Biomedical Engineering Reference
In-Depth Information
radiation. Thus, light can be utilized to heat tissues for therapy or be measured from tissues
to determine temperature.
Temperature Monitoring
The measurement of temperature, in general, has been traditionally performed reliably
and fairly inexpensively using electrically based sensors such as thermistors or thermo-
couples. However, these sensors are not as useful when strong electromagnetic interference
is present, which is common in a hospital-based setting. Also, when attempting to monitor
temperature rise due to laser radiation, these sensors are inappropriate because they can
absorb the laser radiation and thus can act as heat sinks. Therefore, to measure temperature
in the body, several indirect optic and fiber optic approaches have been reported. For exam-
ple, liquid crystal optrodes show a dramatic change with color due to temperature differ-
ences, interferometric sensors change phase or fringes with a change in the path length
due to temperature variations, and the luminescence of many materials depends strongly
on temperature.
The direct type of temperature sensor initially described is known as a thermographic
or radiometer system. The primary application and engineering design for this type of
system has been done by the military for tasks such as detecting vehicles, personnel,
and even ships in total darkness. The formula for the total emission from a blackbody
at temperature
T
is
4
I
¼ es
T
ð
17
:
47
Þ
where
is the Stefan-Boltzmann constant. At room temperature,
objects emit mainly in the far infrared region of the wavelength spectrum, but as temper-
ature rises, the emission appears in the near infrared and finally in the visible spectrum.
Theemissivityisone(
e
is the emissivity and
s
e ¼
1) for biological tissue, and thus the total emission is dependent
only on the temperature
. For the military applications, a thermographic picture gener-
ated from the IR emission can be used to display the temperature, and using this same
technique, the surface temperature of the human body can be monitored for medical
applications. For example, a thermal camera may be used with a TV monitor to reveal
the temperature distribution on the human chest in an effort to reveal the pathologic con-
dition of inflammation and/or breast cancer, since the affected tissue will be slightly
warmer than the healthy tissue. It should be mentioned that although IR imaging is less
reliable than x-ray mammography for breast screening, it is totally passive and the
patients are not exposed to ionizing radiation. The thermographic systems used for this
IR emission measurement require a line of sight between the warm surface and the detector.
When no direct line of sight exists, an infrared-transmitting optical fiber can sometimes be
used to make the connection to the detector. This type of fiber optic radiometer has been
proposed for treatments using microwave heating and laser tissue heating treatments.
T
Light-Induced Heating
As mentioned previously, in addition to monitoring the light radiation from the body,
light itself can be used to heat tissues that absorb the light. Light-induced heating of tis-
sue can be used for a variety of applications, including biostimulation, sealing or welding
