Page 76 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

HEAT TRANSFER APPLICATIONS IN BIOLOGICAL SYSTEMS 53

phenomenon, it has been indicated that the abnormal cutaneous blood flow is related to the diminished
fibrinolytic activity and increased blood viscosity (Engelhart and Kristensen, 1986). LDF has also been
useful for assessing patients with fixed arterial obstructive disease of the lower extremity (Schabauer
and Rooke, 1994). Other cutaneous uses of LDF include postoperative monitoring of digit reattach-
ment, free tissue flap, and facial operations (Schabauer and Rooke, 1994). In the noncutaneous appli-
cation of LDF, it has been reported to measure the retinal blood flow in patients with diabetes mellitus.
LDF has also been used to monitor cerebral blood perfusion (Borgos, 1996). Recently, it was used to
evaluate the brain autoregulation in patients with head injury (Lam et al., 1997).

Temperature Pulse Decay Technique. As described in subsection “Temperature Pulse Decay
(TPD) technique,” local blood perfusion rate can be derived from the comparison between the theo-
retically predicted and experimentally measured temperature decay of a thermistor bead probe. The
details of the measurement mechanism have been described in that section. The temperature pulse
decay technique has been used to measure the in vivo blood perfusion rates of different physical or
physiological conditions in various tissues (Xu et al., 1991, 1998; Zhu et al., 2005). The advantages
of this technique are that it is fast and induces little trauma. Using the Pennes bioheat transfer equation,
the intrinsic thermal conductivity and blood perfusion rate can be simultaneously measured. In some
of the applications, a two-parameter least-square residual fit was first performed to obtain the intrinsic
thermal conductivity of the tissue. This calculated value of thermal conductivity was then used to
perform a one-parameter curve fit for the TPD measurements to obtain the local blood perfusion rate
at the probe location. The error of blood perfusion measurement using the TPD technique is mainly
inherited from the accuracy of the bioheat transfer equation. Theoretical study (Xu et al., 1993) has
shown that this measurement is affected by the presence of large blood vessels in the vicinity of the
thermistor bead probe. Further, poor curve fitting of the blood perfusion rate occurs if the steady state
of the tissue temperature is not established before the heating (Xu et al., 1998).

2.5 HYPERTHERMIA TREATMENT FOR CANCERS AND TUMORS

2.5.1 Introduction
Within the past two decades, there have been important advances in the use of hyperthermia in a
wide variety of therapeutic procedures, especially for cancer treatment. Hyperthermia is used either
as a singular therapy or as an adjuvant therapy with radiation and drugs in human malignancy. It has
fewer complications and is preferable to more costly and risky surgical treatment (Dewhirst et al.,
1997). The treatment objective of current therapy is to raise tumor temperature higher than 43°C for
periods of more than 30 to 60 minutes while keeping temperatures in the surrounding normal tissue
below 43°C. It has been suggested that such elevated temperatures may produce a heat-induced cyto-
toxic response and/or increase the cytotoxic effects of radiation and drugs. Both the direct cell-
killing effects of heat and the sensitization of other agents by heat are phenomena strongly dependent
on the achieved temperature rise and the heating duration.
One of the problems encountered by physicians is that current hyperthermia technology cannot
deliver adequate power to result in effective tumor heating of all sites. The necessity of developing
a reliable and accurate predictive ability for planning hyperthermia protocols is obvious. The treat-
ment planning typically requires the determination of the energy absorption distribution in the tumor
and normal tissue and the resulting temperature distributions. The heating patterns induced by vari-
ous hyperthermia apparatus have to be studied to focus the energy on a given region of the body and
provide a means for protecting the surrounding normal tissues. Over the past two decades, opti-
mization of the thermal dose is possible with known spatial and temporal temperature distribution
during the hyperthermia treatment. However, large spatial and temporal variations in temperature are
still observed because of the heterogeneity of tissue properties (both normal tissue and tumor), spa-
tial variations in specific absorption rates, and the variations and dynamics of blood flow (Overgaard,
1987). It has been suggested that blood flow in large, thermally unequilibrated vessels is the main
cause for temperature nonhomogeneity during hyperthermia treatment, since these large vessels can