Biomedical Signals 1-3(such as bioe

Biomedical Signals 1-3
(such as bioelectric signals). Due to the lowlevel of the magnetic fields to be measured, biomagnetic signals
are usually of very low signal-to-noise ratio. Extreme caution must be taken in designing the acquisition
system of these signals.
5. Biomechanical signals. The term biomechanical signals includes all signals used in the biomedicine
fields that originate from some mechanical function of the biologic system. These signals include motion
and displacement signals, pressure and tension and flow signals, and others. The measurement of biomechanical
signals requires a variety of transducers, not always simple and inexpensive. The mechanical
phenomenon does not propagate, as do the electric, magnetic, and acoustic fields. The measurement
therefore usually has to be performed at the exact site. This very often complicates the measurement and
forces it to be an invasive one.
6. Biochemical signals. Biochemical signals are the result of chemical measurements from the living
tissue or from samples analyzed in the clinical laboratory. Measuring the concentration of various ions
inside and in the vicinity of a cell by means of specific ion electrodes is an example of such a signal.
Partial pressures of oxygen (pO2) and carbon dioxide (pCO2) in the blood or respiratory systemare other
examples. Biochemical signals are most often very low frequency signals. Most biochemical signals are
actually dc signals.
7. Biooptical signals. Biooptical signals are the result of optical functions of the biologic system, occurring
naturally or induced by the measurement. Blood oxygenation may be estimated by measuring the
transmitted and backscattered light from a tissue (in vivo and in vitro) in several wavelengths. Important
information about the fetus may be acquired by measuring fluorescence characteristics of the amniotic
fluid. Estimation of the heart output may be performed by the dye dilution method, which requires the
monitoring of the appearance of recirculated dye in the bloodstream. The development of fiberoptic
technology has opened vast applications of biooptical signals.
1.2 Classification of Biosignals
Biosignals may be classified in many ways. The following is a brief discussion of some of the most
important classifications.
1. Classification according to source. Biosignals may be classified according to their source or physical
nature. This classification was described in the preceding section. This classification may be used when
the basic physical characteristics of the underlying process is of interest, for example, when a model for
the signal is desired.
2. Classification according to biomedical application. The biomedical signal is acquired and processed
with some diagnostic, monitoring, or other goal in mind. Classification may be constructed according to
the field of application, for example, cardiology or neurology. Such classification may be of interest when
the goal is, for example, the study of physiologic systems.
3. Classification according to signal characteristics. From point of view of signal analysis, this is the most
relevant classification method. When the main goal is processing, it is not relevant what is the source of
the signal or to which biomedical system it belongs; what matters are the signal characteristics.
We recognize two broad classes of signals: continuous signals and discrete signals. Continuous signals
are described by a continuous function s(t ) which provides information about the signal at any given
time. Discrete signals are described by a sequence s(m) which provides information at a given discrete
point on the time axis. Most of the biomedical signals are continuous. Since current technology provides
powerful tools for discrete signal processing, we most often transform a continuous signal into a discrete
one by a process known as sampling. A given signal s(t ) is sampled into the sequence s(m) by
s(m) = s(t )|t=mTs m = . . . ,−1, 0, 1, . . . (1.1)
© 2006 by Taylor & Francis Group, LLC
Table 1.1 lists some of the more common biomedical signals with some of their characteristics.