feedings was presented.

The interstitial hyperthermia is an invasive heating method applied by inserting the applicator into the human body. We have been studying on coaxial-slot antennas for interstitial microwave hyperthermia. The characteristics of the square antenna array were theoretically examined. Firstly, the basic structure of the antenna, and a simplified analysis model taking account of the effect of the boundary surface were explained. Then analysis was performed by using the moment method. Finally, the calculated results were discussed. The catheter thickness has much effect on the characteristics and must be considered both in designing and in using the antenna. When the array spacing was increased, the effective heating area became larger and more uniform. As the insertion depth was increased, the effective heating area was also enlarged.

The simulation of a specific absorption rate (SAR) with a temperature distribution becomes more important in the treatment planning for microwave hyperthermia. The simulation technique can also be used to estimate SAR distribution inside human body under hazardous electromagnetic (EM) field circumstances. In the simulation, to use exact permittivity of biological tissues becomes very important to obtain accurate SAR distribution. The permittivity of the medium is very sensitive to the temperature. Therefore, it is considered that the SAR distribution is also very sensitive to the tissue temperature. In this paper, SAR distribution is calculated using FDTD method considering tissue temperature under the electromagnetic (EM) field irradiation. Simulations of temperature distribution are also performed using heat transfer equation. In addition, temperature depending blood flow is taking into account to obtain temperature depending SAR distribution. The results can be used to estimate temperature depending heat generation which can be applied such as microwave hyperthermia treatment.

In the early stage of hyperthermia, a large number of engineering efforts have been done in the development or the improvement of the heating and temperature measuring techniques. However, they were not always satisfactory clinically. Thus, even in this moment, various engineering researches as well as the electromagnetic techniques for hyperthermia should be build up rapidly. This paper describes some of the highlights of developed or ongoing electromagnetic heating techniques in hyperthermia and identities a trend of emerging electromagnetic heating. Furthermore, the author emphasizes that few medical engineering efforts have been done in the boundary field between pure physics and clinics, and the proper way to develop the hyperthermia equipment is the best use of successes in the three essential regions: Physics, Biology and Clinics.

Electromagnetic radiation patterns of planar 915MHz Dual Concentric Conductor (DCC) antennas were investigated with theoretical finite difference time domain (FDTD) analyses and experimental measurements of power deposition in a homogeneous lossy dielectric load. Power deposition (SAR) patterns were characterized by scanning an electric field sensor in front of the radiating aperture 1 cm deep in liquid "muscle tissue" phantom. Results showed close agreement between the theoretical simulations and measured SAR patterns for a 3.5cm square aperture. Additional SAR measurements demonstrated the ability to vary aperture size from 3.5-6cm with minimal change in shape of the power deposition pattern. Both analyses indicated that effective power deposition (50% SARmax) extends to the periphery of the square apertures. These data support the conclusion that the DCC aperture constitutes an improved radiator to be used as the functional building block of larger array applicators which are required for adjustable heating of large superficial tissue regions in the treatment of cancer.

This paper describes that the dielectric characteristics of a catheter around the interstitial antenna have an effect on the wavelength for current, and this effect results in the variation of the SAR (Specific Absorption Rate) distribution around the antenna. A theoretical study of SAR distribution ground a coaxial-slot antenna is performed. Analytical technique used is the moment method. Result and discussion on the effect of material and thickness of the catheter are presented. The wavelength for the current shortens with increasing dielectric constant or decreasing thickness of the catheter. Due to this variation of the wavelength for current, the SAR distributions take various shapes.

A partially ferrites and dielectric loaded water filled waveguide applicator is presented which can be used for microwave heating of tissues. The applicator can change its heating pattern by changing the external DC magnetic field applied to the ferrites. The electromagnetic (EM) field distribution inside the applicator is obtained theoretically and the simulated EM field inside the applicator is checked experimentally using 430MHz. Furthermore, on the basis of the EM field distribution inside the applicator, simulations of SAR distribution inside lossy homogeneous human tissue as muscle are performed using finite difference time domain (FD-TD) method. Simulated data of Specific Absorption Rate (SAR) distribution is compared with the experimental ones. Simulations of temperature distribution are also performed using heat transfer equation. Simulated data of temperature elevation distribution is compared with the experimental ones. The simulated results agree well with the experimental ones and it is confirmed that the heating pattern can be changed by external DC magnetic field applied to the applicator. The results obtained here show that the partially ferrites and dielectric loaded water filled waveguide applicator which operates at 430 MHz can change its heating pattern without changing its setup and can heat local target on the human body for hyperthermia treatment.

A light, thin and flexible applicator using a microstrip patch array for microwave heating is presented and tested in this work. The applicator is made of a flat silicone rubber bag, inside of which flows cooling water. EM coupling feeding is applied, which has no direct contact between the feed and the patch, to improve durability and reliability when it is repeatedly applied to the uneven surface of the heated portion of the human body. Simulations of SAR distribution are performed using the finite difference time domain (FD-TD) method. Simulated data are compared with the experimental ones using cubic and cylindrical phantom models with single and multielement patch applicators. Simulations of temperature distribution are also performed using the heat transfer equation. Simulated data are compared with the experimental ones using cubic and cylindrical phantom models. The simulated results agree well with the experimental ones. The results obtained here show that the multielement flexible microstrip patch applicator which operates at 430MHz can heat a relatively shallow and widespread area on the human body for hyperthermia treatments.

A semicylindrical microstrip applicator system is proposed and designed, both for microwave heating and for noninvasive temperature estimation, in application to hyperthermia treatment. The experimental results showed that the system functions both as a heating device and as a means of noninvasive temperature estimation. Therefore, electrical switching of these two functions makes the system realize both heating and temperature estimation. These functions reduce the pain of hyperthermia therapy for patients. The system is constructed of a water-loaded cylindrical applicator. Thus, the whole system can be made compact compared to conventional applicators. This improvement allows for various merits, such as realizing a surface cooling effect and decreased leakage of electromagnetic (EM) waves. When the applicator is set as an array arrangement, the system can be used as a microwave heating device. The penetration depth can be varied by adjusting phases of the EM wave radiated from each applicator. The experimental results at 430 MHz showed that semicylindrical microstrip applicators can be expected to be valid for tumor heating at depths within 55 mm. Moreover, by measuring transmission power between the two applicators, the system can be used to estimate temperature inside the medium. The transmission power which was measured in the frequency domain was converted in the time domain. By such a method, temperature distribution was calculated by solving simple simultaneous primary equations. The results of the temperature estimation show that the number of estimated temperature segments which have an error within 0.5 is 28 out of 36. The system can be easily used as a temperature measuring applicator as well as a heating applicator.

A dielectric rod waveguide applicator for microwave heating such as microwave hyperthermia is described. The applicator consists of the acrylic cylinder filled with deionized water. By circulating the deionized water, the dielectric rod waveguide applicator acts as a surface cooling device, so that it doesn't need any bolus. This surface cooling device enables the dielectric rod waveguide applicator to control the site of effective heating region along the depth axis. Useful pattern of the circular or spheroidal shape and axially symmetric effective heating region were obtained. Furthermore metal strips provided on the aperture of applicator control the shape of the heating pattern.

A new model for a computer simulation of RF capacitive type hyperthermia has been developed by taking account of the following points. Blood flow is usually determined by many physiological parameters, but is regarded as a function of only blood temperature under some conditions. The temperature dependence of blood flow of tumors and normal tissues is assumed by referring the data obtained by Song et al. and Tanaka. The blood temperature which is elevated by externally applied power significantly affects temperatures of the body and the tumors. The transport of heat from the body surface is studied by considering air convection. These points are examined by experiments on a computer with simple phantom models and real patients. The results of simulation on the patient have shown a good agreement with clinical inspection based on CT images and a temperature of the stomach.