Heat loss and blood flow during hyperthermia in normal canine brain. II: Mathematical model

Brain Cooling With Ventilation of Cold Air Over Respiratory Tract in Newborn Piglets: An Experimental and Numerical Study

We investigate thermal effects of pulmonary cooling which was induced by cold air through an endotracheal tube via a ventilator on newborn piglets. A mathematical model was initially employed to compare the thermal impact of two different gas mixtures, O₂-medical air (1:2) and O₂-Xe (1:2), across the respiratory tract and within the brain. Following mathematical simulations, we examined the theoretical predictions with O₂-medical air condition on nine anesthetized piglets which were randomized to two treatment groups: 1) control group (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}$n = 4$ \end{document}) and 2) pulmonary cooling group (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}$n = 5$ \end{document}). Numerical and experimental results using O₂-medical air mixture show that brain temperature fell from 38.5 °C and 38.3 °C ± 0.3 °C to 35.7 °C ± 0.9 °C and 36.5 °C ± 0.6 °C during 3 h cooling which corresponded to a mean cooling rate of 0.9 °C/h ± 0.2 °C/h and 0.6 °C/h ± 0.1 °C/h, respectively. According to the numerical results, decreasing the metabolic rate and increasing air velocity are helpful to maximize the cooling effect. We demonstrated that pulmonary cooling by cooling of inhalation gases immediately before they enter the trachea can slowly reduce brain and core body temperature of newborn piglets. Numerical simulations show no significant differences between two different inhaled conditions, i.e., O₂-medical air (1:2) and O₂-Xe (1:2) with respect to cooling rate.

Theoretical simulation of temperature distribution in the brain during mild hypothermia treatment for brain injury

Mild or moderate hypothermia (>30 degrees C) has been proposed for clinical use as a therapeutic option for achieving protection from cerebral ischaemia in brain injury patients. In this research, a theoretical model was developed to examine the brain temperature gradients during selective cooling of the brain surface after head injury. The head was modelled as a hemisphere consisting of several layers, representing the scalp, skull and brain tissue, respectively. The dimensions, physical properties and physiological characteristics for each layer, as well as the arterial blood temperature, were used as the input to the Pennes bioheat transfer equation to simulate the steady-state temperature distribution within the brain. Depending on the head surface temperature, a temperature gradient of up to 13 degrees C exists in the brain tissue. The results have shown that the volumetric-averaged brain tissue temperature Tbt,avg for adults and infants can be 1.7 and 4.3 degrees C, respectively, lower than the temperature of the arterial blood supplied to the brain tissue. The location where the probe should be placed to measure Tbt,avg was also determined by the simulation. The calculation suggests that the temperature sensor should be placed 7.5mm and 5.9 mm beneath the brain tissue surface for adults and infants, respectively, to monitor Tbt,avg continuously.

Acute histological effects of interstitial hyperthermia on normal rat brain

Histological changes in the brains of Fischer rats at different times after interstitial heating with various thermal doses were studied. The brains, subjected to sham-heating, and heating at 39 and 40 degrees C for 30 min showed mild capillary congestion and minimal vacuolation at 4, 24 and 72 h. In the brains heated to 41, 42 and 43 degrees C for 30 min, there was local vascular congestion, petechiae, vacuolation and cellular shrinkage with nuclear pyknosis at 4 h; enhanced congestion and petechiae, acute cellular necrosis, infiltration of polymorphonuclear leukocytes and marked vacuolation at the margin at 24h; total coagulative necrosis of all parenchymal and vascular elements, early liquefaction necrosis and vascular hyperplasia at the margin at 72 h; enhanced vascular hyperplasia at the margin at 120 h and 168 h. The threshold thermal dose for the histopathological damage in the rat brain was heating at 41 degrees C for 30 min.

Clinical, physiological and anatomical determinants for radiofrequency hyperthermia

Temperature/time curves and corresponding CT scans of > 200 regional heat treatments with the hyperthermia system BSD-2000 in 43 patients have been analysed. In vivo variables and treatment parameters such as local specific absorption rate SAR, local relative SAR parallel SAR parallel, total power P, local cooling coefficients wb, and local steady-state temperature elevations delta Tss (above systemic temperature) have been determined. For determination of wb the well-known and accepted steady-state approach has been used, which was slightly modified for the purposes of this study. Specifically, comparison of cooling coefficients at the beginning and end of heat treatments were performed in tumours and normal tissues. Other variables are anatomical descriptors from CT scans, score of side effects plim, and various clinical factors. A variance analysis of the dependent variables, specifically delta Tss and parallel SAR parallel, is performed with respect to factors which were estimated as predictive. The intratumoral steady-state temperature elevations are determined by the perfusion-related cooling coefficients and local SAR to almost the same extent. Increase of cooling coefficients in tumours during the heat treatment characterizing the thermoregulatory potential have a slight but less important influence with respect to the achieved temperature elevations. SAR is influenced by several anatomical factors which determine the relative SAR distribution and clinical factors which limit the total power P. However, options for controlling present RHT systems in order to optimize the relative SAR distribution or to avoid hot spot phenomena appear limited. Three-dimensional modelling calculations show that the spatial arrangement of electrical interfaces emerging from bone and fat structures limits SAR control in available RHT technology and is mainly responsible for local power-dependent discomfort (Wust et al. 1994b). Some conclusions are drawn, about how technological development of hyperthermia technology can contribute towards overcoming this problem.

Specific absorption rate levels measured in a phantom head exposed to radio frequency transmissions from analog hand-held mobile phones

Electric fields (E-fields) induced within a phantom head from exposure to three different advanced mobile phone system (AMPS) hand-held telephones were measured using an implantable E-field probe. Measurements were taken in the eye nearest the phone and along a lateral scan through the brain from its centre to the side nearest the phone. During measurement, the phones were positioned alongside the phantom head as in typical use and were configured to transmit at maximum power (600 mW nominal). The specific absorption rate (SAR) was calculated from the in situ E-field measurements, which varied significantly between phone models and antenna configuration. The SARs induced in the eye ranged from 0.007 to 0.21 W/kg. Metal-framed spectacles enhanced SAR levels in the eye by 9-29%. In the brain, maximum levels were recorded at the measurement point closest to the phone and ranged from 0.12 to 0.83 W/kg. These SARs are below peak spatial limits recommended in the U.S. and Australian national standards [IEEE Standards Coordinating Committee 28 (1991): C95.1-1991 and Standards Australia (1990): AS2772.1-1990] and the IRPA guidelines for safe exposure to radio frequency (RF) electromagnetic fields [IRPA (1988): Health Phys 54:115-123]. Furthermore, a detailed thermal analysis of the eye indicated only a 0.022 degrees C maximum steady-state temperature rise in the eye from a uniform SAR loading of 0.21 W/kg. A more approximate thermal analysis in the brain also indicated only a small maximum temperature rise of 0.034 degrees C for a local SAR loading of 0.83 W/kg.

Modified thermal clearance technique for determination of blood flow during local hyperthermia

The thermal clearance method utilizes the rate of temperature decay after the applied power is turned off to estimate the local blood flow. A limitation of this method has been its inability to account for the contribution of thermal conduction to the rate of temperature decay. As a result, the blood flow is generally overestimated. A modification of the thermal clearance method is described in this paper which enables the conduction component to be determined. Profiles of the tissue temperature are obtained in three mutually orthogonal directions about the point where thermal clearance is measured. The Laplacian of the temperature is evaluated from these profiles by the method of finite differences. The tissue thermal conductivity is estimated from literature values. The greatest source of error is the uncertainty in the location of the washout point in each catheter. Strict thermometry requirements must be adopted to reduce the localization error to +/- 0.25 cm. The thermometry catheters should be orthogonal to within +/- 10 degrees and all three catheters should be in contact at the washout point. The methodology was tested in a phantom, studied by use of a computer model, and implemented in the clinic. The experimental error in the conduction component is typically 50%. The resulting error in the blood flow depends on the relative rates of energy removal by blood flow and thermal conduction. When perfusion is the dominant mode of energy removal, the resulting uncertainty in the blood flow is typically in the range 20-30%.

The local tissue cooling coefficient: a unified approach to thermal washout and steady-state 'perfusion' calculations

Several investigators have attempted to utilize either transient 'thermal washout' and/or steady-state temperature data from standard hyperthermia thermometry to obtain information regarding both tumour and normal tissue perfusions, and transient data to measure SAR distributions. This paper reviews that literature, presents a unified theoretical basis for all of the temporal 'perfusion' approaches, and shows that both the steady-state and the 'washout' techniques actually measure the same quantity, which is related to, but not equal to, the tissue perfusion. A new nomenclature for the quantity measured by these techniques is proposed, the local tissue cooling coefficient, a name which avoids any use of the term 'perfusion', in order to avoid unwarranted inferences regarding this quantity. The requirements for relating this quantity to the true tissue perfusion are presented, and possible applications of the local tissue cooling coefficient are reviewed. Finally, the techniques used by various investigators for normalizing SAR data are summarized and discussed, and a standard approach suggested.

Spiral microstrip hyperthermia applicators: technical design and clinical performance

Spiral microstrip microwave (MW) antennas have been developed and adapted for use as clinical hyperthermia applicators. The design has been configured in a variety of forms including single fixed antenna applicators, multi-element arrays, and mechanically scanned single or paired antennas. The latter three configurations have been used to allow an expansion of the effective heating area. Specific absorption rate (SAR) distributions measured in phantom have been used to estimate the depth and volume of effective heating. The estimates are made using the bioheat equation assuming uniformly perfused tissue. In excess of 500 treatments of patients with advanced or recurrent localized superficial tumors have been performed using this applicator technology. Data from clinical treatments have been analyzed to quantify the heating performance and verify the suitability of these applicators for clinical use. Good microwave coupling efficiency together with the compact applicator size have proved to be valuable clinical assets.

Characterization of tumour temperature distributions in hyperthermia based on assumed mathematical forms

Assessing the efficacy of hyperthermia treatments involves three distinct problems: (1) adequately sampling the spatial temperature distribution in a region; (2) defining (a set of) 'descriptors', numerical values which could be used in comparing distinct treatments; (3) testing whether the predictions of prognosis are statistically significant. This paper addresses the first two problems. We use simple assumptions about the tumour geometry and heating pattern to obtain convenient mathematical representations of a temperature distribution, which are then used in defining scalar descriptors such as weighted average temperature TV, and the fraction of tumour volume heated above a given temperature VT/V. Two extreme cases are discussed. In the first, tumour geometry plays the dominant role, and in the second the specific absorption rate (SAR) distribution is assumed to have the greatest influence on the temperature distribution.

Heat loss and blood flow during hyperthermia in normal canine brain. I: Empirical study and analysis

The effects of blood flow and thermal conduction during microwave hyperthermia were investigated in normal canine brain. Heating was accomplished with an external microstrip spiral antenna and temperature measurements were made using a multichannel fluoroptic thermometry system. In order to determine cooling rates, temperature measurements made during cooling were fitted with a model consisting of a constant value and an exponential term. Data from experiments in both perfused and non-perfused brains could be fitted with this simple model. The resulting cooling rates indicated that heat loss by conduction is comparable to that by blood flow. In another series of experiments, temperature measurements were made during several 1 min cooling intervals in which the power was shut off intermittently during a 35 min heating episode. Results were consistent with a 2-3-fold increase in blood flow rate which occurred gradually throughout the course of heating. Parameters that affect the determination of cooling rates are discussed in terms of the bioheat transfer equation. These investigations demonstrate that a simple heat sink model provides a good representation of the cooling data for the thermal distributions obtained.