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Noncontact eddy-current techniques for characterizing superconductors between 5 MHz and 1 GHz are described. Both a transmission and a frequency-modulation technique rely on modification of the mutual inductance between a pair of coupled inductors. The sample is placed between the inductive sensors; the magnitude of eddy currents induced in the sample, a function of conductivity, modifies the mutual inductance. The frequency-modulation method (where the sensors are the inductive section of a series-resonant LC circuit in an oscillator) is particularly useful for determining the critical temperature (T[sub c]) and transition width for superconducting samples. [ABSTRACT FROM AUTHOR]

We show that thermally stimulated luminescence is an important method for detecting insulating impurity phases that commonly occur in high Tc superconductors. The technique is sensitive to impurity phases at <1% level with a probe depth of ∼ 1 μm, which is the region of interest for many superconductor applications. Samples of Y2O3, Y2BaCuO5, YBa2Cu3O6.2, BaCO3, Ba3CuO4, BaCuO2, and YBa2Cu3Ox (x≊7) were investigated. All but the high quality sintered pellets of YBa2Cu3Ox exhibited relatively intense luminescence. The absence of luminescence, and thus of insulating phases, is correlated with low values of rf surface resistance. [ABSTRACT FROM AUTHOR]

Increasing evidence of the beneficial effects of hyperthermia in the treatment of tumors, particularly when combined with radiation therapy or chemotherapy, has created the need for practical methods of localizing heat in arbitrary tissue volumes. Heating superficial transplanted animal tumors in hot-water baths has been of considerable importance in research, but the method has very limited clinical application. Microwave diathermy and ultrasound diathermy are capable of greater penetration than external conduction heating, but are also generally limited to the heating of superficial tissue volumes. A more direct method for localized tissue heating is proposed: localized electric current fields produced with carefully specified electrode configurations. This technique, although often quite invasive, offers the possibility of heat localization in almost any treatment volume specified by the therapist. Variations in electrical and thermal properties of tissues can be exploited to improve the architecture of the treatment plan. Several electrode configurations are illustrated. Limitations of the technique are discussed.

Calculations of transient tissue temperature distributions are enhanced by the use of an algorithm to simulate automatic temperature regulation by negative feedback. This feature is quite useful in treatment planning, enabling the physicist to predict the effects on the treatment of variations in temperature regulation system parameters, such as temperature sensor placement and feedback loop gain.

Experimental studies have shown that (a) tumor cells may be more sensitive to heat than normal cells; (b) hyperthermia inactivates cellular repair mechanisms for radiation damage; and (c) heat may lower the OER for ionizing radiation (anoxic cells are at least as sensitive to hyperthermia as oxygenated cells). Localized hyperthemia produced by localized current fields in the range of 100 kHz-10 MHz by direct contact electrodes offers two major advantages: the eletrode configurations may be manipulated to obtain desired thermal dose distributions, and, since the mode of heating is essentially instantaneous, accurate temperature control can be maintained during treatment.

The performance of lighting timers in animal rooms was tested with a portable light-level recorder. The instrument monitored the light and dark phases under various degrees of illumination. This inexpensive method of electronic surveillance ensured the validity of experiments dependent on photoperiodicity.

We have found that intrastromal diathermy heat to the cornea spares the epithelium and endothelium, while preferentially allowing contraction of the midstromal corneal collagen. Extensive animal studies have demonstrated a potentially useful dosage range for primate corneal diathermy. We have initiated cautious preliminary human studies utilizing the Los Alamos Keratoplasty unit in the last 12 months to reshape the corneas of keratoconus patients who were referred for penetrating keratoplasty. The steepest area of the cone may be flattened from 4 to 57 diopters, thereby allowing a return to contact lens use. We recommend the technique of Los Alamos Keratoplasty as an alternative to penetrating keratoplasty in patients who have been successful in contact lens wear, but whose cone progression precludes continued satisfactory fitting. The procedure appears effective in this preliminary study. The eventual safety and long-them efficacy of this experimental keratorefractive surgical technique requires extensive further study.

A method is presented, based upon finite-difference forms of Laplace's equation, for the iterative calculation of three-dimensional electric field distributions in electrically conductive media. The method, while generally applicable to any conductive media, will be presented with emphasis on its use for the prediction of power density in tissue when radio-frequency hyperthermia is utilized in the treatment of cancer. A computer code which performs these calculations has been written in BASIC so that is may be adapted to relatively inexpensive desktop computers for use in treatment planning. Example calculations of the distributions of electric potential, gradient, and power density with specific electrode configurations are presented. Applications and limitations of the technique are discussed.

Development is underway on a hyperthermia applicator intended for complete implantation and long-term use. Radio frequency energy is transmitted from an external antenna to a closely coupled subdermal antenna. This internal antenna is connected via a transmission line to deeply implanted electrodes. Changes in temperature at the electrodes result in a change in tissue resistivity which modifies the complex impedance seen at the external antenna terminals. This variation in antenna impedance (magnitude and/or phase angle) can, in principle, be utilized to indirectly monitor and regulate tissue temperature at the electrode location. Test results from conductive-gel tissue phantom experiments are presented.

Tests have been performed in saline phantoms on a 6.78 MHz hyperthermia applicator designed for complete implantation; no conductors would penetrate the skin following the implant procedure. Energy is coupled across the skin from an external to an internal loop antenna: the latter is connected by transmission line to conventional r.f. tissue electrodes which are typically either parallel plates or arrays of cylindrical pins. In phantoms the technique produces deep, localized heating at a selected site without significant hearing at the 'subdermal' receiving antenna near the surface. In addition, the hyperthermia applicator may have potential as an indicator of temperature for the tissue volume at the deep site, since complex impedance at the external antenna is a function of tissue resistivity/temperature at the electrodes. Specifically, temperature at the deep site can be controlled effectively by regulation of the phase angle between voltage and current at the external antenna. A separate passive implantable electromagnetic-reflectance thermometer, energized by external electromagnetic fields, has been designed. This device, also tested in gel and water phantoms, is energized and interrogated by external r.f. fields and tracks a thermocouple to within +/- 0.1 degree C over the range of interest in clinical hyperthermia. The intended application for these devices is repeated, long-term hyperthermic treatment and thermometry of deep-seated malignant tumours following a single invasive procedure.