The Performance of a New Direct Contact Applicator for Microwave Diathermy

A direct contact applicator, specifically designed for microwave diathermy at the Industrial, Scientific Medical (ISM) frequency of 2.45 GHz was evaluated by studying near-field patterns in free space, thermographic heating patterns in phantoms of simulated fat and muscle tissue, and associated leakage radiation. The main features are a circular waveguide with a short conical flare horn output section surrounded by an annular choke and two sets of dual posts to generate far-field circular polarization. The significant near field components of the therapeutic beam are in a transverse plane, parallel to the aperture. Heating patterns on the exposed surface of muscle phantoms and inside fat-muscle phantoms are spatially similar and relatively uniform. The leakage level is 0.8 mW/cm/sup 2/ per 100 W of forward power for direct contact and 4 mW/cm/sup 2/ per 100 W of forward power for a 1-cm air gap between aperture and planar phantoms. The uncertainty of these leakage measurements is /spl plusmn/2 dB. This investigation demonstrates the technical feasibility of a safe and effective direct contact microwave diathermy applicator operating at 2.45 GHz. The applicator is a viable candidate for hyperthermia appllications.     

A 2450-MHz Slab-Loaded Direct Contact Applicator with Choke

A Teflon-slab-loaded direct contact microwave diathermy applicator has been developed. It produces minimal leakage radiation during effective heating of simulated planar tissue models. 'The use of TEM mode excitation results in heating patterns which are more uniform than the patterms of comparable waveguide applicators without dielectric loading.     

Preliminary studies: far-field microwave dosimetric measurements of a full-scale model of man.

Measurements of microwave heating were made in a full-size, upright human model. The 75-Kg model, composed of electrically simulated muscle, was placed in the far-zone of a standard-gain horn inside an absorber-lined chamber. Pulsed energy at 1.29 GHz was obtained from a military radar transmitter (AN/TPS-1G) and produced radiation at 6-14 mW/cm2 average power density at the location of the model. Microwave heating at the front surface was measured at nine locations on the phantom. Measurements at several depths within the phantom were also made at a central location to gain information on the depth-of-penetration of the microwave energy. Results of the frontal surface measurements and of the penetration study permitted a calculation of the approximate whole-body average specific absorption rate (SAR) when the model's long axis was parallel to the E-field vector. For a normalized power density of 1 mW/cm2 at a frequency of 1.29 GHz, the whole-body average SAR approximated 0.03 W/Kg. This result agrees well with theoretical predictions based on absorption in prolate spheroidal models of man.     

A new coaxial TEM radiofrequency/microwave applicator for non-invasive deep-body hyperthermia

At the moment great efforts are being made to develop non-invasive heating systems which produce controlled local or regional deep-body hyperthermia. Electromagnetic interference techniques (10-80 MHz) with several separated applicators or with multiple applicators can produce deep-body heating. In our institute a coaxial frequency-independent TEM radiofrequency/microwave applicator has been designed. This applicator can produce a theoretically optimal interference maximum in the centre of the body. The applicator is very simple to construct and inexpensive. To test the design with the equipment available, a scaled prototype of the TEM applicator has been developed. The prototype has a diameter of 20 cm and operates at 434 MHz. The applicator has been tested on several phantom materials. The measured absorbed power distributions are in good agreement with the calculated theoretical distributions. The theoretical calculations of the absorbed power distributions of a 10-80 MHz TEM deep-body applicator are very encouraging.     

New Low-Profile Applicators for Local Heating of Tissues

A new type of low-profile applicator is described giving efficient power transfer to tissue with low leakage. It consists of a resonant element spaced from a ground plane by low-loss material having high relative permittivity and some relative permeability. This combination is covered by similar material which can be placed in direct contact with the tissue to be heated, or separated from it by a water bolus as required. Laboratory measurements using phantom materials show that these applicators are well matched and produce well defined and relatively uniform heating profiles. Applicators having maximum dimensions of 3-15 cm covering the frequency range 1000-100 MHz, respectively, are described. Penetration depths of at least 4 cm at 100-200 MHz are achieved. Tests on anesthetised pigs and some clinical assessment have confirmed the usefulness of this compact low-frequency type of applicator and have shown that performance compares favorably with that of other recently reported types. The design can be optimized for treatment of specific sites, and suitable choice of dimensions and electrical characteristics of the layer material allow the low-profile concept to be extended over a wide range of operating frequencies.     

Absorption of microwave energy by muscle models and by birds of differing mass and geometry

Spheres composed of phantom muscle of radius 1.5, 2, 3, 4, 6 and 8 cm, as well as birds (parakeets, quail, pigeons, chickens, turkeys) were exposed to far-field plane waves at power densities of incident radiation between 182 and 560 mW/cm2 and at frequencies of 775, 915 and 2450 MHz. Specific absorption rate (SAR) patterns were determined by thermographic techniques for both spheres and birds. The measured SAR patterns in spheres were comparable to those from theoretical predictions. The SAR patterns in birds, however, varied markedly from those obtained from spheres of comparable mass. The results indicate that the geometrically complex animal is not represented by simple geometric models for making absorption studies. Thermograms of birds exposed in the flying position indicated that the SAR is high in the wings. The behavioral response of the birds to the exposure was variable. Threshold power densities for biological or behavioral reactions were determined for each bird at all three frequencies. The lowest power density associated with reactivity by the chicken was 5.8 mW/cm2 (corresponding to SARs of 3.1 W/kg in the head and 3.9 W/kg in the neck) at 775 MHz.     

The use of a current sheet applicator array for superficialhyperthermia: incoherent versus coherent operation

There are a number of potential advantages to be gained by using an array of applicators in hyperthermia treatments compared with single applicator systems, These advantages include the possibility of greater spatial control of power deposition and conformability to nonplanar sites. Arrays of applicators can be driven either coherently or incoherently. In the case of coherent operation, an added advantage is the ability to steer power deposition by varying the phases of the antennas. In this study, the authors investigated the relative merits of the two modes of operation when a 2×2 planar array of current sheet applicators is used. The effective field size (EFS) of the array was calculated using a Gaussian beam representation of the applicators on a layered model in which the fat layer had its thickness varied. Good agreement was obtained between the square of the electric field distribution (E²) and quantitative experimental results. It is shown that when the planar array is used with a fat layer greater than about 2 mm present, it should be driven incoherently as this results in a significantly larger EFS than that obtained when the array is driven coherently     

Computer-aided design of two dimensional electric-type hyperthermiaapplicators using the finite-difference time-domain method

A hyperthermia applicator design tool consisting of a finite-difference time-domain (FDTD) technique in combination with a graphical display of electric fields and normalized linear temperature rise is described. This technique calculates, rather than assumes, antenna current distributions; it includes mutual interactions between the body and the applicator, and it calculates driving-point impedance and power delivered to the applicator. Results show that the fundamental limitation of 2-D electric-type applicators is overheating of the fat by normal components of the electric field, which exist because of near fields and capacitive coupling with the muscle. Two factors which contribute to the capacitance are the muscle conductivity and the small antenna size in air. Two examples of applicators designed to avoid fat overheating are described: a 27-MHz segmented dipole for heating large tumors to 7 cm depth, and a 100-MHz dipole for small tumors to 5 cm depth. The first uses a water bolus, and the second uses a water bolus with low-permittivity strips to reduce normal fields at the antenna ends. The results of this study describe fundamental limitations of electric field applicators, and illustrate the use of a powerful applicator design tool that allows rapid evaluation of a wide range of ideas for applicators which would require months and years to test experimentally.     

Multifrequency radiometric determination of temperature profiles ina lossy homogeneous phantom using a dual-mode antenna with integralwater bolus

In the treatment of cancer, microwave hyperthermia has been established as an efficient adjunctive procedure to radiation therapy and chemotherapy. Wider acceptance of this method awaits schemes to measure volumetric temperatures noninvasively in human tissue for control of the heating process. This effort describes the design and performance of a new microstrip applicator intended for homogeneous heating of superficial tissue while at the same time monitoring temperature of the underlying tissue by noninvasive radiometric sensing of black-body radiation from the heated volume. Radiometric capabilities are assessed in terms of accuracy of up to six measured brightness temperatures applied in an inversion algorithm from which one-dimensional depth temperature profiles are generated. Based on radiometric signals recorded over the 1-4-GHz range, the temperature accuracy determined from statistical analysis of 200 realizations of the process is better than ±0.2°C to a depth of 5 cm in phantom. Aperture heating uniformity is assessed with electric field scans in a homogeneous muscle phantom. As long as sufficiently thin (< 5 mm) water boli are used, SAR distributions at 1-cm depth in phantom extends effectively just outside the aperture perimeter, making this microstrip antenna an excellent building block element of larger multi-antenna array applicators     

Average SAR and SAR Distributions in Man Exposed to 450-MHz Radiofrequency Radiation

Fifth-scale phantom models were exposed to 2450-MHz electromagnetic fields to obtain the average specific absorption rate (SAR) and SAR distribution in man exposed to 1 mW/cm/sup 2/ 450-MHz radiofrequency radiation for various polarizations and body positions. The average SAR was measured calorimetrically and SAR distribution was determined thermographically using an interactive computer system, The mean SAR, as averaged over the body, remained relatively constant at 0.050 W/kg, with a standard deviation of +-0.007 W/kg for all polarizations and body postures considered in the study. Peak SAR values were as high as 0.650 W/kg, occuring typicaly in the wrist.     

Low-frequency RF hyperthermia. IV. A 27 MHz hybrid applicator forlocalized deep tumor heating

A 27 MHz dual-device applicator of novel design, which is aimed to heat noninvasively with improved safety tumor masses at depth, is proposed. A substantially localized temperature gain is obtained by superimposing two delocalized low RF frequency and phase-coherent current distributions, which are launched to constructively interfere over a limited region emcompassing the tumor volume. An hybrid applicator (HA) has been developed, integrated one capacitive and one inductive heating device, and has been assessed on a 20 cm diameter cylindrical fat-muscle phantom. The interference pattern is characterized by a deep broad SAR maximum and by the disappearance of the central null SAR value typical of single inductive devices. An 80% SAR useful therapeutic volume (UTV) of a near-cylindrical shape of about 800 cm3 is obtained with a penetration of 6-8 cm for the phantom surface, with a noncritical axial length of approximately 21 cm. The UTV may be somehow controlled in size and penetration. These results are obtained with the tissue-like medium surrounding the UTV heated uniformly and safely to a temperature pedestal below the therapeutic temperature with about half RF power values to each of the heating devices.     

Numerical computation of fat layer effects on microwave near-fieldradiation to the abdomen of a full-scale human body model

Numerical computation results of fat layer effects on the microwave near field radiation to the abdomen of a three-dimensional (3-D) full-scale human body model are presented. The human body is modeled as a 3-D homogeneous muscle phantom with a fat layer covering the abdomen part. The dipole wire-antenna located proximate to the abdomen is used as the microwave radiation source at 915 MHz. This is to study the effects on hyperthermia heating by using the microwave applicator (at 915 MHz) or the near-field exposure from the proximate handset antenna to the human body at ISM band wireless communication band (902-928 MHz). Coupled integral equations (CIE) and the method of moments (MoM) are employed to numerically compute electromagnetic (EM) energy deposition specific absorption rate (SAR) from the radio frequency (RF) antenna applicator into the proximate fat layer covered abdomen. The antenna input impedance (proximate to the body), return loss (RL), and the resonant antenna length (proximate to the body) will also be numerically determined to increase the microwave power delivered into the body. The study of fat layer effects is important for microwave hyperthermia applications. It is also important for the investigation of the potential health hazard from the near-field radiation of a wireless communication antenna     

Feasibility of ultrasound hyperthermia with waveguide interstitialapplicator

Interstitial and intracavitary ultrasonic hyperthermia applicators facilitate well-controlled power deposition in tissues. In this paper, analysis of temperature elevation and experimental results in tissue phantom, animal tissue in vivo and animal tissue in vitro are presented for a waveguide applicator intended for treatment of brain tumors. It consisted of a G18 hypodermic needle attached via a conical velocity transformer to a 12.7-mm-diameter piezoelectric disk operated at 1.0 MHz. The axial acoustic pressure distribution had a standing-wave pattern with the four cycles/cm spatial periodicity. This periodicity was absent in the temperature distribution in tissue phantoms. The simulations based on a solution to the effective heat conductivity equation indicated that the hyperthermic range can be reached within a 4- and a 10-mm radius around the applicator for a 21- and a 60-mm sample diameters, respectively, with reasonable input power. The first number corresponded closely to the 5-mm radius observed in porcine brain in vivo and the second one came close to the 9-mm radius in porcine brain in vitro. The presented results demonstrate the potential of the ultrasound waveguide interstitial applicator for hyperthermia of small volume tumors     

High performance single frequency fiber grating-basederbium/ytterbium-codoped fiber lasers

The device characteristics of Er³⁺,Yb³⁺ single frequency fiber lasers are reported. A 5-cm long 1550-nm distributed feedback fiber laser with 4 mW output power is shown to have excellent specifications in terms of optical linewidth, signal-to-noise ratio (SNR), relative intensity noise, side-mode suppression and polarization purity. For higher power applications, a 1.5 cm single frequency Er³⁺,Yb³⁺ grating-based fiber laser with 60 mW output power and a net efficiency of 12% is demonstrated     

Preclinical evaluation of submillimeter diameter microwave interstitial hyperthermia applicators

Ultra miniature coaxial cable has been used, with microscopic techniques, to fabricate interstitial hyperthermia applicators having diameters of 0.20 mm, 0.33 mm, and 0.58 mm; commercial applicators have a diameter of 1.1 mm. Animal studies with the 0.33 mm diameter applicators have shown that they produce less local tissue trauma than the larger-diameter devices. All of these applicators operate at 915 MHz and have similar heating patterns because they use the conventional monopole design and the catheters have been approximately scaled to the dimensions of each size applicator. We have measured the heating (SAR) patterns of these applicators in tissue-simulating phantoms, both singly and in arrays, using a miniature electric field probe. As an intermediate step to patient trials, we have examined the ability of these applicators to provide effective heating of perfused tissue, using pig thigh and liver as models. Test results suggest that the durability and power handling capabilities of our submillimeter applicators are adequate for use in patients. These new applicators should be useful in the percutaneous treatment of deep-seated tumors and in intraoperative treatments. The applicators also permit intraluminal or intravascular access to tumors.     

W-band investigation of material parameters, SAR distribution, and thermal response in human tissue

This investigation is divided into three parts. First, the W-band dielectric properties of different biological tissues are determined. Then, the electromagnetic field in the human eye and skin is simulated for plane-wave exposure. An analytical method is used to investigate the specific absorption rate (SAR) inside a layered model of the human skin between 3-100 GHz. Furthermore, the SAR inside a detailed model of the human eye is investigated numerically by the finite-difference time-domain method for a frequency of 77 GHz. Maximum local SAR values of 27.2 W/kg in skin tissue and 45.1 W/kg in eye tissue are found for 77 GHz and an incident power density of 1 mW/cm/sup 2/. In the third part of the investigation, the temperature changes of superficial tissue caused by millimeter-wave irradiation are measured by a thermal infrared imaging system. The exposure setup is based on a horn antenna with a Gunn oscillator operating at 15.8-dBm output power. The measurements showed a maximum temperature increase of 0.7/spl deg/C for a power density of 10 mW/cm/sup 2/ and less than 0.1/spl deg/C for 1 mW/cm/sup 2/, both in human skin (in vivo), as well as in porcine eye (in vitro). The comparison of the temperature measurements with a thermal bio-heat-transfer simulation of a layered skin model showed a good agreement.     

Contact flexible microstrip applicators (CFMA) in a range from microwaves up to short waves

Contact flexible microstrip applicator (CFMA) is a new light-weight microstrip applicator type for superficial and deep local hyperthermia. Typical specimens are developed for operation at frequencies of 434, 70, 40, and 27 MHz. The main common features of CFMA, namely, their flexibility and light weight, as well as their aperture dimensions slightly depend on the operating frequency. Two antenna types are used in CFMAs: inductive antennas with a radiating plane electrical dipole at microwaves, and coplanar capacitive antennas, providing depression of the normal component of the electrical field in the very high-frequency (VHF) and high-frequency (HF) range. The flexibility of the applicators enables one to conform them with curved surfaces. In a bent state of the applicators there arises a focusing effect of energy deposition in deeper located tissues due to linear polarization of the irradiated electromagnetic (EM) field, inherent in CFMA. All CFMA are integrated with silicon water boluses which serve as a matching element, so as a skin cooling agent. Due to this and to the predominance of the tangential electrical component in the radiated EM field, no fat overheating effects are noticed, as a rule. The aperture of the developed applicators overlap the range 160-630 cm2 providing effective heating field sizes (EFSs) 64-400 cm2, respectively. The most bulky CFMAs with an aperture of (21 x 29) cm2 operating at the frequency of 434 MHz weigh 0.8 kg and 2.5 kg at 27 MHz. Phenomenological analysis of the radiating systems, as well as experimental evaluation of the applicators are presented. CFMAs operating at frequencies of 434 and 40 MHz are used in clinical practice. CFMA at 70 and 27 MHz are subjected to laboratory clinical investigations.     

A perfused tissue phantom for ultrasound hyperthermia

A perfused tissue phantom, developed as a tool for analyzing the performance of ultrasound hyperthermia applicators, was investigated. The phantom, consisting of a fixed porcine kidney with thermocouples placed throughout the tissue, was perfused with degassed water by a variable flow rate pump. The phantom was insonated by an unfocused multielement ultrasound applicator and the temperatures in the phantom were recorded. The results indicate that for testing protocols where tissue phantoms are needed, the fixed kidney preparation offers an opportunity to use a more realistic phantom than has previously been available to assess the heating performance of ultrasound hyperthermia applicators.     

Ultrasound applicators with internal water-cooling for high-powered interstitial thermal therapy

Internal water-cooling of direct-coupled ultrasound (US) applicators for interstitial thermal therapy (hyperthermia and coagulative thermal therapy) was investigated. Implantable applicators were constructed using tubular US sources (360 angular acoustic emittance, approximately 7 MHz) of 10 mm length and 1.5, 1.8, 2.2, and 2.5 mm outer diameter (OD). Directional applicators were also constructed using 2.2 mm OD tubes sectored to provide active acoustic sectors of 90 degrees and 200 degrees. A water-cooling mechanism was integrated within the inner lumen of the applicator to remove heat from the inner transducer surface. High levels of convective heat transfer (2100-3800 W/m2K) were measured for practical water flow rates of 20-80 mL/min. Comparative acoustic measurements demonstrated that internal water-cooling did not significantly degrade the acoustic intensity or beam distribution of the US transducers. Water-cooling allowed substantially higher levels of applied electrical power (> 45 W) than previous designs (with air-cooling or no cooling), without detriment to the applicators. High-temperature heating trials performed with these applicators in vivo (porcine liver and thigh muscle) and in vitro (bovine liver) showed improved thermal penetration and coagulation. Radial depth of coagulation from the applicator surface ranged from 12 to 20 mm for 1-5 min of sonication with 28-W applied power. Higher powers (41 W) demonstrated increased coagulation depths (approximately 9 mm) at shorter times (15 s). Thermal lesion dimensions (angular and axial expanse) produced with directional applicators were controlled and directed, and corresponded to the active zone of the transducer. These characteristic lesion shapes were also generally unchanged with different sonication times and power, and were found to be consistent with previous coagulation studies using air-cooled applicators. The implementation of water-cooling is a significant advance for the application of ultrasound interstitial thermal therapy (USITT), providing greater treatment volumes, shorter treatment times, and the potential for treatment of highly perfused tissue with shaped lesions.     

Optimal power deposition with finite-sized, planar hyperthermiaapplicator arrays

Improved hyperthermia applicator technology is allowing finer spatial power resolution within the heated tissue volume. Effective utilization of these planar applicator arrays requires an understanding of the interrelationships between the lateral dimensions of the tumor and the applicators, the power field produced by the applicators, the amount of surface cooling, the tumor tissue blood perfusion, and the normal tissue blood perfusion. These interrelationships are investigated using three-dimensional power patterns and temperature fields produced by optimizing the power amplitudes of the individual applicators located within an array of small, but finite, planar applicators. Five major conclusions are obtained. First, optimization works and is effective in determining optimal power fields. Second, for optimal treatments the lateral dimensions of a single superficial applicator need to extend beyond the tumor boundary. Third, surface cooling is needed to reduce the high normal tissue temperatures at shallow depths. Fourth, finer power resolution becomes more important as the tumor size decreases, but, little improvement in the temperature field is achieved beyond a 3 x 3 array configuration. Fifth, increasing the normal blood perfusion rate can decrease the temperature on the tumor boundary if direct power deposition on that boundary is unavailable.