Selected Research Papers of Jonathan Valvano
 
 Nonlinear Conductance-Volume Relationship for Murine Conductance Catheter Measurement System

The conductance catheter system is a tool to determine instantaneous left ventricular volume in vivo by converting measured conductance to volume. The currently adopted conductance-to-volume conversion equation was proposed by Baan, and the accuracy of this equation is limited by the assumption of a linear conductance-volume relationship. The electric field generated by a conductance catheter is nonuniform, which results in a nonlinear relationship between conductance and volume. This paper investigates this nonlinear relationship and proposes a new nonlinear conductance-to-volume conversion equation. The proposed nonlinear equation uses a single empirically determined calibration coefficient, derived from independently measured stroke volume. In vitro experiments and numerical model simulations were performed to verify and validate the proposed equation.

 Evidence of Time-Varying Myocardial Contribution by In Vivo Magnitude
and Phase Measurement in Mice

Cardiac volume can be estimated by a conductance catheter system. Both blood and myocardium are conductive, but only the blood conductance is desired. Therefore, the parallel myocardium contribution should be removed from the total measured conductance. Several methods have been developed to estimate the contribution from myocardium, and they only determine a single steady state value for the parallel contribution. Besides, myocardium was treated as purely resistive or mainly capacitive when estimating the myocardial contribution. We question these assumptions and propose that the myocardium is both resistive and capacitive, and its contribution changes during a single cardiac cycle. In vivo magnitude and phase experiments were performed in mice to confirm this hypothesis.

 Thermal Conductivity and Diffusivity of Biomaterials Measured with Self-Heated Thermistors

Thermal Properties measured with self-heat thermistors, and includes theory, instrumentation, calibration, and results measured from 3 to 45 C. International Journal of Thermophysics, 6 (3), 301-311, 1985.

A Small Artery Heat Transfer Model for Self-Heated Thermistor Measurements of Perfusion in the Kidney Cortex

A small artery model (SAM) for self-heated thermistor measurements of perfusion in the canine kidney is developed based on the anatomy of the cortex vasculature. In this model interlobular arteries and veins play a dominant role in the heat transfer due to blood flow. Effective thermal conductivity, kss , is calculated from steady state thermistor measurements of heat transfer in the kidney cortex. This small artery and vein model of perfusion correctly indicates the shape of the measured kss versus perfusion curve. It also correctly predicts that the sinusoidal response of the thermistor can be used to measure intrinsic tissue conductivity, km , in perfused tissue. Although this model is specific for the canine kidney cortex, the modeling approach is applicable for a wide variety of biologic tissues.  Journal of Biomechanical Engineering. 116, 71-78, Feb. 1994.

Bioheat Properties of Biomaterials

The transport of thermal energy in living tissue is a complex process involving multiple phenomenological mechanisms including conduction, convection, radiation, metabolism, evaporation, and phase change. The equilibrium thermal properties presented in this chapter were measured after temperature stability had been achieved.

2-D Finite Difference Modeling of Microwave Heating in the Prostate

Accurate prediction of temperatures in the prostate undergoing thermally-based treatments is crucial to assessing efficacy and safety. A two-dimensional transient finite difference model for predicting temperatures in prostate undergoing microwave heating via a transurethral fluid-cooled catheter is presented. Unconditional stability and good accuracy are achieved by using the alternating direction implicit method. A transverse section of the prostate centered at the urethra is modeled in cylindrical coordinates. The model geometry consists of a hollow silicone cylinder, representing the catheter, surrounded by multiple regions of tissue. Cold fluid flowing through the catheter minimizes the temperature in the periurethral tissue. This flow is modeled as a convective boundary condition at the surface between the catheter lumen and wall. The outer surface of the tissue is assumed to remain at baseline temperature. Microwave heating has both a radial and angular dependence. In order to maximize the heat to the target tissue, the microwave field emitted from the transurethral catheter focuses heat away from the rectum. Different perfusion situations within the prostate are simulated. Pennes' perfusion term is assumed to model the effect of perfusion on heat transfer. Results of the numerical model are compared to phantom experiment results. The model parameters which provided the best fit for the phantom was extended to model canine prostate.

Treatment of Benign Prostatic Hyperplasia

The treatment of benign prostatic hyperplasia (BPH) has implications which affect the majority of the adult male population. Although benign compared to prostate cancer, clinical symptoms can dramatically alter the quality of life. The hyperplastic tissue can cause constriction of the urethra and thus affect voiding of urine. Factors to consider for thermally-based treatments of the prostate include minimization of thermal injury to the urethra and rectum, and maximal delivery of thermal energy to target tissue. Minimizing temperature rise in the urethra allows for minimal or no anesthesia, and has been shown to reduce post-operative complications. Protection of the rectal wall is imperative since injury can lead to clinical complications as severe as a rectal fistula. Due to its location immediately dorsal to the prostate, the ventral aspect of the rectal wall is susceptible to overheating when a uniform radiating microwave heat source is applied transurethrally to treat the prostate.

Interactive 6811 Simulator for Microcontroller Software Interfacing

This paper presents a microcontroller hardware/software simulator which is used in a laboratory setting to educate undergraduate electrical engineering students. The specific objectives of the course include microcomputer architecture, assembly language programming, data structures, modular programming techniques, debugging strategies, hardware/software interfaces and embedded microcontroller applications. In this paper, I present both basic concepts and specific implementations which create an effective learning environment for my students. In particular, I wrote a DOS-based interactive simulator for the Motorola 6811. The application runs on a standard IBM-PC compatible with minimal requirements: Intel 386DX, 640K RAM, VGA color monitor, and 2 Megabytes of hard drive space. The student develops Motorola 6811 software which is cross-assembled and simulated. The major features of this interactive programming environment include user-configurable interactive external I/O devices, multiple display windows, extensive information available describing the activity both inside and outside the processor, elaborate protection against and explanation of programming errors, effective mechanisms for setting breakpoints, and user-defined “scan points” which allow the user program to interact with the graphics display.

Analysis of the Weinbaum-Jiji Model of Blood Flow in the Canine Kidney Cortex for Self-Heated Thermistors

The Weinbaum-Jiji equation can be applied to situations where: 1) the vascular anatomy is known; 2) the blood velocities are known; 3) the effective modeling volume includes many vessels; and 4) the vessel equilibration length is small compared to the actual length of the vessel. These criteria are satisfied in the situation where steady-state heated thermistors are placed in the kidney cortex. In this paper, the Weinbaum-Jiji bioheat equation is used to analyze the steady state response of four different sized self-heated thermistors in the canine kidney. This heat transfer model is developed based on actual physical measurements of the vasculature of the canine kidney cortex. In this model, parallel-structured interlobular arterioles and venules with a 60 µm diameter play the dominant role in the heat transfer due to blood flow. Continuous power is applied to the thermistor, and the instrument measures the resulting steady state temperature rise. If an accurate thermal model is available, perfusion can be calculated from these steady-state measurements. The finite element simulations correlate well in shape and amplitude with experimental results in the canine kidney. In addition, this paper shows that the Weinbaum-Jiji equation can not be used to model the transient response of the thermistor because the modeling volume does not include enough vessels and the vessel equilibration length is not small compared to the actual length of the vessel. Journal of Biomechanical Engineering, 116, 201-207, May 1994.

Modeling of Temperature Probes in Convective Media

This paper discusses the dynamic behavior of probes embedded in convective media during temperature measurements. In certain conditions the temperature measured by a probe can be written as the convolution of the true temperature with the impulse response of the probe. We present a general method to find the natural response of any kind of probe, and then we present results for a more realistic 1-D model for the thermistor probe in a thermodilution catheter. The results of these analyzes can be applied to enhance the dynamic response of temperature measurements made by probes in convective media. 17th Southern Biomedical Engineering Conference, Feb. 7, 1998.

Measurement of the Dynamic Response of a Contact Probe Thermosensor in Conductive Media

This paper describes a method for characterizing the step response of a thermistor probe embedded in a low-conductivity solid. We define the “step response” as the dynamic response of a finite-size thermosensor instantaneously plunged into an infinite homogeneous conductive solid. The final goal of this research is to evaluate and enhance the time-dependent response of contact-type thermosensors. We will use the step response as the parameter for optimizing the probe time-dependent behavior. Although our research focuses on thermistors, the results could be applied to other contact-type sensors like thermocouples and RTD’s.

Methodology for Modeling the Response of Temperature Probes in Convective Media

This paper discusses the dynamic behavior of probes embedded in convective media during temperature measurements. It will be shown that in certain conditions the temperature measured by a probe can be written as the convolution of the true temperature with the impulse response of the probe. We present a general method to find the natural response of any kind of probe, and then we present results for a more realistic 1-D model for the thermistor probe in a thermodilution catheter. The results of these analyzes can be applied to enhance the dynamic response of temperature measurements made by probes in convective media.

Thermal Properties by Kenneth Holmes

The following physiological properties were compiled by Professor Kenneth R. Holmes <krholmes@ux1.cso.uiuc.edu> and were published in part previously. The tabulation includes values for both the native thermal conductivity of biomaterials (Appendix A) and the blood perfusion rates for specific tissues and organs (Appendix B). Original sources are documented in the dedicated list of references at the end of each appendix. Knowledge of the perfusion behavior of tissues is important in that the flow of blood can have a direct quantitative effect on the temperature distribution within living tissue.

Real Time Data Acquisition and Control

This paper presents a laboratory environment for the development of real time data acquisition and control on the IBM-PC platform. The laboratory station involves the integration of low-cost computer technology with powerful software components which empower the student to efficiently and effectively construct real time systems. The software base integrates an editor, a spreadsheet, and a real time programming environment built around Druma FORTH. We have written multiple FORTH libraries to assist the student in the translation of engineering concept into creation. Real time events are managed using a rich set of FORTH software routines which guarantee that time-critical software is executed on schedule. The real time color-VGA graphic library includes many types of windows. We have developed an extendible debugging tool called PROSYM (PROfiler and SYMbolic debugger.) PROSYM provides a simple set of primitives with a high expressive power that may be used singly or may be combined to construct customized debugging tools. In addition to providing basic debugging functions, PROSYM supports an event-action model of debugging. We have evaluated this development system on the full range of PC platforms from the original PC-XT to the newest 486 systems. The environment has been used for two years by Biomedical and Electrical Engineering graduate students performing both teaching and research projects. Gulf-Southwest Section of the American Society of Engineering Education, Austin, pp. 597-604, 1993.