Microwave hyperthermia induced by a phased interstitial antennaarray

An interstitial microwave antenna array for hyperthermia cancer treatment is investigated. The purpose is to generate both uniform and controlled nonuniform temperature distributions in biological tissue by modulating the phases of the signals applied to each antenna. The array has four antennas positioned on the corners of a 2 cm square. The distributions of absorbed power within the arrays are computed and then converted into temperature distributions through a heat conduction simulation. The temperature patterns over phantom muscle are presented in both the lateral plane (perpendicular to the antennas) and the axial plane (parallel to the antennas). It is found that by proper phase modulation of radiofrequency signals applied to each antenna, a uniform heating can be produced in the entire array volume     

Development of a family of RF helical coil applicators which produce transversely uniform axially distributed heating in cylindrical fat-muscle phantoms

Uniform transverse heating, without excessive surface heating, in phantoms which simulate human limbs or trunks has long been sought after as a precursor for predictable hyperthermia treatment of deep-seated cancerous tissues. A family of helical coils developed at the National Center for Devices and Radiological Health has induced transversely uniform axially distributed heating in the muscle portion of simulated fat-muscle, cylindrical arm- and thigh-sized phantoms without excessively heating the simulated fat. The final design parameters for the coils were derived using different sized coil/phantom combinations and electric (E)-and magnetic (H)-field strength measurement scans. Transverse heating patterns, in the form of thermographic pictures, will be the principal format for presentation of the experimental data in this paper. Independent nonperturbing thermometry data are also included to enhance the accuracy of the thermographic results obtained.     

Measurement of nonuniform current density by magnetic resonance

A noninvasive tissue current measurement technique and its use in measuring a nonuniform current density are described. This current density image is created by measuring the magnetic field arising from these currents and taking its curl. These magnetic fields are proportional to the phase component of a complex magnetic resonance image. Measurements of all three components of a quasistatic nonuniform current density in a phantom are described. Expected current density calculations from a numerical solution for the magnetic field which was created by the phantom are presented for comparison. The results of a numerical simulation of the experiment, which used this field solution and which included the effects of slice selection and sampling, are also presented. The experimental and simulated results are quantitatively compared. It is concluded that the principle source of systematic error was the finite slice thickness, which causes blurring of boundaries.     

Experimental characterization of helical coils as hyperthermiaapplicators

A series of helical-coil hyperthermia applicators have been designed for treating human limbs. Several experiments to determine their operating characteristics were conducted using muscle-equivalent, cylindrical, and lower-body-shaped phantoms. It was found that this kind of applicator has to be operated at resonances which are both sharp and load-dependent. This can have significant clinical implications, since changes in the position of the patient and/or the tissue dielectric properties with temperature can produce a severe mismatch. Moreover, even though the patterns of energy deposition were found to be relative transversely uniform and axially belt-shaped within the cylindrical phantoms, they were strongly dependent on the shape of the phantom and of the coil for the more realistic human-shaped phantom. Intense local heating was observed whenever the winding of the helical coil was within a few millimeters of the surface of the human-shaped phantom. The tests with the human-shaped phantom showed that there can be significant energy deposition outside of the region intended for treatment     

Manipulation of Central Axis Heating Patterns with a Prototype, Three-Electrode Capacitive Device for Deep-Tumor Hyperthermia

Measurements of initial temperature elevations produced by a prototype, three-electrode capacitive heating device operating at 13.56 MHz were made at selected points along vertical and horizontal axes in beef phantoms. Adjustment of the net power supplied by the three generators driving the upper and lower two electrodes of the device permitted significant manipulation of the distribution of initial temperature elevations along the vertical "central" axis of the beef phantoms. For example, in a 17 cm thick phantom, the ratio of initial temperature elevations at central axis depths of 1 and 8 cm below the upper surface was reduced from 3.2 to 0.5, through appropriate redistribution of power to the three electrodes.     

An analytical scatter correction for singles-mode transmission data in PET

We present an analytical scatter correction, based upon the Klein-Nishina formula, for singles-mode transmission data in positron emission tomography (PET) and its implementation as part of an iterative image reconstruction algorithm. We compared our analytically-calculated scatter sinogram data with previously validated simulation data for a small animal PET scanner with 68 Ge (a positron emitter) and 57 Co (approximately 122-keV photon emitter) transmission sources using four different phantom configurations (three uniform water cylinders with radii of 25, 30, and 45 mm and a nonuniform phantom consisting of water, Teflon, and air). Our scatter calculation correctly predicts the contribution from single-scattered (one incoherent scatter interaction) photons to the simulated sinogram data and provides good agreement for the percent scatter fraction (SF) per sinogram for all phantoms and both transmission sources. We then applied our scatter correction as part of an iterative reconstruction algorithm for PET transmission data for simulated and experimental data using uniform and nonuniform phantoms. For both simulated and experimental data, the reconstructed linear attenuation coefficients (mu-values-values) agreed with expected values to within 4% when scatter corrections were applied, for both the 68 Ge and 57 Co transmission sources. We also tested our reconstruction and scatter correction procedure for two experimental rodent studies (a mouse and rat). For the rodent studies, we found that the average mu-values for soft-tissue regions of interest agreed with expected values to within 4%. Using a 2.2-GHz processor, each scatter correction iteration required between 6-27 min of CPU time (without any code optimization) depending on the phantom size and source used. This extra calculation time does not seem unreasonable considering that, without scatter corrections, errors in the reconstructed mu-values were between 18%-45% depending on the phantom size and transmission source used.     

Low-Frequency RF Twin-Dipole Applicator for Intermediate Depth Hyperthermia

In studies on heating deep-seated tumors, various attempts have been made to develop radiofrequency applicators and to confine the controlled volumes into limited sizes at variable useful depths. Results of the present investigation show that the conduction-current mechanism dominates the heating with magnetic dipoles working at frequencies as low as 27 MHz, and that two single-magnetic dipoles forming a loosely coupled pair (twin-dipole applicator) fed by low-frequency, in-phase currents, give a better performance than a single dipole of the same size, due to the phase coherence of the superimposing fields. A number of single-dipole and twin-dipole applicator working at 27 MHz have been developed and given the fundamental tests on phantoms simulating muscle and fat tissues. The results obtained show the feasibility of the proposed applicator to produce a penetration depth up to 7 cm and a power deposition pattern showing a well-defined maximum, which may undergo a controlled shift of a few centimeters in depth. Moreover, the surface overheating may be easily controlled. A circuit design is described that improves the matching and the uniformity of the power deposition pattern. A preliminary calculation in the quasi-static-fields approximation of the electric field induced by the twin-dipole applicator in air is also described.     

RF heating of implanted spinal fusion stimulator during magneticresonance imaging

Radio frequency (RF) heating of an implanted spinal fusion stimulator (SpF) during magnetic resonance imaging (MRI) was studied on a full-size human phantom. Heating during MRI scans (GE Signa 4X, 1.5 T) was measured with RF-transparent fiberoptic sensors. With the implant correctly connected, the maximum temperature rises were less than 2°C during the 26 min that the scans were at maximum RF power. At the tip of a broken stimulator lead (connecting the SpF generator and its electrodes), the maximum temperature rise was 11-14°C. Regular 4-min scans of the spinal cord produced similar temperature rises at the broken tip. After the generator and the leads were removed, heating at the electrode connector tip was less than 1.5°C. The control temperature rises at the same locations, without the stimulator, were less than 0.5°C. This study shows that spinal fusion stimulator heating is within the Food and Drug Administration safety guideline of 2°C. However, if a lead wire is broken, it is unsafe during MRI scans. Radiological examinations will be necessary to ensure the integrity of the implant     

A review of selected biological effects and dosimetric data useful for development of radiofrequency safety standards for human exposure.

This report examines the bases for developing radiofrequency exposure standards which can be related to the thermogenic properties of electromagnetic fields. A review of selected biological effects, including dosimetric data and simulation of human thermodyanmic characteristics that are pertinent to standards development, is presented. Based on the analogy of thermal-stress standards that have been developed for hot industrial environments, limits on increases of body temperature are proposed as criteria for limiting exposure to radiofrequency fields, i.e., occupational exposures involving deep heating of the whole body should not increase core temperature in excess of 1 degree C. Since energy deposition from exposure to some RF fields is likely to be non-uniform and may be high in tissues that are not adapted to high rates of absorption or dissipation of thermalizing energy, means are needed to adjust focal thermal loading against the whole-body averages. A limit on core temperature is inadequate when focal elevations of temperature are close to the limits for protein denaturation, as may well occur even though the core temperature may rise less than 1 degree C. Safety limits for the general population are also discussed and here the permissible thermal load should be low enough to cause no more than an insignificant increase in core temperature. Areas needing further research to reduce the uncertainties in developing safe exposure limits for man are delineated. Even in highly adverse environmental conditions the gross thermal load and consequential heat stress from exposure to radiofrequency fields at the 10 mW/cm2 level will be small compared with that generated by any physical effort. On the basis of available data, it is concluded that the safe value for continuous exposure to 10 mW/cm2, widely used in Western countries, appears to provide an adequate margin of safety for both occupational and environmental exposure for frequencies above about 1 GHz. This limit may well be too high (perhaps by an order of magnitude) for some frequencies below 1 GHz where body resonances cause a significant increase in energy deposition and where local temperature rises occur. At the same time the present averaging period of 0.1 h seems unjustifiably short.     

An Analysis of Minimally Perturbing Temperature Probe and Thermographic Measurements in Microwave Diathermy

Temperature measurements in fat-muscle phantoms using thermography and a minimally perturbing temperature probe were investigated. Two microwave applicators (915- and 2450-MHz) were used to induce the heating in the phantom. Discrepancies between data taken with the thermographic camera versus the probe were measured. These discrepancies were shown to be primarily caused by a 40-s time delay in performing temperature measurements with the thermographic camera, which resulted in additional thermal diffusion in the phantom.     

Magnetic field sensitivity of variable thickness microbridges inTBCCO, BSCCO, and YBCO

We describe results of a study comparing the magnetic field sensitivities of variable thickness bridge (VTB) arrays fabricated in TBCCO, BSCCO, and YBCO thin films. Identical structures were patterned in a variety of films, and the bridges were thinned by four different methods. Analysis of the data yields experimental evidence as to the suitability of these types of films for devices such as the superconducting flux flow transistor (SFFT) which is based on this geometry. The volt-ampere characteristics of the arrays were measured in low uniform magnetic fields (⩽130 G) and in nonuniform fields (⩽5 G) produced by a nearby control line. For these films in this geometry, no measurable effect of the control line magnetic field was observed. Large values of transresistance and current gain could only be attained through a thermal mechanism when the control line was driven normal. Upper bounds for (magnetically generated) transresistance (⩽5 mΩ) and current gains (⩽0.005) have been inferred from the uniform field data assuming a standard best-case device geometry. All volt-ampere curves followed closely a power law relationship (V~I ⁿ), with exponent n ~1.2-10. We suggest materials considerations that may yield improved device performance     

Improving the use of vibro-acoustography for brachytherapy metal seed imaging: a feasibility study

Vibro-acoustography method is explored for detecting and imaging brachytherapy metal seeds in gel phantoms. In a previous paper, we have shown that some immersed objects' resonance frequencies could be detected by vibro-acoustography. Here, we use this idea to optimize the vibro-acoustic excitation of two different sized brass seeds implanted in an agar gel phantom. In the experiments, the best excitation vibration frequencies were determined either by calculating fundamental resonance frequencies for each of the seeds or the experimental optimal resonance frequency of the gel. The resulting vibro-acoustography images demonstrate remarkable contrast in acoustic emission amplitude compared with images obtained at nonresonance frequencies. Results suggest the possible application of vibro-acoustography for directing prostate brachytherapy seed implantation treatment.     

Heating patterns in biological tissue phantoms caused by millimeterwave electromagnetic irradiation

Distribution of millimeter wavelength electromagnetic energy absorption in surface layers of biological tissue models was studied using methods of Infrared Thermography. 0.1 mm thin-layer phantoms were irradiated in the near field using different types of horn antennas in the 37-78 GHz frequency range. Heating patterns were recorded during microwave irradiation, and surface SAR distributions mere calculated. The temperature resolution was better than 0.05 K. It was found that horn antennas produced nonuniform heating patterns in irradiated objects. These nonuniform patterns were due to a geometrical resonance resulting from a secondary wave-mode interaction between an irradiated object and the corresponding critical cross-section of the horn antenna. Local SAR values in hot spots exceeded the spatially averaged values by over 10 times, and the widths of these hot spots at 5 times the average SAR were often 1 mm or less. The location, quantity, number and size of the local field absorption maxima of irradiated objects strongly depended on the frequency of electromagnetic irradiation, with equivalent Q-factors of 500 or more. These findings provide an explanation for a number of frequency-dependent effects of millimeter wave electromagnetic irradiation     

Current crowding effect on thermal characteristics of Ni/doped-Si contacts

Continuous scaling of integrated circuit elements results in an increase of current density and associated Joule heating in the interconnects. The self-heating effect that leads to a temperature rise at interconnects may become a source of thermal reliability problems. This work reports an experimental and computational investigation of the current crowding effect on thermal characteristics of metal/doped-Si contacts. The temperature rise at the contacts is determined from the Seebeck potential measured in a microfabricated test structure. It is found that a nonuniform current distribution introduces a much higher heating power density than a uniform one at the contact window. The increase of the contact temperature is proportional to the power density at the Ni/doped-Si contact. The dependence of sheet resistance of the doped-Si layer, contact resistivity, and contact size on the contact power density is also discussed.     

Microwave heating of malignant mouse tumors and tissue equivalent phantom systems.

Studies were conducted on the free-field heating of mammary tumors on the flank of mice and of tissue equivalent phantoms. A 200 watt, P.M., 2450 MHZ source was marginal in heating 1 cm tumors from room ambient to 38 degrees C. A technique was developed using warm air to raise the thermal baseline of the tumors to body core temperature and microwaves to produce significant hyperthermal levels. This allowed the production tumor temperatures of 45 degrees with greatly reduced microwave power and with significantly more uniform temperatures. However, the non-uniformity in temperature achieved with monodirectional microwave heating is still considered to be excessive: +/- 1 degrees C.     

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 3-D impedance method to calculate power deposition in biologicalbodies subjected to time varying magnetic fields

A previously described two-dimensional impedance method (ibid., vol.31, p.644-51, 1984) for modeling the response of biological bodies exposed to time-varying electromagnetic fields has been extended to three dimensions. This method is useful at those frequencies where the quasistatic approximation is valid and calculates the fields, current densities, and power depositions in the bodies. Solutions using this method for homogeneous spheres in plane waves are presented and compared to the analytic solutions for the same configuration. Solutions for a man exposed to a uniform radio-frequency magnetic field at 30 MHz, are presented, as well as for a man exposed to either circularly or linearly polarized magnetic fields at 63 MHz, uniform within a portion of his body and linearly decreasing outside of that portion, which approximates the exposure in some nuclear-magnetic-resonance imaging devices     

A source of thermocouple error in radiofrequency electric fields

When thermocouples are exposed to radiofrequency fields, the unequal resistances of the two wires cause electrical currents to flow through the junction. The junction is thereby heated, resulting in erroneously high temperature readings. Experimental documentation for the effect and a theoretical explanation are presented. Methods to minimise the error are also suggested.     

Detection of vesicoureteral reflux using microwave radiometry-system characterization with tissue phantoms

Microwave (MW) radiometry is proposed for passive monitoring of kidney temperature to detect vesicoureteral reflux (VUR) of urine that is externally heated by a MW hyperthermia device and thereafter reflows from the bladder to kidneys during reflux. Here, we characterize in tissue-mimicking phantoms the performance of a 1.375 GHz radiometry system connected to an electromagnetically (EM) shielded microstrip log spiral antenna optimized for VUR detection. Phantom EM properties are characterized using a coaxial dielectric probe and network analyzer (NA). Power reflection and receive patterns of the antenna are measured in layered tissue phantom. Receiver spectral measurements are used to assess EM shielding provided by a metal cup surrounding the antenna. Radiometer and fiberoptic temperature data are recorded for varying volumes (10-30 mL) and temperaturesg (40-46°C) of the urine phantom at 35 mm depth surrounded by 36.5°C muscle phantom. Directional receive pattern with about 5% power spectral density at 35 mm target depth and better than -10 dB return loss from tissue load are measured for the antenna. Antenna measurements demonstrate no deterioration in power reception and effective EM shielding in the presence of the metal cup. Radiometry power measurements are in excellent agreement with the temperature of the kidney phantom. Laboratory testing of the radiometry system in temperature-controlled phantoms supports the feasibility of passive kidney thermometry for VUR detection.     

International developments in controlled thermonuclear fusion

Basic processes and fundamental technological requirements of controlled thermonuclear fusion have been widely discussed. Briefly, they include: heating an ultrapure, low-density deuterium and tritium plasma to superhigh temperatures; stably containing the extreme temperature plasma by magnetic fields for a duration adequate to fuse the nuclei; diminishing particle losses occurring through diffusion and instabilities to acceptable levels; gaining useful fusion products conveying energy sufficiently in excess of thermal and radiant energy losses; and, finally, converting the energy released to useful electric power. Recent progress has been encouraging. Numerous and diverse plasmas have been produced and improved understanding and experimental confirmation of stable magnetic containment have been obtained. Also, increased plasma density, temperature, and containment times in a few experimental systems have been achieved. However, complex difficulties, such as new types of instabilities, raise formidable barriers to a workable reactor concept.