Steady-state Temperature Increase Research Articles

Heating of tissues by microwaves: a model analysis.

We consider the thermal response times for heating of tissue subject to nonionizing (microwave or infrared) radiation. The analysis is based on a dimensionless form of the bioheat equation. The thermal response is governed by two time constants: one (tau1) pertains to heat convection by blood flow, and is of the order of 20-30 min for physiologically normal perfusion rates; the second (tau2) characterizes heat conduction and varies as the square of a distance that characterizes the spatial extent of the heating. Two idealized cases are examined. The first is a tissue block with an insulated surface, subject to irradiation with an exponentially decreasing specific absorption rate, which models a large surface area of tissue exposed to microwaves. The second is a hemispherical region of tissue exposed at a spatially uniform specific absorption rate, which models localized exposure. In both cases, the steady-state temperature increase can be written as the product of the incident power density and an effective time constant tau(eff), which is defined for each geometry as an appropriate function of tau1 and tau2. In appropriate limits of the ratio of these time constants, the local temperature rise is dominated by conductive or convective heat transport. Predictions of the block model agree well with recent data for the thresholds for perception of warmth or pain from exposure to microwave energy. Using these concepts, we developed a thermal averaging time that might be used in standards for human exposure to microwave radiation, to limit the temperature rise in tissue from radiation by pulsed sources. We compare the ANSI exposure standards for microwaves and infrared laser radiation with respect to the maximal increase in tissue temperature that would be allowed at the maximal permissible exposures. A historical appendix presents the origin of the 6-min averaging time used in the microwave standard.

Synchrotron white beam thermal loading on polycapillary X-ray optics

Measurements and calculations have been performed of thermal loading effects on polycapillary X-ray optics due to synchrotron bending magnet white-beam exposure. Steady-state temperature increases of up to 190°C at the input end of the optic have been measured during exposure to a beam intensity of 11 W/cm 2; extended exposure at these intensities produces little or no performance degradation. Time to reach steady-state thermal equilibrium after exposure to the X-ray beam has been found to be of the order of 1 min. Theoretical modeling of thermal behavior agrees with experiment. Theoretical modeling predicts that slight misalignment of the optic will not affect the thermal load. A substantial decrease in the temperature rise is expected if the air atmosphere is replaced with helium or if air flow is maintained over the optic.

Vibration Analysis of Stiffened Laminated Plates Including Thermally Postbuckled Deflection Effect

Vibration characteristics of stiffened laminated plates are investigated considering postbuckled deflection effect due to thermal load. The finite element method is used for the analysis of thermal postbuckling and natural vibration of stiffened plates in the postbuckling range. The finite element model is based on the first order shear deformable plate theory (FSDT) for a skin panel and Timoshenko beam theory for stiffeners. Von Karman strain-displacement relation is used to account for a large deflection. Critical buckling temperature and the corresponding mode shape are determined from Euler buckling problem. In order to solve the thermal-postbuckling problem, the initial nonlinear stiffness is determined from an estimated deflection of scaled buckling mode shape. The converged deflection at any temperature change is obtained using the Newton-Raphson method. The vibration analyses of stiffened laminated plates in the postbuckling range are performed using the tangent stiffness obtained from the converged deflection. The effects of the stiffener size, the number of stiffeners, and lamination scheme on vibration characteristics are studied for stiffened laminated plates subject to steady-state uniform temperature increase.

Vibration behaviors of thermally postbuckled anisotropic plates using first-order shear deformable plate theory

Vibration behaviors of thermally postbuckled anisotropic plates are investigated. The finite element method is used for the analysis of thermal postbuckling and natural vibration of thermally postbuckled plates. The finite element model is based on the first-order shear deformable plate theory (FSDT) and von Karman strain-displacement relation to account for large deflection. Critical buckling temperature and the corresponding mode shape are determined from Euler buckling problem. In order to solve the thermal-postbuckling problem, the initial nonlinear stiffness is determined from estimated deflection of scaled buckling mode shape. The converged deflection at any temperature change is obtained using the Newton-Raphson method. The vibration analysis of thermally postbuckled plates are performed using the tangent stiffness obtained from the converged deflection. The effect of fiber orientation angle and aspect ratio on postbuckling and vibration behaviors are studied for simply supported laminated plates subject to steady-state uniform temperature increase.

The monopole-source solution for estimating tissue temperature increases for focused ultrasound fields

The monopole-source solution to the problem of estimating tissue temperature rise generated by a focused ultrasound beam is presented. The acoustic pressure field generated by a focused, continuous-wave ultrasound source using the acoustic monopole-source method is developed. The point-source solution to the linear bio-heat transfer equation is used to calculate the axial, steady-state temperature increase for both circular and rectangular apertures. The results of the circular aperture are compared with the temperature increase calculated using the heated-disc method and are shown to be in substantial agreement. Finally, the temperature increase generated by the circular aperture is compared to that of the rectangular aperture for the same source power, aperture surface area, operating frequency, and medium properties, and it is shown that the rectangular source generates temperature increases less than those of the circular source under these conditions.

Infrared radiometry of thermally insulated skin for the assessment of skin blood flow

The temperature increase of thermally insulated skin provides useful information about its blood flow and the blood temperature. The measurement of skin temperature by a contact thermometer, such as a thermistor, is not accurate, because it depends on the pressure exerted on the skin by the thermometer. In order to have reproducible measurement of the skin temperature, noncontact temperature measurement is preferable. Suitable insulation is achieved by using a cylinder of lowthermal-conductivity material, covered by polyethylene layers, which is applied on the skin. The polyethylene layers permit partial transmission of the infrared radiation through it. Preliminary results show that both the transient and the steady-state temperature increase can be obtained from measurements of radiation transmitted through the thermal insulation that was applied to the skin, and that the steady-state temperature increase is more closely related to tissue blood flow than the uncovered-skin temperature is.

Disordering of platinum clusters

Platinum clusters less than 4 nm in diameter appear to “melt” when exposed to the electron beam in a scanning transmission electron microscope. The diffraction patterns of these disordered clusters show dynamic features which suggest that some micro-crystalline structure may still remain. The calculated steady-state temperature increase due to the interaction of the electron beam with the cluster and substrate is not sufficient to cause the cluster to melt even with melting-point depression considered. The clusters remain amorphous for an average of 20 minutes on a carbon substrate, and for 24 hours when supported on silica. The cause of “melting” (or disordering) and the recrystallization mechanism are, at present, unknown.