Jonathan W. Valvano

Thermal conductivity and diffusivity of biomaterials measured with self-heated thermistors

This paper presents an experimental method to measure the thermal conductivity and thermal diffusivity of biomaterials. Self-heated thermistor probes, inserted into the tissue of interest, are used to deliver heat as well as to monitor the rate of heat removal. An empirical calibration procedure allows accurate thermal-property measurements over a wide range of tissue temperatures. Operation of the instrument in three media with known thermal properties shows the uncertainty of measurements to be about 2%. The reproducibility is 0.5% for the thermal-conductivity measurements and 2% for the thermal-diffusivity measurements. Thermal properties were measured in dog, pig, rabbit, and human tissues. The tissues included kidney, spleen, liver, brain, heart, lung, pancreas, colon cancer, and breast cancer. Thermal properties were measured for 65 separate tissue samples at 3, 10, 17, 23, 30, 37, and 45°C. The results show that the temperature coefficient of biomaterials approximates that of water.

A 14-b 100-MS/s Pipelined ADC With a Merged SHA and First MDAC

A low-power 14-b 100-MS/s analog-to-digital converter (ADC) is described. The prototype ADC achieves low-power consumption and small die area by sharing an opamp between two successive pipeline stages. Further reduction of power and area is achieved by completely merging the front-end sample-and-hold amplifier (SHA) into the first multiplying digital-to-analog converter (MDAC) using the proposed opamp and capacitor sharing technique. The ADC, implemented in a 0.18-mum dual-gate-oxide (DGO) CMOS technology, achieves 72.4-dB signal-to-noise and distortion ratio, 88.5-dB spurious free dynamic range, and 11.7 effective number of bits at full sampling rate with a 46-MHz input while consuming 230-mW from a 3-V supply.

A Self-Heated Thermistor Technique to Measure Effective Thermal Properties From the Tissue Surface

A microcomputer based instrument to measure effective thermal conductivity and diffusivity at the surface of a tissue has been developed. Self-heated spherical thermistors, partially embedded in an insulator, are used to simultaneously heat tissue and measure the resulting temperature rise. The temperature increase of the thermistor for a given applied power is a function of the combined thermal properties of the insulator, the thermistor, and the tissue. Once the probe is calibrated, the instrument accurately measures the thermal properties of tissue. Conductivity measurements are accurate to 2 percent and diffusivity measurements are accurate to 4 percent. A simplified bioheat equation is used which assumes the effective tissue thermal conductivity is a linear function of perfusion. Since tissue blood flow strongly affects heat transfer, the surface thermistor probe is quite sensitive to perfusion.

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.

Volume Catheter Parallel Conductance Varies Between End-Systole and End-Diastole

In order for the conductance catheter system to accurately measure instantaneous cardiac blood volume, it is necessary to determine and remove the contribution from parallel myocardial tissue. In previous studies, the myocardium has been treated as either purely resistive or purely capacitive when developing methods to estimate the myocardial contribution. We propose that both the capacitive and the resistive properties of the myocardium are substantial, and neither should be ignored. Hence, the measured result should be labeled admittance rather than conductance. We have measured the admittance (magnitude and phase angle) of the left ventricle in the mouse, and have shown that it is measurable and increases with frequency. Further, this more accurate technique suggests that the myocardial contribution to measured admittance varies between end-systole and end-diastole, contrary to previous literature. We have tested these hypotheses both with numerical finite-element models for a mouse left ventricle constructed from magnetic resonance imaging images, and with in vivo admittance measurements in the murine left ventricle. Finally, we propose a new method to determine the instantaneous myocardial contribution to the measured left ventricular admittance that does not require saline injection or other intervention to calibrate.

Optical and thermal properties of nasal septal cartilage

The aim of the study was to measure the spectral dependence of optical absorption and reduced scattering coefficients and thermal conductivity and diffusivity of porcine nasal septal cartilage. Values of optical and thermal properties determined in this study may aid in determining laser dosimetry and allow selection of an optical source wavelength for noninvasive diagnostics for laser‐assisted reshaping of cartilage.

Self-heated thermistor measurements of perfusion

A microcomputer-based control system applies a combination of steady-state and sinusoidal power to a thermistor probe which is inserted into the tissue of interest. The steady-state temperature response is an indication of the effective thermal conductivity (k/sub eff/), which includes a component due to intrinsic conduction plus a convective component due to the tissue blood flow near the probe. By careful choice of the excitation frequency, the sinusoidal temperature response can be used to measure intrinsic thermal conductivity (k/sub m/) in the presence of blood flow. Optimal sinusoidal heating frequency depends on the thermistor size. Experimental results show that perfusion is linearly related to the difference k/sub eff/ minus k/sub m/. The instrument can measure tissue thermal conductivity with an accuracy of 2% and resolve changes in perfusion of 10 mL/100g-min. The maximum error in measured perfusion is about 30%. When tissue trauma due to probe insertion is minimized, the self-heated thermistor method gives a reliable indication of local tissue blood flow.<<ETX>>

A 14b 100MS/s Pipelined ADC with a Merged Active S/H and First MDAC

The prototype ADC is implemented in 0.18mum dual gate-oxide (DGO) CMOS technology and achieves 72.4dB SNR and 88.5dB SFDR at 100MS/s with a 46MHz input while consuming 230mW from a 3V supply. Recently, power saving has been achieved by removing the explicit active S/H. Instead of removing the S/H, this work solves these drawbacks by merging the active S/H amplifier with the first MDAC (SMDAC). Thus, the ADC achieves low-power operation without sacrificing speed or accuracy.

Electrical Conductivity and Permittivity of Murine Myocardium

A classic problem in traditional conductance measurement of left ventricular (LV) volume is the separation of the contributions of myocardium from blood. Measurement of both the magnitude and the phase of admittance allow estimation of the time-varying myocardial contribution, which provides a substantial improvement by eliminating the need for hypertonic saline injection. We present in vivo epicardial surface probe measurements of electrical properties in murine myocardium using two different techniques (a digital and an analog approach). These methods exploit the capacitive properties of the myocardium, and both methods yield similar results. The relative permittivity varies from approximately 100 000 at 2 kHz to approximately 5000 at 50 kHz. The electrical conductivity is approximately constant at 0.16 S/m over the same frequency range. These values can be used to estimate and eliminate the time-varying myocardial contribution from the combined signal obtained in LV conductance catheter measurements, thus yielding the blood contribution alone. To study the effects of albumin on the blood conductivity, we also present electrical conductivity estimates of murine blood with and without typical administrations of albumin during the experiment. The blood conductivity is significantly altered (p < 0.0001) by administering albumin (0.941 S/m with albumin, 0.478 S/m without albumin).

An Isolated Rat Liver Model for the Evaluation of Thermal Techniques to Quantify Perfusion

An isolated, thermally regulated, perfused rat liver model system is presented. The model was developed to evaluate thermal methods to quantify perfusion in small volumes of tissue. The surgically isolated rat liver is perfused with an isothermal oxygenated Krebs-Ringer bicarbonate buffer solution via the cannulated portal vein. A constant-pressure head variable-resistance scheme is utilized to control the total flow to the liver. Total flow is quantified by hepatic vein collection. The spatial distribution of perfusion within the liver is determined using two independent methods. In the first method, radio-labelled microspheres are injected into the portal vein, and the regional flow distribution is determined from the relative radioactivity of each section of tissue. In the second method, the tissue is thermally perturbed, and the time constant of the tissue temperature recovery is measured. The regional distribution is determined from the relative time constants of each section of tissue. Both methods require the measurement of total liver flow to determine the absolute perfusion at each point. Results obtained by the two methods were well correlated (0.973). The rat liver system offers a stable, controllable, and measurable perfusion model for the evaluation of new perfusion measurement techniques.