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IN THIS VOLUME


Heat Transfer

2002;():1-5. doi:10.1115/IMECE2002-33657.

We developed a technology to measure the shear modulus of tissue-like materials using Magnetic Resonance (MR) imaging. In this technique, called MR elastography or MRE, the sample is vibrated at 100–300 Hz while MR images are made. The shear wave generated by the vibration will propagate into the material at a speed proportional to the square root of the shear modulus. We compared the shear modulus of gel samples measured in this way with the modulus derived from a static compression test and assuming the material is linearly elastic. The results show good agreement of the two methods provided the shear modulus of the material is below approximately 10 kPa. Differences in the two measurements are explainable by differences in the compression rate of the two techniques.

Commentary by Dr. Valentin Fuster
2002;():7-15. doi:10.1115/IMECE2002-33664.

Biological materials in both cryopreservation and cryosurgery are composed of various chemicals and experience a wide range of temperature change. Therefore, their thermal properties including specific heat, latent heat (including water/ice and eutectic phase change) and thermal conductivity are expected to change significantly during freezing/thawing. The effects of thermal properties on heat transfer in cryopreservation/cryosurgery were studied experimentally and numerically. Thermal properties of various biological aqueous solutions were measured over a wide temperature range (−150~30°C). To estimate the effect of thermal property changes on the heat transfer, numerical simulations of both cryopreservation (cooled from outside) and cryosurgery (cooled from inside) geometries were performed with constant and temperature-dependent properties. The results show that the constant-property case significantly under-predicts the heat transfer over the temperature-dependent-property case regardless of the geometry.

Commentary by Dr. Valentin Fuster
2002;():17-21. doi:10.1115/IMECE2002-33683.

In this paper, a new technique, using a tiny thermistor with 0.3~0.5mm in diameter to determine thermal conductivity of biomaterials in wide temperature range, has been developed. Based on steady spherical heat transfer in an infinite homogeneous medium, thermal conductivity of the measured medium can be determined by power applied and temperature rise of the thermistor. Compared with recommended values, maximum measurement errors of standard samples, aqueous glycol and CaCl2 solutions, water and ice, are 5.1% in temperature range 233~313K. The thermal conductivities of rabbit’s liver, kidney, heart and carotid artery in temperature range 233~293K are determined. Error caused by measurement parameters, effects of the finite scale of the measured medium and the decoupler between the thermistor and the medium are analyzed.

Commentary by Dr. Valentin Fuster
2002;():23-27. doi:10.1115/IMECE2002-33688.

Transient one-dimensional distribution of cryoprotectant concentration in pseudobiological tissues (agar) was measured noninvasively using magnetic resonance imaging (MRI). Cryoprotectants were dimethyl sulfoxide (DMSO) and glycerol, common cryoprotectants penetrating cells. Attenuation of MRI image intensity due to volumetric fraction of solution and relaxation times was also investigated. Apparent diffusivity of each cryoprotectant as a function of agar concentration was determined from the inverse problem analysis. The diffusivity decreased with an increase in agar concentration. This method was also applied to the liver tissues of chicken.

Commentary by Dr. Valentin Fuster
2002;():29-35. doi:10.1115/IMECE2002-33693.

This paper describes a numerical procedure conducted to estimate thermo-physical properties of the human tissue during hyperthermia treatment of a cancerous region. The estimation algorithm is based on the solution of an inverse heat conduction problem. The Gauss-Newton method is used to estimate simultaneously the volumetric heat capacity, the thermal conductivity, and the volumetric blood rate (blood perfusion) in the bio-heat transfer equation during a hyperthermia treatment cycle. The treatment quality of hyperthermia is analyzed by the computation of the thermal dose which is obtained from the resulting temperature field in the tissue. The importance of an accurate estimation of the thermo-physical properties of the tissue lies in that they are the most important factors for achieving a high precision heating cycle which results in an optimized treatment. The inverse analysis is based on the temperature measurements taken inside the cancerous tissue region during the transient heating process. An experimental optimization procedure is conducted to make the estimated parameters as accurate as possible. Several numerical tests were performed and show that the developed method provides an accurate estimation of thermo-physical properties in a very short practical time. As the blood perfusion is very sensitive to the temperature variation in the tissue, this estimation tool can be implemented during one cycle treatment which results in an on-line thermo-physical parameter correction while the treatment is performed.

Commentary by Dr. Valentin Fuster
2002;():37-38. doi:10.1115/IMECE2002-33647.

Approximately 100,000 burn patients require hospital admissions each year in the United States. About 90% of those patients survive to face the long term consequences of burn injury [1]. The primary cause of long term disability in burn survivors is hypertrophic scarring. These thick, deforming scars physically impair movement and cause major psychological morbidity. Hypertrophic scarring is particularly severe in young children [2].

Topics: Wounds
Commentary by Dr. Valentin Fuster
2002;():39-43. doi:10.1115/IMECE2002-33668.

In this study, the in situ protein denaturation in Dunning AT-1 rat prostate cells was studied using FTIR and DSC. The denaturation information from FTIR showed a shift from α helix (amide-I band at 1655cm−1 ) to β sheet (amide I band at 1620 cm−1 ). The relative beta sheet area change between 20 °C and 70 °C was used to dynamically scale the thermal denaturation process during heating at 2 °C/min. DSC scans (at scanning rates of 2°C/min and 5°C/min) of the heat flow due to protein denaturation were recorded from 20 °C to 70 °C. The range over which protein denaturation occurs (from 45 °C to 70 °C) was consistent in both studies. By calorimetric experiments with DSC, the enthalpy change during protein denaturation was found to be 27.3±4.0J/g J/g and 25.9±0.5J/g, respectively. In addition, the activation energy and frequency factor were measured by kinetic experiments with both DSC and FTIR. A first order irreversible Arrhenius model was fit to the kinetic data using a flexible tolerance method. The activation energy (E) and frequency factor (A) between 20–70 °C was found to be 107.8kJ/mole and 3.4·1014 l/s for the FTIR study and 120.9kJ/mole and 3.6·1016 l/s for DSC analysis at a scanning rate of 2°C/min, respectively. At a scanning rate of 5°C/min, the DSC results gave an activation energy of 144.5kJ/mole and a frequency factor of 4.1·1020 l/s.

Commentary by Dr. Valentin Fuster
2002;():45-46. doi:10.1115/IMECE2002-33678.

HSP70 is well known for its major role in cardiac ischemia protection. The purpose of this study was to determine the HSP70 expression kinetics for new protocol design in cardiac surgery, based on HSP70 protection function in clinical applications. Bovine aortic endothelial cells (BAEC) were used in experiments. Cells were heated at 42°C at different time intervals up to 5 hours and subsequently incubated at 37°C for up to 48 hours. Western blot and quantitative protein analysis were performed to measure HSP70 expression. The expression kinetics is a function of thermal stress time as well as poststress time. At least three stages were identified for the kinetics curve: increasing, maximum plateau and decreasing regions. The peak HSP70 concentration is 10 times the basal level for western blot analysis in BAECs. Two hours incubator heating followed by twelve hours post-heating falls in the plateau region. This research result provides information applicable to evaluation of energy sources and heating methods to induce optimal HSP70 expression in a target tissue.

Commentary by Dr. Valentin Fuster
2002;():47-49. doi:10.1115/IMECE2002-33684.

Long term preservation of mouse sperm in a desiccated state using sugars like trehalose may offer attractive economic benefits in the management of rapidly increasing transgenic mouse strains. The goal of the current study was to evaluate the protective effect of intracellular trehalose on sperm nucleus by predicting the long-term nuclear degradation kinetics of desiccated spermatozoa using an Arrhenius model whose parameters are obtained from high temperature-short time storage studies. B6D2F1 sperm isolated in an EGTA supplemented tris-HCl buffer (with or without 0.5M intracellular trehalose) were convectively dried with inert nitrogen gas in a controlled manner to moisture content >5%. The samples were then vacuum packed and stored at 22, 37, 45, 60 and 90°C for 1, 3 or 7 days. Following rehydration, the sperm sample was assayed for DNA damage using the sperm chromatin structure assay (SCSA). Results indicate significantly (p>0.05) lower DNA degradation for cells dried with intracellular trehalose at 45, 60 and 90°C for 1, 3 or 7 days compared to cells dried without trehalose. Based on a 10% increase in the index of injury, the calculated activation energy and frequency factors were 10.33 kcal/mole and 5.4×105 hr−1 respectively for cells dried in EGTA solution only. The corresponding numbers for cells dried in EGTA solution supplemented with 0.5M trehalose were 5.7 kcal/mole and 43.73 hr−1 . Based on these parameters the time required for 10% DNA degradation are 279 and 759 hours for samples desiccated in plain EGTA vs. trehalose supplemented EGTA. These results indicate the beneficial effect of intracellular trehalose for the long-term storage of desiccated sperm.

Topics: Storage , DNA
Commentary by Dr. Valentin Fuster
2002;():51-54. doi:10.1115/IMECE2002-33690.

Application of sub-ablative levels of heat to collagenous tissues has important therapeutic applications in medicine, such as tissue welding, thermokeratoplasty, skin resurfacing and treatment of joint instability. Sub-ablative heating produces collagen denaturation with desired tissue shrinkage yet detrimental comprises in mechanical properties of the tissue. In this paper, results of preliminary analyses with Optical Coherence Tomography (OCT) and Magnetic Resonance Imaging (MRI) are compared with a numerical model of tissue denaturation.

Commentary by Dr. Valentin Fuster
2002;():55-57. doi:10.1115/IMECE2002-33697.

Estimating effective thermal damage process coefficients for the first order kinetic model of damage processes is not difficult when the temperature is held constant for a substantial period. Laser coagulation experiments, however, are of short duration and, because of non uniform beam profiles, exhibit important heat transfer effects: the thermal histories are transient by nature and spatial temperature gradients are significant. An attempt is made to obtain useful estimates of activation energy, E, and collision frequency factor, A, directly from the transient history at the boundary of the zones of white coagulation and red hemorrhagic coagulation in liver in the rat, as identified by histologic studies. The Trial Region Method proves to be effective for transient data, in that a useful estimate of A and E can be obtained even from a single temperature curve. However, the results are very sensitive to small uncertainties in the actual value of Ω for the transient thermal histories to which the method is applied.

Commentary by Dr. Valentin Fuster
2002;():59-61. doi:10.1115/IMECE2002-33703.

The temperature dependence of the rate of denaturation of Type I collagen due to heating is described well by chemical kinetics via the Arrhenius equation or transition state theory (TST) [1, 2, 3], each of which requires two material parameters. Nevertheless, many have sought to find a single convenient metric, such as one with units of temperature, to describe thermal denaturation of collagen. Comparing the results of studies that measured denaturation and cell death for a variety of biological samples shows that the parameters for either the Arrhenius equation (i.e. activation energy Ea and the frequency factor A) appear correlated over the range of temperatures for which biological materials are tested [4]. It has also been suggested that denaturation is a first-order phase change (i.e., melting) and thus should be characterized by a melting or denaturation temperature Td [5].

Topics: Temperature , Heating
Commentary by Dr. Valentin Fuster
2002;():63-67. doi:10.1115/IMECE2002-32044.

Brain temperature control is important in clinical therapy, because moderate temperature reduction of brain temperature increases the survival rate after head trauma. A factor that affects the brain temperature distribution is the cerebral blood flow, which is controlled by autoregulatory mechanisms. To improve the existing thermal models of brain, we incorporate the effect of the temperature over the metabolic heat generation, and the regulatory processes that control the cerebral blood perfusion and depend on physiological parameters like, the mean arterial blood pressure, the partial pressure of oxygen, the partial pressure of carbon dioxide, and the cerebral metabolic rate of oxygen consumption. The introduction of these parameters in a thermal model gives information about how specific conditions, such as brain edema, hypoxia, hypercapnia, or hypotension, affect the temperature distribution within the brain. Existing biological thermal models of the human brain, assume constant blood perfusion, and neglect metabolic heat generation or consider it constant, which is a valid assumption for healthy tissue. But during sickness, trauma or under the effect of drugs like anesthetics, the metabolic activity and organ blood flow vary considerably, and such variations must be accounted for in order to achieve accurate thermal modeling. Our work, on a layered head model, shows that variations of the physiological parameters have profound effect on the temperature gradients within the head.

Commentary by Dr. Valentin Fuster
2002;():69-81. doi:10.1115/IMECE2002-32045.

Rapid cooling of the brain in the first minutes following the onset of cerebral ischemia is a potentially attractive preservation method. This computer modeling study was undertaken to examine brain-cooling profiles in response to various external cooling methods and protocols, in order to guide the development of clinical cooling devices. The criterion of successful cooling is the attainment of a 33.0°C average brain temperature within 30 minutes of treatment. Comparison of the finite element model results with a formal mathematical solution, give confidence that the simulation methods are sound. The cooling simulations considered to date all indicate that no one means of external cooling of the head or neck is sufficient to cool the brain in a reasonable period of time (30 minutes). Neither ice packs applied to head or neck, or cooling helmets can satisfy the 33.0°C target temperature specification. This central conclusion of insubstantial cooling is supported by the modest enhancements reported in experimental investigations of externally applied cooling. The key problem is overcoming the protective effect of warm blood perfusion, which reaches the brain via the uncooled carotid arterial supply and effectively blocks the external cooling wave from advancing to the core of the brain. This suggests that other cooling means should be explored requiring a realistic simulation of cooling of other pertinent parts of the human anatomy.

Commentary by Dr. Valentin Fuster
2002;():83-85. doi:10.1115/IMECE2002-32046.

Radio-frequency (RF) ablation is a minimally invasive procedure that has the potential for widespread use in hepatic cancer therapy. In the procedure, RF current is applied to the tissue, resulting in the conversion of electrical to heat energy and thus, a rise in temperature, with the goal of eventual tumor necrosis. Potential complications from the procedure include insufficient heating of large tumors, resulting in tumor recursion, as well as excessive thermal damage to healthy tissue. Mathematical models are valuable in predicting the temperature rise within the organ during RF ablation, thereby enhancing the success rate of the procedure. Eventually, models can be used to guide ablation procedures, by predicting the optimal set of operational parameters e.g., catheter probe geometry and placement, given patient-specific information. The present study focuses on the analysis of temperature rise within a reconstructed model of a realistic three-dimensional (3D) section of a porcine liver during RF ablation. This study calculates the effect of blood flow through arteries as well as perfusion through the liver on the time-dependent temperature distribution near the RF ablation probe (Figure 1). For a time duration of 30 min of an ablation procedure, a temperature of about 80°C could be achieved over a diameter of about 4 cm with the present RF probe. As an initial step, the present study includes isotropic hepatic tissue and blood properties.

Commentary by Dr. Valentin Fuster
2002;():87-88. doi:10.1115/IMECE2002-32047.

Medical literature indicates that permanent tissue damage can occur when exposed to temperature elevations of as little as 5°C for times as short as a few seconds. Clinically, it has also been observed that during surgical osteotomies (bone cutting procedures) significant amounts of heat are typically generated. We present experimental data that suggests critical conditions for thermal injury are regularly exceeded during simulated surgical procedures using cadeveric metatarsal bones.

Topics: Surgery , Wounds
Commentary by Dr. Valentin Fuster
2002;():89-98. doi:10.1115/IMECE2002-32048.

In this paper, we report on the characterization of microwave therapy of normal porcine kidneys both in vitro and in vivo. This technology is being developed for eventual use in the treatment of small renal cell carcinoma (RCC) by minimally invasive procedures. During experiments, microwave energy was applied through an interstitial microwave probe (Urologix, Plymouth, MN) to the kidney cortex with occasional involvement of the kidney medulla. The thermal histories at several locations were recorded. After treatment, the kidneys were bisected and small tissue slices were cut out at approximately the same depth as the thermal probes. The tissue slices were further processed for histological study. Both cellular injury and the area of microvascular stasis were quantitatively evaluated by histology. Absolute rate kinetic models of cellular injury and vascular stasis were developed and fit to this data. A 3-D finite element thermal model based on the Pennes Bioheat equation was developed and solved using a commercial software package (ANSYS, V5.7). The Specific Absorption Rate (SAR) of the microwave probe was measured experimentally in tissue equivalent gel-like solution. The thermal model was first validated by the measured in vitro thermal histories. It was then used to determine the blood perfusion term in vivo.

Commentary by Dr. Valentin Fuster
2002;():99-106. doi:10.1115/IMECE2002-32055.

This paper presents experimental and numerical studies of transient heat transfer inside the uterus during application of a PFC (perfluorochemical) fluid into the endometrium cavity in order to achieve cryoablation. The numerical prediction is based on a 1-D finite difference method of the bio-heat equation using the Crank Nicolson scheme. The numerical method is first validated by a 1-D physical model by measuring temperature history at several locations within a silicone rubber sheet. Good agreement, thus positive predictability, was obtained by comparing numerical predictions with the experimental data obtained from eight intact, hysterectomized uteri during cryoablation.

Commentary by Dr. Valentin Fuster

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