Investigation of the Thermal and Injury Behavior During Microwave Thermal Therapy of Porcine Kidney

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.

Thermostability Studies of Desiccated Murine Spermatozoa Nuclear DNA to Predict the Beneficial Effect of Trehalose in Long Term Storage

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.

A Tiny Detector Made of a Self-Heated Thermistor for Determining Thermal Conductivity of Biomaterials in Temperature Range 233~313K

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.

Experimental and Numerical Studies of Heat Transfer in Human Uteri During Cryoablation

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.

Heat Shock Protein 70 Expression Kinetics

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.

Measurement of Cryoprotectant Concentration Using MRI in Pseudobiological Tissues and Determination of Diffusivity Based on Inverse Problem Analysis

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.

Heat-Induced Denaturation of Collagenous Tissue: A Comparison of Numerical Simulations With OCT and MRI Data

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.

Multi-Parameter Estimation in Hyperthermia Problem by Using an Optimal Choice of Descent Parameters in Iterative Methods

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.

Potential for Thermal Injury During Surgical Osteotomies

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.

Finite Element Analysis of Radio-Frequency Ablation in a Reconstructed Realistic Hepatic Geometry

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.