Signals and systems - Biomedical Engineering Desk Reference - page 63

observed at some distance (on a clear day), and relayed
onward if necessary. Information processing can also be
accomplished mechanically, as in the early numerical
processors constructed by Babbage. More recently,
mechanically based digital components have been
attempted using variations in fluid flow. Modern elec-
tronics provides numerous techniques for modifying
electrical signals at very high speeds. The body also uses
electrical energy to carry information when speed is
important. Since the body does not have many free
electrons, it relies on ions, notably Na þ ,K þ , and Cl ,as
the primary charge carriers. Outside the body, electri-
cally based signals are so useful that signals carried by
other energy forms are usually converted to electrical
energy when significant transmission or processing tasks
are required. The conversion of physiological energy to an
electric signal is an important step, often the first step, in
gathering information for clinical or research use. The
energy conversion task is done by a device termed
a transducer, specifically a biotransducer.
A transducer is a device that converts energy from one
form to another. By this definition, a light bulb or a motor
is a transducer. In signal processing applications, the
purpose of energy conversion is to transfer information,
not to transform energy as with a light bulb or a motor. In
physiological measurement systems, all transducers are
so-called input transducers: they convert nonelectrical
energy into an electronic signal. An exception to this is
the electrode, a transducer that converts electrical energy
from ionic to electronic form. Usually, the output of
a biotransducer is a voltage (or current) whose amplitude
is proportional to the measured energy. Figure 2.3-2
shows

intestine during surgical procedures. The mechanical
transducers used in the device are called strain gages and
they change their electrical resistance when stretched
even slightly.
The energy that is converted by the input transducer
may be generated by the physiological process itself, may
be energy that is indirectly related to the physiological
process, or may be energy produced by an external
source. In the latter case, the externally generated energy
interacts with, and is modified by, the physiological
process, and it is this alteration that produces the mea-
surement. For example, when externally produced x-rays
are transmitted through the body, they are absorbed
by the intervening tissue, and a measurement of this
absorption is used to construct an image. Most medical
imaging systems are based on this external energy

approach.
Images can also be constructed from energy sources
internal to the body as in the case of radioactive emis-
sions from radioisotopes injected into the body. These
techniques make use of the fact that selected, or tagged,
molecules will collect in specific tissue. The areas where
these radioisotopes collect can be mapped using a gamma
camera or, with certain short-lived isotopes, better lo-
calized using positron emission tomography (PET).
Many physiological processes produce energy that can
be detected directly. For example, cardiac internal
pressures are usually measured using a pressure trans-
ducer placed on the tip of a catheter introduced into the
appropriate chamber of the heart. The measurement of
electrical activity in the heart, muscles, or brain provides
other examples of the direct measurement of physio-
logical energy. For these measurements, the energy is
already electrical and only needs to be converted from
ionic to electronic current using an electrode. These
sources are usually given the term ExG, where the x
represents the physiological process that produces the
electrical energy: ECG, electrocardiogram; EEG, elec-
troencephalogram; EMG, electromyogram; EOG, elec-
trooculogram; ERG, electroretinogram; and EGG,
electrogastrogram. An exception to this terminology is
the galvanic skin response, GSR, the electrical activity
generated by the skin. Typical physiological measure-
ments that involve the conversion of other energy forms
to electrical energy are shown in Table 2.3-1 . Figure 2.3-3
shows the early ECG machine where the interface be-
tween the body and the electrical monitoring equipment
was buckets filled with saline ( Figure 2.3-3E ).
The biotransducer is often the most critical element in
the system because it constitutes the interface between
the subject or life process and the rest of the system. The
transducer establishes the risk, or invasiveness, of the
overall system. For example, an imaging system based on
differential absorption of x-rays, such as a CT (computed
tomography) scanner, is considered more invasive than

a device to measure the movements of the

Figure 2.3-2 A device used to measure small movements of the
intestine during surgery. These movements can be used to assess
the viability of a segment of intestine. The device consists of an
inflexible lower plate and a flexible upper plate. Movement of the
upper plate is detected by two strain gages placed on its upper
and lower surfaces. Strain gage transducers change their
resistance in response to small changes in length. Subsequent
electronics detect these resistance changes.