In this paper, we present the design, fabrication and calibration of new micro machined electrical probe and
experimental studies on liver tissues using this probe. The probe was fabricated by photolithography and mounted in
a catheter with 1.5mm in diameter, which can be used to measure local impedance of the biological tissues. After the
calibration of the impedance at 500k Hz against different concentrations of saline water, the electrical conductivity
can be obtained from the measured impedance value. The micro electrical probe was first used to investigate the
effect of temperature elevation on the electrical conductivity liver tissues by different heating methods. Also, the
electrical conductivity change caused by directional placement and perfusion rate was investigated on a perfused pig
liver model. The experimental results show that the local electrical conductivity varies location to location and that
the electrical conductivity has a strong directional dependence. Also by varying the perfusion rate, the probe shows
that the local electrical conductivity varies linearly with the square root of perfusion rate. These results may be of
great value to many biomedical applications.
Using micro-fabrication techniques a micro thermal probe has been developed in our laboratory to measure the thermal
conductivity of biological tissues. This paper presents our latest experimental results which demonstrate the usefulness
of the micro thermal probe in mapping the complicated perfusion field inside biological tissues. A perfused pig liver
model has been constructed to simulate in vivo conditions. The portal vein and hepatic artery of a porcine liver were
intubated and connected to a perfusion circuit. Saline water was perfused through the liver driven by a peristaltic pump.
By varying the pumping rate of the perfused model, we measured the effective thermal conductivity at different
perfusion rates in different locations. The results show that the effective thermal conductivity varies with the square root
of the perfusion rate. Also, by rotating the micro probes, we observed a strong directional dependence of the effective
thermal conductivity, revealing that perfusion is not a scalar but a vector field.
Investigators reporting RF ablation (RFA) studies often use different initial and dynamic conditions, often in
porcine or bovine liver models. This study examines the effects of initial temperature, prior freezing, and perfusion in
these models. Understanding how these variables affect RFA size provides some basis for comparing data from different
studies. We obtained porcine and bovine livers from a slaughterhouse and divided them into experimental groups each
with discrete initial temperatures set in the range of 12 to 37°C. The livers were used either the day of harvest or frozen
within 1-3 days prior to RFA treatment. A perfused liver model was developed to simulate human blood flow rates and
allowed accurate control of the temperature and flow rate. Saline (0.9%) was substituted for blood. The non-perfused
liver model group included bovine and porcine tissue; whereas the perfused liver model group included only porcine
tissue. One experiment included porcine livers that were perfused at different flow rates and with different saline
concentrations. Harvested tissue from this group was examined under a light microscope and the level of edema was
assessed using image processing software. The results demonstrate no significant difference in RF lesion sizes between
porcine and bovine livers. Freezing the tissue prior to treatment has no significant effect but the initial temperature does
significantly affect the size of ablation. The ablation size in perfused liver is similar to in vivo results (earlier study) but is
significantly smaller then non-perfused liver. Morphological analysis indicates that perfusion, freezing, and saline
concentration cause significant tissue edema.
Radiofrequency ablation (RFA) has been used for a variety of clinical treatments including treatment of non-resectable
liver tumors with good clinical success. Liver pretreatment with injected saline increases the volume of the RFA
treatment and is a potential tool for strategically treating larger tumors. Understanding the electrical conductivity of the
affected tissue is required to improve the applicator performance and to accurately control the ablation area. We have
developed a micro two-electrode probe capable of measuring the local electrical conductivity of tissues at different
temperature levels and recording the transient change of electrical conductivity with saline pretreatment. An optical
temperature sensor was attached on the probe tip for real-time temperature monitoring to capture the dynamic effects of
temperature changes. Three methods which were implemented by water bath and a commercial RF ablation applicator
(Cool-tip RF ablation system) were used to heat the hepatic tissues. The results show that at elevated temperatures the
electrical conductivity increases by a factor of two compared to the values at the body temperature and different heating
methods cause different levels of electrical conductivity change. The preliminary measurements of the local electrical
conductivity after the saline injection indicate a dynamic pattern in electrical conductivity. The results serve to provide
guidance for accurate prediction of RFA area when using saline injection pretreatment.
A micro thin-film thermal conductivity probe is developed to measure thermal conductivity of biological tissues based
on the principle of traditional hot-wire method. The design of this new micro probe consists of a resistive line heating
element on a substrate and a RTD based temperature sensor. The transient time response of the heating element depends
on the thermal conductivity of the surrounding medium and the substrate. A theoretical analysis of the transient
conduction for this configuration where the heater source is sandwiched between two materials (the substrate and the
surrounding medium) shows that the composite thermal conductivity calculated from the temperature versus time
response is simply the average of the thermal conductivity of the two materials. The experiments conducted to measure
thermal conductivity of Crisco and agar gel show a good match with the theoretical and numerical analyses. The
technique demonstrates the potential of the microprobe for in vivo measurements of thermal conductivity of biological
tissues.
Knowledge of heat transfer in biological bodies has many therapeutic applications involving either increasing or
lowering tissue temperature. Radio-Frequency (RF) energy deposition is a method for increasing the temperature of
diseased tissue above 55°C to thermally ablate it. The resulting elevated tissue temperature is due to RF energy
deposition as well as tissue thermodynamics. However, it is difficult to separate these two processes on any lab bench or
in vivo model, hence computer simulation is a valuable tool for the separation and examination of these two phenomena.
Classically, the Pennes' bio-heat equation coupled with electrical field equations in a finite element analysis (FEA)
environment provides the governing structure for computer simulations that model energy deposition in biological
tissues. In the present work we have modified the computer simulation to allow an artificial partitioning of RF energy
deposition and tissue thermal diffusion. An internal cooled RF electrode (CoolTipTM) is analyzed using this partitioning
method. This method provides useful knowledge for optimizing the control of RF energy delivery to target tissue.
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