Haralambos N. Kritikos

Hot Spots Generated in Conducting Spheres by Electromagnetic Waves and Biological Implications

The distribution of the heating potential generated by an incident electromagnetic plane wave on a conducting sphere simulating the human head was investigated. It was found that for a sphere of 10-cm radius having the same electrical characteristics as those of biological tissues, no hot spots are generated inside. While at lower frequencies the heating is relatively uniform with some polarization effects, for frequencies above 1000 MHz only skin heating takes place. For a sphere of the same size but of conductivity ?= 10 mmho/cm (which for f>1000 is lower than that of biological tissues) hot spots occur inside for f>1000 MHz. Intense hot spots also occur inside spheres of radius 5 cm having the same electrical characteristics as those of biological tissues in the frequency region of 250 MHz

Effect of Surface Cooling and Blood Flow on the Microwave Heating of Tissue

The one-dimensional heat transport equation has been solved for a semi-infinite plane of tissue irradiated by microwave radiation, to determine the effects of thermal convection due to blood flow and transfer of heat from the tissue surface into space on the steady-state temperature distribution in the tissue. For microwaves in the 1 to 10-GHz range, the effective heating depth is 1 to 2 cm, and can be much deeper than the penetration depth of the radiation in the tissue. The position of the maximum tissue temperature can be varied by a centimeter or so by cooling the surface. The results suggest that microwave irradiation of simulated biomaterials is not likely to provide accurate estimates of the actual microwave-induced temperature distribution in tissue.

The Distribution of Heating Potential Inside Lossy Spheres

The distribution of the heating potential inside a lossy sphere having the same electrical characteristics as those of brain tissue was investigated in the frequency region of 10 MHz to 12 GHz. The conditions under which a potential hot spot appears inside the sphere and its shape were determined and were displayed in a radius vs. frequency diagram. The results show that hot spots appear inside only for spheres with radii 8 cm > R > 0.1 cm and only in the frequency range of 300 < f < 12,000 MHz. It was found that the heating potential is always non-uniformly distributed, and in cases where it was maximum inside the sphere it was larger by a factor of approximately ten to the values calculated by the plane slab or the average cross section model.

Potential Temperature Rise Induced by Electromagnetic Field in Brain Tissues

The differential temperature rise in a spherical region stimulating a potential hot spot in the central region of a human head has been determined, takdng into account heat conduction as well as heat convection due to blood flow. The results apply in general to ali cases where the dimensions of the model and the frequency of the incident electromagnetic wave is such that a hot spot is formed near the central region. For a heat deposition rate of 10 mW/g in a hot spot of 1 cm radius, a differential temperature rise over the ambient tissues and blood is approximately 0.5°C. This corresponds to the case of a differential absorption cross section of 1 cm-1 for a 5 cm radius sphere exposed to an incident power density of 10 mW/cm2. For larger models of mans head the differential temperature rise is smaller.

Formation of Hot Spots in Multilayer Spheres

A theoretical study of the distribution of the normalized heating potential resulting from a plane wave incident in a multilayered sphere simulating a human head with skin, fat, bone and brain tissue layers has been undertaken. It was found that for spheres of radii 10 cm and 5 cm a relative peak of the normalized heating potential occurs in the vicinity of the center of the sphere. For the case of the 5 cm sphere in the range of frequencies 400 MHZ to 2.76 GHZ the maximum value of the heating potential occurs at the center suggesting the possibility of a hot spot. A comparison between the multilayer and a single layer model shows that the maximum values of the heating potential are the same.

Variations in the period of the sunspot cycle

Using a wavelet analysis approach, subtle variations in the solar cycle period were extracted from the yearly means of the sunspot numbers. For the nominal eleven-year solar cycle, the resulting estimates of the cycles period are in good agreement with those given by Friis-Christensen and Lassen. Longer period structures in the solar-cycle, including the Gleissberg cycle, are also delineated by the wavelet analysis.

Modern spectral climate patterns in rhythmically deposited argillites of the Gowganda Formation (Early Proterozoic), southern Ontario, Canada

Abstract Rhythmically deposited argillites of the Gowganda Formation (ca. 2.0–2.5 Ga) probably formed in a glacial setting. Drop stones and layered sedimentary couplets in the rock presumably indicate formation in a lacustrine environment with repeating freeze–thaw cycles. It is plausible that temporal variations in the thickness of sedimentary layers are related to interannual climatic variability, e.g. average seasonal temperature could have influenced melting and the amount of sediment source material carried to the lake. A sequence of layer couplet thickness measurements was made from high-resolution digitized photographs taken at an outcrop in southern Ontario, Canada. The frequency spectrum of thickness measurements displays patterns that resemble some aspects of modern climate. Coherent periodic modes in the thickness spectrum appear at 9.9–10.7 layer couplets and at 14.3 layer couplets. It is unlikely that these coherent modes result from random processes. Modern instrument records of regional temperature and rainfall display similar spectral patterns, with some datasets showing significant modes near 14 yr in both parameters. Rainfall and temperature could have affected sedimentary layering in the Gowganda argillite sequence, and climate modulation of couplet thickness emerges as the most likely explanation of the observed layering pattern. If this interpretation is correct, the layer couplets represent predominantly annual accumulations of sediment (i.e. they are varves), and the thickness spectrum provides a glimpse of Early Proterozoic climatic variability. The presence of interannual climate patterns is not unanticipated, but field evidence presented here may be of some value in developing a climate theory for the Early Proterozoic.

SAR ocean image decomposition using the Gabor expansion

Demonstrates the utility of the Gabor expansion as a new tool in geophysical research. The Gabor expansion provides good time-frequency (or space-wavenumber) localization and is ideally suited to represent nonstationary processes. The properties of this tool are demonstrated by expanding an FM-chirp waveform, and azimuth cuts taken from two different SAR ocean images. The effects of filtering in Gabor phase space are also investigated. >

Optimal signal-to-noise ratio for linear arrays by the Schwartz inequality

Abstract A procedure has been developed which determines the excitation parameters of a linear array of isotropic sources such that the signal-to-noise ratio is maximized. The signal is considered to be a point source and the noise has a known distribution in space.

A time-frequency analysis method for radar scattering

A time-frequency analysis method to study electromagnetic scattering is presented and demonstrated using canonical objects. The time-frequency analysis method utilizes the Bargmann transform to formulate the signal representation in phase space. The use of the Bargmann transform leads to an attractive parametric signal representation in terms of complex polynomials, and elliptical filters can be constructed to crop or extract selected areas of the phase plane. The signal representation and filtering operations are demonstrated using scattering responses from spheres and thin wires, and the prominent scattering features are identified and extracted.