Biomedical Engineering Reference
In-Depth Information
•
Bioimpedance signals:
The electrical impedance of tissue contains important in-
formation concerning, for example, its makeup, blood volume, and endocrine ac-
tivity. These signals are normally obtained by injecting low-current (
20 mA)
sinusoidal electrical signals into the body at frequencies between 50 kHz and
1 MHz and monitoring the relationship between the current and the voltage.
•
Bioacoustic signals:
Many biological phenomena generate acoustic outputs, and
these are indicative of the function being performed. A good example of this is the
“lub-dub” sound produced by the pumping heart. Other sounds are generated by
blood flow, air flow, and the transit of solids, liquids, and gas through the digestive
system.
•
Biomagnetic signals:
All of the organs in which electrical activity occurs generate
magnetic fields as a result of this activity. These organs include the brain, heart,
and lungs as well as the skeletal muscles. Unfortunately, the amplitude of these
signals is very small, and they are difficult to measure.
•
Biomechanical signals:
These signals include motion and displacement as well
as pressure, tension, and flow within the organism. Measurement of these requires
the use the sensors and transducers discussed in Chapter 2. Unlike electrical and
magnetic signals, these signals generally do not propagate (with the exception of
pressure) and so are mostly measured at the source.
•
Biochemical signals:
These result from chemical activity within the organism
that can alter its chemical composition in both subtle and gross ways. Chemical
composition is measured using the sensors discussed in Chapter 2 and can include
gases such as CO
2
and O
2
as well as dissolved solids like glucose and various salts.
•
Bio-optical signals:
These are signals related to the optical reflectance or trans-
mission of the tissue. For example, blood oxygen levels may be determined from
the relationship between the reflectance of infrared (IR) and visible wavelengths.
Other information may be gathered using fluorescence characteristics based on
injected dye materials.
The signals can be classified with regard to their source or application or in terms
of the signal characteristics. Biological signals are classified as either continuous or dis-
crete. Continuous signals include temperature, pressure, and chemical concentration, and
discrete signals include electrical impulses generated by individual nerve cells.
These signals can be divided into broad classes depending on the rate and nature of
the variations (Carr, 1997).
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5.2.1 Bioelectric Signals
The idea that electricity is generated by the body was first introduced by Luigi Galvani
in 1786, but it was not until 1903 when William Einthoven was able to measure these
potentials using an improved string galvanometer, that their true potential as a diagnostic
tool became apparent. This potential was further enhanced by the invention of the vacuum
tube amplifier a few years later.
Bioelectric potentials are actually ionic voltages produced as a result of electrochem-
ical activity in some types of specialist cells. Conductive solutions consisting of dissolved
salts surround the cells in the body. The principal ions in solution are sodium (Na
+
), potas-
sium (K
+
), and chloride (Cl
−
). The semipermeable membranes of nerve and muscle cells
