Analysis of the SAR distributions in three-layered bio-media in direct contact with a water-loaded modified box-horn applicator

This paper presents theoretical analysis of the specific absorption rate (SAR) distribution in three-layered bio-media (skin, fat, and muscle layers) in direct contact with a modified box-horn applicator in which the horn is flared in both the E- and H-planes for hyperthermia treatment of cancer. The modified box-horn is assumed to be filled with water to provide a better impedance match to the bio-media. The present analysis is based on a plane-wave spectral technique. The spatial distribution of SAR in skin, fat, and muscle layers are computed and presented at 433 and 2450 MHz. Higher SAR value in each of the bio-layers, higher penetration depth, and lower resolution in the muscle layer are observed at 433 MHz in comparison to those at 2450 MHz for the water-loaded modified box-horn. The effect of substituting a pyramidal horn in place of an H-plane sectoral horn in the design of the box-horn on the SAR distribution is also examined. The results have been validated against published results in the literature.     

Mutual Coupling Between Box-Horn Elements of a Phased Array Terminated in Three-layered Bio-Media

The ldquoplane wave spectral techniquerdquo is used to investigate theoretically the extent of mutual coupling between two box-horn elements of a phased array terminated in three-layered bio-media (skin, fat, and muscle layers). The results obtained have been validated against published results in the literature. Each box-horn of the array is assumed to be filled with water to provide good impedance match to the bio-media. Also, it reduces the dimensions of each box-horn which makes it suitable for array configuration. The effects of relative position of elements, spacing between elements, frequency, aperture size of each element, relative phase and amplitude of excitation of the elements on mutual coupling coefficients are numerically investigated and presented for an array of two box-horns terminated in bio-media. The specific absorption rate (SAR) distributions along the direction of the array axis in muscle layer of the bio-media in direct contact with a collinear array of two box-horns both with and without mutual coupling considerations are also computed and presented. The results of the present research work can provide useful design guidelines for the development of prototypes of box-horn array for hyperthermia treatment of tumors. Bio-media, box-horn, hyperthermia, mutual coupling, phased array, plane wave spectra.     

A Direct-Contact Microwave Lens Applicator with a Microcomputer-Controlled Heating System for Local Hyperthermia

A direct-contact lens applicator for local microwave hyperthermia is proposed and developed with a computer-controlled microwave heating system. The applicator is a practical one that can converge the radiated electromagnetic field to deposit its energy deep in human tissues. The experimental results, which agree well with the theoretical ones, show that the applicator which operated at 2450 MHz could heat at twice the depth at which a simple and conventional waveguide applicator could heat. The experimental results using a developed computer system that supplies microwave energy and circulated cooling water to the developed applicator show that the fluctuations of temperature at the heating location in the human tissue model were maintained within +-0.3° C of the set temperature. The results of the phantom model and the animal experiment using the system with the applicator show that the maximum depth of noninvasive heating was more than 30 mm below the surface. These results are available for the clinical hyperthermia treatment of cancer.     

Development and Testing of a 2450-MHz Lens Applicator for Localized Microwave Hyperthermia (Short Paper)

A new type of applicator with a convergent lens for localized microwave hyperthermia is developed. A lens applicator of direct contact type was designed to conduct actual and progressive experiments with phantoms of simulated fat and muscle tissues heated at 2450 MHz. The experimental results showed that the heating power penetration depth increased 40 percent with this applicator as compared to a simple rectangular waveguide applicator with the same size aperture that had generally been used for microwave hyperthermia. Our applicator had a concave-shaped aperture and was designed to contact well with the heating medium whose shape was cylindrical like a human body.     

Experimental investigation of an adaptive feedback algorithm forhot spot reduction in radio-frequency phased-array hyperthermia

A computer-controlled adaptive phased array radiofrequency hyperthermia system for improved therapeutic tumor heating is experimentally investigated. Adaptive array feedback techniques are used to modify the electric-field in hyperthermia experiments with a homogeneous saline phantom target. A hyperthermia phased-array antenna system has been modified to implement adaptive nulling and adaptive focusing algorithms. The hyperthermia system is a ring phased-array antenna applicator with four independently controlled RF transmitter channels operating at a CW frequency of 100 MHz. The hyperthermia phased array is made adaptive by software modifications which invoke a gradient-search feedback algorithm that controls the amplitude and phase of each transmitter channel. The gradient-search algorithm implements the method of steepest descent for adaptive nulling (power minimization) and the method of steepest ascent for adaptive focusing (power maximization). The feedback signals are measured by electric-field short-dipole probe antennas. The measured data indicate that with an adaptive hyperthermia array it may be possible to maximize the applied electric field at a tumor position in a complex scattering target body and simultaneously minimize or reduce the electric field at target positions where undesired high-temperature regions (hot spots) occur     

Combined microwave heating and surface cooling of the cornea

We investigated a nonsurgical means of reshaping the cornea to correct hyperopia, keratoconus, or myopia. The object was to heat the central stroma of the cornea to the shrinkage temperature of collagen, 55-58 degrees C. The heating device was an open-ended, coaxial, near-field applicator driven at 2450 MHz; it incorporates cooling of the cornea surface by flow of saline. We investigated the system theoretically by computing the 2-D, axisymmetric temperature distribution with the finite element method. We investigated the system experimentally by heating excised steer corneas. Histology showed the system could shrink the stroma to a depth of 0.6 mm while sparing the epithelium in 75% of cases; the diameter of shrinkage was 1.3 mm. Theory predicted a significantly deeper and narrower region of shrinkage than was observed.     

Absorbed Power Distributions from Coherent Microwave Arrays for Localized Hyperthermia

Absurbed power distributions in a homogeneous muscle-like tissue due to a planar coherent array consisting of 16 small direct contact applicators at 434 and 915 MHz are calculated, assuming various relative phases and amplitudes are compared with that of a single aperture source at the same frequency with the same overall dimensions. The array applicator may offer improvement in field size or, when focused, a small improvement in penetration, but in practice the performance is very dependent upon bolus thickness. An additional advantage of the array applicator is the ability to modify absorbed power distributions during use by changing the amplitudes of individual applicators.     

The calculated and measured temperature distribution of a phasedinterstitial antenna array [invasive applicators]

The power deposition pattern of four antennas, positioned on the corners of a 2-cm square array with different driving phases, is computed under the assumption of negligible coupling between the antennas. The spatial SAR (specific absorption rate) distribution is calculated by modeling each interstitial applicator as an insulated, asymmetric dipole. For comparison with the heating patterns measured by a thermal video system, the calculated SAR distributions are converted into temperature patterns through an electric network simulation of the heating in artificial muscle tissue. At each nodal point of a grid in the thermal system, the absorbed microwave power (or SAR times density), thermal resistivity, heat capacitance, and temperature are simulated, respectively, as current source, electrical resistance, electrical capacitance and potential. Therefore, solving the equivalent electric network on a computerized simulation routine (SPICE) yields the temperature distribution. In both the axial and transverse planes, the resulting temperature distributions from the antenna array, with various driving phases, agree very well with the measured temperature patterns     

Optimization of SAR distributions in liver and lung regionsirradiated by the H-horn annular phased array hyperthermiasystem

The author discusses a type of annular phased array (APA) system-the H-horn APA. The phase and amplitude control of power deposition patterns for this system have been theoretically analyzed at a frequency of 200 MHz. The formulas for calculating the electric field (E-field) and specific absorption rate (SAR) for this APA system have been derived and can be applied to other types of APA systems. Models on computerized tomography (CT) scans from liver and lung regions have been used for predicting optimization of the E-field and SAR patterns in the case of the relative phase and amplitude changes. It is shown that the phase and amplitude control of SAR patterns results in a more selective and effective heating of tumors situated eccentrically and deeply within the body. The numerical results indicate that it is possible to shift the absorbed power patterns by electrically varying the phase and/or the amplitude of the sources and manually moving the positions of the horns     

A spherical-section ultrasound phased array applicator for deeplocalized hyperthermia

Computer simulation shows that a new ultrasound phased-array with nonplanar geometry has considerable potential as an applicator for deep localized hyperthermia. The array provides precise control over the heating pattern in three dimensions. The array elements form a rectangular lattice on a section of a sphere. Therefore, the array has a natural focus at its geometric center when all its elements are driven in phase. When compared to a planar array with similar dimensions, the spherical-section array provides higher focal intensity gain which is useful for deep penetration and heat localization. Furthermore, the relative grating-lobe level (with respect to the focus) is lower for scanned foci synthesized with this array (compared to a planar array with equal center-to-center spacing and number of elements). This could be the key to the realization of phased-array applicator systems with a realistic number of elements. The spherical-section array is simulated as a spot-scanning applicator and, using the pseudo-inverse pattern synthesis method, to directly synthesize heating patterns overlaying the tumor geometry. A combination of the above two methods can be used to achieve the desired heating pattern in the rapidly varying tumor environment.     

915-MHz Phased-Array System for Treating Tumors in Cylindrical Structures

A phased-array 915-MHz microwave system consisting of four 13x13 cm square applicators was constructed and tested for its design in heating both deep and superficial tumors in cylindrical structures such as the upper and lower extremities or neck. Since each applicator produced SAR patterns in cylindrical phantoms in a plane through the array similar to those produced by a plane wave, a theoretical analysis of the SAR patterns due to the superposition of four plane waves incident on a cyIindrical tissue was done. By altering the orientation of the E-field (either parallel or perpendicular to the axis of the cylinder), as well as the phase and amplitude of the incident waves, various distinct SAR patterns were predicted. Thermograms used to experimentally verify the SAR patterns produced by the four applicators showed similar results with those predicted by the plane-wave analysis. A patient with recurrent melanomas on a lower leg was subjected to a clinical trial of hyperthermia in which high tissue temperatures were produced by utilizing two sets of the phased-array system in series. A therapeutic temperature of 43°C in tumor tissues was confirmed by invasive thermometry with high-resistance thermistors.     

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.     

The performance of a new 915-MHz direct contact applicator with reduced leakage

The technical feasibility of commercially developing a safe and effective direct contact diathermy applicator operating at the industrial, scientific, medical (ISM) frequency of 915 MHz is demonstrated. The basic design consists of a circular waveguide which is internally loaded with two orthogonal pairs of forward ridges to obtain circular polarization and two rear ridges with a probe to excite the guide. Two prototype designs are considered: the small applicator (15 cm diameter) has one annular choke covered with a 2.5-cm thick microwave absorber, and the large applicator (25 cm diameter) has two additional concentric chokes to limit leakage radiation. The performance of the applicators was evaluated in terms of the requirements of a ORH microwave diathermy test protocol to control stray radiation and deliver a thermally effective absorbed dose rate to simulated muscle tissue of a phantom with a 1-cm or 2-cm fat layer. The net power required to deliver a thermally effective 235-W/kg specific absorption rate (SAR) to such a planar phantom was determined. For this net power, leakage levels considerably less than 5 mW/cm2 (at 5 cm from applicator-phantom boundary) were obtained for the applicators in direct contact with the phantom. If a small spacing (1 cm) between these applicators and planar phantoms is introduced, the net power required to deliver an effective SAR to a phantom and the associated leakage can become excessive. For the small applicator, the required net power for inducing an SAR of 235 W/kg in muscle tissue of a 1-cm fat layer phantom is about 330 W and the leakage is about 120 mW/cm2. For a 2-cm fat layer phantom, these values are somewhat higher. For the large applicator, using a 1-cm fat layer phantom, the values are about 200 W and about 17 mW/cm2. Again, for a 2-cm fat layer phantom, these values are somewhat higher.     

Theoretical analysis of H-Horn annular phased array system for heating deep-seated tumors

This paper discusses a type of annular phased array system--H-Horn APA. The phase and amplitude control of power deposition patterns for this system are theoretically analyzed at a frequency of 200 MHz. The formulas for calculating E-field and SAR for this APA system are derived, and can be applied to other type APA systems. Models of computerized tomography (CT) scans from liver and lung regions have been used, respectively, for predicting optimization of E-field and SAR patterns in the case of the relative phase and amplitude changes. It is shown that the techniques of the phase and amplitude control of SAR patterns result in more selectively and effectively heating of tumors situated eccentrically and deeply within bodies of patients. The APA hyperthermia described in this paper shows great promise, and it looks very useful for developing clinical applications.     

Theoretical and experimental analysis of air cooling forintracavitary microwave hyperthermia applicators

An intracavitary microwave antenna array system has been developed and tested for the hyperthermia treatment of prostate cancer at Thayer School of Engineering and Dartmouth-Witchcock Medical Center. The antenna array consists of a choked dipole antenna inserted into the urethra and a choked dipole antenna eccentrically embedded in a Teflon obturator inserted into the rectum. To prevent unnecessary heating of the healthy tissue that surrounds each applicator, an air cooling system has been incorporated into the rectal applicator. The air cooling system was designed and modeled theoretically using a numerical solution of heat and momentum equations within the applicator, and an analytical solution of the Pennes bioheat equation in tissue surrounding the applicator. The 3D temperature distribution produced by the air-cooled rectal applicator was measured in a perfused canine prostate     

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     

热疗用微波辐射器长度对其性能影响的理论分析

本文分析了一种带冷却水的用于人体体腔内部热疗的绝缘偶极子微波辐射器的特性。利用辐射器上的电流分布导出辐射阻抗和驻波经与辐射器长度的关系式，探讨了体腔周围被加热生物组织中分布的比吸收率，进而根据热平衡方程得到辐射器长度对温度场分布的影响，为体腔内部热疗用微波辐射器的设计提供了理论依据。     

Development of a 915-MHz Direct-Contact Applicator for Therapeutic Heating of Tissues

The design of a 915-MHz diathermy dielectric-loaded applicator with a TE/sub 10/-mode aperture field distribution is described. The lightweight porous dielectric used for loading the applicator allows for the transmission of refrigerated air through the cavity to provide surface cooling so therapeutic temperature can be produced in deep tissues without excessive heating of surface tissues. The design is based on theoretical calculations previously developed by the authors which predict optimal size of the aperture and field distribution that would provide the best heating patterns in deep layers of tissue. Experimental evaluations of the heating of tissues of models and human beings are discussed.     

Microwave Heating Effect on the Dog Thyroid Gland

Dog thyroid glands were exposed in vivo to 2450 MHz, CW microwave fields for 2 h using a dielectrically loaded waveguide applicator. Specific absorption rates of 58, 131, and 190 W/kg in the center of the thyroid gland resulted in temperatures of 38-39°C, 40-42°C, and 44-46°C, respectively. An increase in the heated gland's thyroid hormone, thyroxine (T4) and triiodothyronine (T3), release rate was observed. This result demonstrates that the dog thyroid gland can be directly stimulated by microwave heating.     

水冷式微波偶极子辐射器在生物介质中的SAR分布计算

本文从电磁场理论模型出发，分析地腔内水冷式微波偶极子辐射器在生物介质中的近场辐射特性，计算得到有水冷时的比吸收率（ＳＡＲ）分布形态。与无水冷时的ＳＡＲ分布比较表明，水冷可改善热区分布，增加治疗深度。该结果与工程经验和临床实际基本相符。