Biomedical Engineering Reference
In-Depth Information
11.2.5 Bioacoustic Signals
Bioacoustic signals are a special subset of biomechanical signals that involve vibrations
(motion). Many biological events produce acoustic noise. For instance, the flow of blood
through the valves in the heart has a distinctive sound. Measurements of the bioacoustic
signal of a heart valve can be used to determine whether it is operating properly. The respi-
ratory system, joints, and muscles also generate bioacoustic signals that propagate through
the biological medium and can often be measured at the skin surface by using acoustic
transducers such as microphones and accelerometers.
11.2.6 Biooptical Signals
Biooptical signals are generated by the optical or light induced attributes of biological
systems. Biooptical signals can occur naturally, or in some cases, the signals may be intro-
duced to measure a biological parameter with an external light medium. For example,
information about the health of a fetus may be obtained by measuring the fluorescence
characteristics of the amniotic fluid. Estimates of cardiac output can be made by using
the dye dilution method that involves monitoring the concentration of a dye as it recircu-
lates through the bloodstream. Finally, red and infrared light are used in various applica-
tions, such as to obtain precise measurements of blood oxygen levels by measuring the
light absorption across the skin or a particular tissue.
EXAMPLE PROBLEM 11.1
What types of biosignals would the muscles in your lower legs produce if you were to sprint
across a paved street?
Solution
Motion of the muscles and the external forces imposed as your feet hit the pavement produce
biomechanical signals. Muscle stimulation by nerves and the contraction of muscle cells produce
bioelectric signals. Metabolic processes in the muscle tissue could be measured as biochemical
signals.
11.3 CHARACTERISTICS OF BIOSIGNALS
Biological signals can be classified according to various characteristics of the signal,
including the waveform shape, statistical structure, and temporal properties. Two broad
classes of signals that are commonly encountered include continuous and discrete signals.
Continuous
signals are defined over a continuum of time or space and are described by con-
tinuous variables. The notation
x
(
t
) is used to represent a signal,
x
, that varies as a function
of continuous time,
. Signals that are produced by biological phenomena are almost always
continuous signals. Some examples include voltage measurements from the heart (see
Figure 11.1), arterial blood pressure measurements (see Figure 11.2), and measurements
of electrical activity from the brain.
t
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