Biomedical Engineering Reference
In-Depth Information
11.1 INTRODUCTION
Biological signals, or biosignals, are space, time, or space-time records of a biological
event such as a beating heart or a contracting muscle. The electrical, chemical, and mechan-
ical activity that occurs during this biological event often produces signals that can be
measured and analyzed. Biosignals, therefore, contain useful information that can be used
to understand the underlying physiological mechanisms of a specific biological event or
system and that may be useful for medical diagnosis.
Biological signals can be acquired in a variety of ways—for example, by a physician who
uses a stethoscope to listen to a patient's heart sounds or with the aid of technologically
advanced biomedical instruments. Following data acquisition, biological signals are ana-
lyzed in order to retrieve useful information. Basic methods of signal analysis, such as
amplification, filtering, digitization, processing, and storage, can be applied to many
biological signals. These techniques are generally accomplished with simple electronic cir-
cuits or with digital computers. In addition to these common procedures, sophisticated dig-
ital processing methods are quite common and can significantly improve the quality of the
retrieved data. These include signal averaging, wavelet analysis, and artificial intelligence
techniques.
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1.2 PHYSIOLOGICAL ORIGINS OF BIOSIGNAL
S
11.2.1 Bioelectric Signals
Nerve and muscle cells generate bioelectric signals that are the result of electrochemical
changes within and between cells (see Chapter 5). If a nerve or muscle cell is stimulated by
a stimulus that is strong enough to reach a necessary threshold, the cell will generate an
action potential. The action potential, which represents a brief flow of ions across the cell
membrane, can be measured with intracellular or extracellular electrodes. Action potentials
generated by an excited cell can be transmitted from one cell to adjacent cells via its axon.
When many cells become activated, an electric field is generated that propagates through
the biological tissue. These changes in extracellular potential can be measured on the sur-
face of the tissue or organism by using surface electrodes. The electrocardiogram (ECG),
electrogastrogram (EGG), electroencephalogram (EEG), and electromyogram (EMG) are
all examples of this phenomenon (Figure 11.1).
11.2.2 Biomagnetic Signals
Different organs, including the heart, brain, and lungs, also generate weak magnetic
fields that can be measured with magnetic sensors. Typically, the strength of the mag-
netic field is much weaker than the corresponding physiological bioelectric signals. Bio-
magnetism is the measurement of the magnetic signals that are associated with specific
physiological activity and that are typically linked to an accompanying electric field from
a specific tissue or organ. With the aid of very precise magnetic sensors or SQUID (super-
conducting quantum interference device) magnetometers, it is possible to directly monitor
