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.     

Direct computation of ultrasound phased-array driving signals froma specified temperature distribution for hyperthermia

This paper presents a new method which obtains ultrasound hyperthermia applicator phased-array element driving signals from a desired temperature distribution. The approach combines a technique which computes array element driving signals from focal point locations and intensities with a new technique which calculates focal point locations and power deposition values from temperature requirements. Temperature specifications appear here as upper and lower bounds within the tumor volume, and a focal point placement algorithm chooses focal patterns capable of achieving the temperature range objective. The linear algebraic structure of the method allows rapid calculation of both the phased-array driving signals and an approximate temperature field response. Computer simulations verify the method with a spherical section array (SSA) for a variety of temperature specifications and blood perfusion values. This scheme, which applies to any phased-array geometry, completes an essential step in both treatment planning and feedback for hyperthermia with ultrasound phased-array applicators.     

Induction of hyperthermia using an intracavitary multielementultrasonic applicator

In this paper, the possibility of inducing controlled hyperthermia in rectal or vaginal wall tumors using an intracavitary ultrasonic applicator was investigated. A computer model that took into account the thermal and ultrasonic properties of tissues and surface cooling was used to optimize the transducer parameters to obtain desirable temperature distributions for different perfusion situations in the tumor. Also, an applicator that consisted of a cylindrical array of five independently controllable ultrasonic transducers was developed. This array was then tested in degassed water to determine the functional characteristics. This same applicator, modified to include water cooling of the tissue surface, was tested in vivo in dogs. The temperature distributions were found to be promising and with modifications this approach will be used in clinical treatments of suitable tumors.     

Coupling phenomena in concentric multi-applicator phased arrayhyperthermia systems

The coupling between the waveguide applicators of a four-element phased array hyperthermia system irradiating a three-layered cylindrical tissue model of circular cross section is analyzed theoretically. The fields inside the tissue layers are expressed in terms of cylindrical vector wave functions satisfying the corresponding wave equations, while the fields inside each waveguide are expanded in terms of guided and evanescent normal modes. Then, by implementing the appropriate boundary conditions, a system of four coupled integral equations is derived in terms of the unknown electric field distributions on the open waveguide apertures. This system is solved by expanding the unknown electric field on each aperture into waveguide normal modes and by applying a Galerkin's procedure. The self reflection coefficient and the mutual coupling coefficients are then determined and numerical results for a four-element phased array hyperthermia system are computed and presented for different waveguide applicator sizes and settings     

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.     

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.     

A Predictive-Adaptive, Multipoint Feedback Controller for Local Heat Therapy of Solid Tumors

Uniform heating of tumor tissue to therapeutic temperatures without damaging surrounding normal tissue is required for optimal local heat therapy of cancer. This paper describes an algorithm for on-line computer control that will allow the therapist to minimize the standard deviation of measured intratumoral temperatures. The method is applicable to systems incorporating multiple surface and/or interstitial applicators delivering microwave, radiofrequency, or ultrasonic power and operating under control of a small computer. The essential element is a novel predictive-adaptive control algorithm that infers relevant thermal parameters from the responses of multiple temperature sensors as each of the power applicators is briefly turned off. Applied power and effective perfusion are estimated from transient slope changes of the temperature-time curves for each sensor. By substituting these values into a system of linear equations derived from the bio-heat transfer equation, the small computer can calculate the optimal allocation of power among the various applicators ("knob settings") to generate most uniform intratumoral temperature distribution with the desired mean, or minimum, tumor temperature.     

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.     

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.     

Numerical simulation of magnetic induction heating of tumors with ferromagnetic seed implants

Calculations based on the bioheat transfer equation have been carried out to determine the temperature distributions to be expected from the use of inductively heated ferromagnetic implants to heat deep-seated tumors. Two types of ferromagnetic implants are considered: constant power seeds, for example, those constructed from Type 430 stainless steel; and constant temperature seeds which pass through a Curie transition to the nonmagnetic state at a specified temperature. The temperature distributions are studied as a function of the size of the implant array, its geometrical relationship to the tumor, the density of implants within the array, and the blood perfusion characteristics of the tumor and its surrounding normal tissue. Two tumor models are considered: a uniformly perfused model which is indistinguishable from the surrounding normal tissue, and an annular perfusion model with a necrotic core surrounded by intermediately and highly perfused shells. Temperature distributions are considered acceptable if the minimum temperature in the tumor is greater than 42°C and the maximum temperature does not exceed a maximum allowable value (either 48 or 60°C). The results of over 200 combinations of the above parameters are presented in a compact format. General conclusions drawn are that the tumor should lie entirely within the implanted array if the tumor periphery is to be heated adequately, and that the constant temperature seeds, which are self-regulating in temperature, give better tumor temperature distributions.     

Automatic temperature controller for multielement array hyperthermia systems

This paper concerns the optimization and performance analysis of an automatic control algorithm for managing power output of large multielement array hyperthermia applicators. Simulation and corresponding measurement of controller performance in a solid tissue equivalent phantom model is utilized for analysis of controller response to dynamically varying thermal load conditions that simulate clinical treatments. The analysis leads to an optimum controller which demonstrates the ability to achieve a uniform and stable temperature profile over a large surface area regardless of surrounding thermal load. This paper presents several advancements to the performance of a previously published control routine, including: 1) simplified simulation techniques for thorough characterization of controller performance; 2) an optimization procedure leading to an improved hybrid control algorithm for maintaining optimal performance during periods of both "rising" and "steady-state" temperature; 3) performance analysis of a control algorithm tailored for large area hyperthermia treatments with a mulitelement array applicator. The optimized hybrid controller is applied to the conformal microwave array (CMA) hyperthermia system previously developed for heating large area surface disease such as diffuse chestwall recurrence of breast carcinoma, and shown to produce stable, uniform temperatures under the multielement array applicator for all thermal load conditions.     

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     

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.     

Body conformable 915 MHz microstrip array applicators for largesurface area hyperthermia

The optimal treatment with hyperthermia of superficially located tumors which involve large surface areas requires applicators which can physically conform to body contours, and locally alter their power deposition patterns to adjust for nonuniform temperature caused by tissue inhomogeneities and blood flow variations. A series of 915 MHz microstrip array applicators satisfying these criteria have been developed and clinically tested. Clinical and engineering design tradeoffs for practical devices are discussed. Measurements taken in tissue equivalent phantoms and a summary of our clinical experiences with these microstrip arrays are presented.     

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.     

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.     

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     

Use of the impedance method to calculate 3-D power depositionpatterns for hyperthermia with capacitive plate electrodes

We have used the three-dimensional impedance method to calculate the power deposition in the human pelvic region due to a radio frequency current at 13.56 MHz applied through round or oval capacitive-plate applicators. We compare the energy deposited in several tumor sites when energy is applied using a variety of applicator sizes, locations, and boluses of various conductivities. We show detailed maps of the power deposition in these cases for selected regions of the body and suggest useful configurations of applicators for heating tumors in several locations in the body.     

Optimal Temperature Control with Phased Array Hyperthermia System

A strategy for controlling the temperature distribution in tissue, irradiated by a phased array of RF applicators, described. Using the amptitudes and relative phases from the individual applicators as control inputs, feedback control is established from a corresponding number of measured tissue temperatures. Optimal control theory based on a state-space model of the thermal process is used for designing the multi-variable self-toning controller. Simulations of the two-dimensional temperature distribution in cylindrical homogeneous muscle tissue indicate that it is possible to place a temperature maximum near a given point and to maintain therapeutic temperatures in a specified tumor area while the temperatures in the surrounding tissue are lower.