Page 70 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

HEAT TRANSFER APPLICATIONS IN BIOLOGICAL SYSTEMS 47

techniques. The disadvantages of MRI thermometry include limited temporal resolution (i.e., quasi-
real time), high environmental sensitivity, high material expenditure, and high running costs.
Infrared thermography is based on Planck’s distribution law describing the relationship between
the emissive power and the temperature of a blackbody surface. The total radiative energy emitted
by an object can be found by integrating the Planck equation for all wavelengths. This integration
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gives the Stefan-Boltzmann law E(T) =εσT . The thermal spectrum as observed by an infrared-
sensitive detector can be formed primarily by the emitted light. Hence, the formed thermal image is
determined by the local surface temperature and the emissivity of the surface. If the emissivity of the
object is known, and no intervening attenuating medium exists, the surface temperature can be quan-
tified. Quantification of skin temperature is possible because the human skin is almost a perfect
blackbody (ε = 0.98) over the wavelengths of interest. A recent numerical simulation of the temper-
ature field of breast (Hu et al., 2004) suggests that image subtraction could be employed to improve
the thermal signature of the tumor on the skin surface. Drug-induced vascular constriction in the
breast can further enhance the ability of infrared thermography in detecting deep-seated tumor.
Qualitative thermography has been successfully used in a wide range of medical applications (Jones,
1998) including cardiovascular surgery (Fiorini et al., 1982), breast cancer diagnoses (Gautherie and
Gros, 1980; Lapayowker and Revesz, 1980), tumor hyperthermia (Cetas et al., 1980), laser angio-
plasty, and peripheral venous disease. Clinical studies on patients who had breast thermography
demonstrated that an abnormal thermography was associated with an increased risk of breast cancer
and a poorer prognosis for the breast cancer patients (Gautherie and Gros, 1980; Head et al., 1993).
Infrared tympanic thermometry has also been developed and widely used in clinical practice and
thermoregulatory research as a simple and rapid device to estimate the body core temperature
(Matsukawa et al., 1996; Shibasaki et al., 1998).

2.4.2 Thermal Property (Thermal Conductivity and Thermal Diffusivity)
Measurements
Knowledge of thermal properties of biological tissues is fundamental to understanding heat transfer
processes in the biological system. This knowledge has increased importance in view of the concerns
for radiological safety with microwave and ultrasound irradiation, and with the renewed interest in
local and regional hyperthermia as a cancer therapy. The availability of a technique capable of accu-
rately characterizing thermal properties of both diseased and normal tissue would greatly improve
the predictive ability of theoretical modeling and lead to better diagnostic and therapeutic tools.
The primary requirement in designing an apparatus to measure thermal conductivity and diffu-
sivity is that the total energy supplied should be used to establish the observed temperature distrib-
ution within the specimen. For accurate measurements, a number of structural and environmental
factors, such as undesired heat conduction to or from the specimen, convection currents caused by
temperature-induced density variations, and thermal radiation, must be minimized. Biomaterials pre-
sent additional difficulties. The existing literature on biological heat transfer bears convincing evi-
dence of the complexity of heat transfer processes in living tissue. For example, thermal properties
of living tissue differ from those of excised tissue. Clearly the presence of blood perfusion is a major
factor in this difference. Relatively large differences in thermal conductivity exist between similar
tissues and organs, and variations for the same organ, are frequently reported. Such variations sug-
gest the importance of both defining the measurement conditions and establishing a reliable mea-
surement technique.
The thermal property measurement techniques can be categorized as steady-state methods and
transient methods. They can also be categorized as invasive and noninvasive techniques. In general,
determining thermal properties of tissue is conducted by an inverse heat transfer analysis during
which either the temperatures or heat transfer rates are measured in a well-designed experimental
setup. The major challenge is to design the experiment so that a theoretical analysis of the tempera-
ture field of the experimental specimen can be as simple as possible to determine the thermal prop-
erty from the measured temperatures. It is usually preferred that an analytical solution of the
temperature field can be derived. In the following sections, those principles are illustrated by several
widely used techniques for measuring tissue thermal conductivity or diffusivity. Their advantages
and limitations will also be described.