Page 69 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

46 BIOMECHANICS OF THE HUMAN BODY

and/or taking off clothes to increase the overall heat transfer coefficient h. Therefore, one can use the
model to accurately delineate important clinical scenarios such as heat stroke, and predict body tem-
perature elevations during heavy exercise and/or heat exposures.

2.4 TEMPERATURE,THERMAL PROPERTY, AND BLOOD
FLOW MEASUREMENTS

2.4.1 Temperature

The control of human body temperature is a complex mechanism involving release of neurotrans-
mitters and hormones, redistributing blood flow to the skin, respiration, evaporation, and adjusting
metabolic rate. The control mechanism can be altered by certain pathologic (fever) and external
(hyperthermia treatment) events. Consequently, temperature is an important parameter in the diag-
nosis and treatment for many diseases. Elevated local tissue temperature can be an indication of
excess or abnormal metabolic rates. Inflammation is the body’s response to attacks and a mechanism
for removing foreign or diseased substances. Exercise also induces an increase in local temperature
of skeletal muscles and joints. Some diagnostic procedures involve the measurement of tempera-
tures. Thermal images of the breast surface have been used to detect the presence of malignant
tumors. Temperature measurement is also critical in many therapeutic procedures involved in either
hyperthermia or hypothermia.
Temperature-measuring devices can fall into two categories, invasive and noninvasive. Invasive
temperature sensors offer the advantages of small size, fast response time, extreme sensitivity to tem-
perature changes, and high stability. However, they have generally involved a limited number of
measurement locations, uncertainties about the anatomic placement of thermometry devices, inter-
action with the energy field applied, periodic rather than continuous temperature monitoring, and, in
some cases, surgical exposure of the target tissue for placement of the temperature probes.
Invasive temperature devices include thermocouples, thermistor beads, optical fiber sensors, etc.
A thermocouple consists of two pieces of dissimilar metal that form two junctions. In the wire, an
electric potential difference is formed if there exists a temperature difference between the two junc-
tions. This potential difference can be measured with a high resolution voltmeter and translated to
temperature with a fairly simple means of calibration. A thermocouple usually has a good long-term
stability, responds very quickly to changes in temperature due to its small thermal capacity, and can
be constructed in a manner that allows a good resolution. Another kind of invasive device, the ther-
mistor bead, is made by depositing a small quantity of semiconductor paste onto closely spaced
metal wires. The wire and beads are sintered at a high temperature when the material forms a tight
bond. The wires are then coated with glass or epoxy for protection and stabilization. The resistors
generally exhibit high thermal sensitivity. This characteristic sensitivity to temperature change can
result in a change of thermistor resistance of more than 50 Ω/°C. Unlike a thermocouple or a ther-
mistor bead, the fiber optic temperature probe does not interfere with an electromagnetic field. It has
been used to measure tissue temperature rise induced by microwave and/or radio frequency heating
(Zhu et al., 1996b, 1998). However, it is relatively big in size (~1.5 mm in diameter) and has a lower
temperature resolution (~0.2°C).
Noninvasive temperature-measuring techniques include MRI thermometry, infrared thermogra-
phy, etc. Because of the theoretical sensitivity of some of its parameters to temperature, MRI has
been considered to be a potential noninvasive method of mapping temperature changes during ther-
apies using various forms of hyperthermia. MRI imaging has the advantage of producing three-
dimensional anatomic images of any part of the body in any orientation. In clinical practice, MRI
characteristic parameters such as the molecular diffusion coefficient of water, the proton spin-lattice
(T ) relaxation time (Parker et al., 1982), and the temperature-dependent proton resonance fre-
1
quency (PRF) shift have been used to estimate the in vivo temperature distribution in tissues. MRI
provides good spatial localization and sufficient temperature sensitivity. At the present time, it also
appears to be the most promising modality to conduct basic assessments of heating systems and