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Biomedical subjects

M J Hagmann

Publications and source records attributed to M J Hagmann.

At least 19 recordsLinked to original sources
PubMed2001

Simulations of photon-assisted tunneling using the Fokker-Planck equation to model the scattering of electrons within the emitting metal tip.

A method to simulate photon-assisted tunneling is developed, and applied to model laser-assisted field emission from metals. Our simulations show that most of the exchange of quanta between the electrons and the radiation occurs within the emitting metal tip. In typical experiments (lambda = 670 nm with tungsten metal) the depth of penetration for the radiation is four times the mean free path for electrons at the Fermi level, so it is necessary to allow for scattering. We use a Floquet expansion with the time-dependent Schrödinger equation to allow for the exchange of quanta between the electrons and the radiation field. Multiparticle effects are modeled with the density functional theory within the local density approximation for the Kohn-Sham exchange and correlation, and the Fokker-Planck formulation is used to determine the effects of scattering on the energy distribution of the electrons.

Journal Article↗
PubMed2001

Prototypes using metal, carbon fiber and composite field emission sources modulated by a laser beam.

Field emission of electrons from a variety of metallic, carbon fiber and composite metal-insulator micropoint cathodes was employed in this study. Tungsten, carbon fiber and ZrC tips, were studied using a field emission microscope. These cathodes were characterized and the current-voltage (I-V) characteristics were determined. A variety of surface treatment procedures were carried out to increase the stability of emission. These electron sources were mounted in sealed prototype field emission tubes, while others were tested under medium, high and UHV conditions. The emission current switch-on phenomenon was found with all non-metallic cathodes. The emitters were then subjected to a square wave-modulated, maximally focused laser diode beam (lambda = 658 nm, 30mW). The beam impedance (approximately 1 Gohms) and the anode capacitance (approximately 10 pF) act as a low-pass filter.

Journal Article↗
PubMed1993

Noninvasive measurement of current in the human body for electromagnetic dosimetry.

Minimally perturbing, resistive, nonferrous probes were developed for noninvasively measuring hazardous currents induced in the human body by electromagnetic fields at 1 to 200 MHz. Each probe has a resistive toroidal coil that is placed around the leg or other body member. An electrostatic shield is required to limit capacitive coupling. A new shielded test fixture provides TEM fields for calibration with a VSWR less than 1.1 from 1 to 200 MHz. A man-sized phantom was exposed to the near-field of a vertical monopole antenna at 29.9 MHz, and the value of the current measured in the leg with our probe is in reasonable agreement with measured heating. Analyses and experiments show that commercial ferrous current probes modify the circuit in which they are used, changing the current being measured. Less change is caused by our nonferrous current probes.

Bias↗
PubMed1987

Criteria for accurate usage of block models.

The results of block model calculations for multiple discretizations of a dielectric sphere and circular cylinder suggest that it is essential that the cells must be arranged for a best-fit of the body being modeled, the matrix elements must be reasonably accurate, and the cells must be small enough so that the pulse-function basis approximation is not blatantly unreasonable. When these criteria are approximately satisfied the remaining errors appear to be mainly due to imperfect representation of the shape of the object being modeled. It appears that the accuracy can be improved by using discretizations having cells of reduced size near the surface of the object. Geometric factors are defined which allow testing the potential accuracy of a solution without dimensioning or inverting a large matrix. Several unique procedures for discretization are also described that have the potential of partially mitigating the errors due to inaccurate representation of the shape of a scatterer.

Humans↗
PubMed1981

Effects of millimeter-wave radiation on monolayer cell cultures. II. Scanning and transmission electron microscopy.

Both thermal and athermal effects of millimeter-wave radiation of BHK-21/C13 cells were sought using scanning and transmission electron microscopy in conjunction with an in vitro technique that allows direct exposure of monolayer cultures to high average power densities. Culture dishes were irradiated by placing them on the open end of an E- or U-band wave guide. This technique exposes different regions of the cell monolayer lying along the longer axis of the wave guide aperture to varying power densities ranging from zero at each edge to twice the average power density at the center. Cell ultrastructure was unaffected by microwave radiation for 1 hour (41.8 or 74.0 GHz, average power densities = 320 or 450 mW/cm2, respectively) with or without cooling by rapid recirculation of the culture medium. Temperature in recirculated cultures was held at 37.2 degrees C, and that in noncooled cultures never exceeded 42 degrees C during irradiation at either power density. In contrast, cell morphology was affected by microwave exposure whenever irradiation conditions were altered so that the temperature of the monolayer reached or exceeded 44.5 degrees C. Ultrastructural alterations included breakage of cell processes, progressive detachment of cells from the substrate, increased clumping of heterochromatin in the nuclei, and the appearance of large empty vesicles in the cytoplasm. Such morphological changes resulted from either application of higher average power densities or irradiation at the power densities described above at a higher ambient temperature (greater than or equal to 38.5 degrees C).

Animals↗
PubMed1980

Millimeter wave absorption spectra of biological samples.

A solid-state computer-controlled system has been used to make swept-frequency measurements of absorption of biological specimens from 26.5 to 90.0 GHz. A wide range of samples was used, including solutions of DNA and RNA, and suspensions of BHK-21/C13 cells, Candida albicans, C krusei, and Escherichia coli. Sharp spectra reported by other workers were not observed. The strong absorbance of water (10--30 dB/mm) caused the absorbance of all aqueous preparations that we examined to have a water-like dependence on frequency. Reduction of incident power (to below 1.0 microW), elimination of modulation, and control of temperature to assure cell viability were not found to significantly alter the water-dominated absorbance. Frozen samples of BHK-21/C13 cells tested at dry ice and liquid nitrogen temperatures were found to have average insertion loss reduced to 0.2 dB/cm but still showed no reproducible peaks that could be attributed to absorption spectra. It is concluded that the special resonances reported by others are likely to be in error.

Absorption↗
PubMed1980

Plane-wave spectrum approach for the calculation of electromagnetic absorption under near-field exposure conditions.

The exposure of humans to electromagnetic near fields has not been sufficiently emphasized by researcher. We have used the plane-wave-spectrum approach to evaluate the electromagnetic field and determine the energy deposited in a lossy, homogeneous, semi-infinite slab placed in the near field of a source leaking radiation. Values of the fields and absorbed energy in the target are obtained by vector summation of the contributions of all the plane waves into which the prescribed field is decomposed. Use of a fast Fourier transform algorithm contributes to the high efficiency of the computations. The numerical results show that, for field distributions that are nearly constant over a physical extent of at least a free-space wavelength, the energy coupled into the target is approximately equal to the resulting from plane-wave exposed.

Absorption↗
PubMed1980

Electromagnetic absorption in a multilayered slab model of tissue under near-field exposure conditions.

The electromagnetic energy deposited in a semi-infinite slab model consisting of skin, fat, and muscle layers is calculated for both plane-wave and near-field exposures. The plane-wave spectrum (PWS) approach is used to calculate the energy deposited in the model by fields present due to leakage from equipment using electromagnetic energy. This analysis applies to near-field exposures where coupling of the target to the leakage source can be neglected. Calculations were made for 2,450 MHz, at which frequency the layered slab adequately models flat regions of the human body. Resonant absorption due to layering is examined as a function of the skin and fat thicknesses for plane-wave exposure and as a function of the physical extent of the near-field distribution. Calculations show that for fields that are nearly constant over at least a free-space wavelength, the energy deposition (for skin, fat, and muscle combination that gives resonant absorption) is equal to or less than that resulting from plane-wave exposure, but is appreciably greater than that obtained for a homogeneous muscle slab model.

Absorption↗
PubMedDate not supplied

Measurements of the self-sustained enhancement of field emission by carbon fiber microemitters.

Two types of self-sustained enhancement in field emission by carbon fibers are described. In the first, the field is increased until the emission current switches from zero to between 1 and 10 microA. Next the field is reduced, but not so far that the current would drop. Then the current remains for several hours to several days, with transient increases from the 10 microA to between 14 and 22 microA. It is believed that the transients are caused by the activation of new microtips on the fiber surface. These effects were noted when the carbon fiber tip was mounted in a closed glass vacuum bulb pumped by barium getters, and also in a vacuum system using the combination of a molecular drag pump and ion pumps. The second type of enhancement occurs under ultrahigh vacuum conditions, during in situ thermal treatment of the carbon fiber tip while the emission current is about 2.5 microA. A specially built cathode assembly enables heating the tip to approximately 725 degrees C. After continuous heating at 570 degrees C for 20 to 35 h, the current suddenly increases to between 13 and 25 microA. This enhancement is reversible if the emitted current is kept at the newly increased value for at least 30 min. The current-voltage characteristics at several temperatures were recorded and analyzed. Similar field-forming phenomena were previously observed with Molybdenum and ZnO-W tips.

Journal Article↗
PubMedDate not supplied

Human leg heating using a mini-annular phased array.

The energy deposition pattern within an isolated human leg heated with a mini-annular phased array (MAPA) hyperthermia applicator has been determined. The non-tumor-bearing lower portion of a human leg amputated at the hip due to the presence of a large tumor in the thigh was "fixed" in a 50% ethanol in 0.9% saline solution. Subsequent to this fixation process, the leg was rehydrated in 0.9% saline and heated four times using a MAPA operating at 122 MHz. Specific absorption rates and electric field strengths were calculated from the rates of change of temperature with time measured at 143 different anatomical locations within the leg. When the leg was coaxial with the MAPA and the MAPA was axially positioned midway between the knee and the ankle, the points of maximum heating were skewed away from the center of the MAPA, towards the ankle of the leg and along the central axis of the MAPA. Significant temperature rise was measured inside the bone and the fat as well as inside the muscle of the leg. Bone heating was reduced when the leg was shifted away from the MAPA axis.

Amputation, Surgical↗