Biomedical Engineering Reference
In-Depth Information
lost function) devices as solutions to problems in
healthcare or the life sciences.
a wide range of phenomena that will occur in biosystems,
it is important to have a common language that can be
used to describe and model bioelectric, biomechanical,
and biochemical phenomena.
2.1a.2 Fundamental aspects
of BME
2.1a.3.1 A framework for problem
solving
Any BME device includes one or more measurement,
modeling, or manipulation tasks. By measurement, we
mean sensing properties of the physical, chemical, or bi-
ological system under consideration. Realizing that no
property can be measured exactly is an important concept
for biomedical engineers to understand. For this reason, an
appreciation of statistics is required and will be covered
later in this text. Principles ofmeasurement also include an
understanding of variability and sources of noise; sensing
instruments and accuracy; and resolution and re-
producibility as characterizations of measurements.
Manipulation in this engineering sense means inter-
acting with a system in some way. For the most part,
biomedical engineers will interact with the human body or
biological system by constructing diagnostic or therapeu-
tic interventions. The process of developing an inter-
vention system starts with requirements and constraints,
which, in turn, lead to specifications for the particular
intervention and then to design, fabrication, and testing.
The task of engineering modeling is the process by
which a biomedical engineer expresses the principles of
physics, chemistry, and biology in a mathematical state-
ment that describes the phenomena or system under
consideration. The mathematical model is a precise state-
ment of how the system interacts with its environment. A
model is a tool that allows the engineer to predict how the
system will react to changes in one of the system parameters.
Engineering models aremathematical statements using
one or more of four areas of continuous mathematics: al-
gebra, calculus, differential equations and statistics. Other
than the pure modeling tasks, any of the other significant
BME projects highlighted above use models, signal
processing, or control systems that are implemented in
a computer system. In these cases, one must describe the
behavior algebraically, as an integral, differential equation
or as an expression of the variability in analogs of these
continuous mathematical models; the challenge is to
preserve the accuracy and resolution to the highest degree
possible and perform stable computation. This is the
purpose of numerical methods.
The problem-solving framework is based on first identi-
fying the conservation law that governs the observed
behavior. There are four steps to developing the solution
to a problem in BME
1. Identify the system to be analyzed: A system is any
region in space or quantity of matter set aside for
analysis; it's the part of the universe in which we are
interested. The environment is everything not inside the
system. The system boundary is an infinitesimally thin
surface, real or imagined, that separates the system
from its environment. The boundary has no mass, and
merely serves as a delineator of the extent of the system.
2. Determine the extensive property to be accounted
for: An extensive property doesnothaveavalueat
a point, and its value depends on the extent or size of
the system; e.g., it is proportional to the mass of the
system. The amount of an extensive property for
a systemcan be determined by summing the amount of
extensive property for each subsystem that
comprises the system. The value of an extensive prop-
erty for a system only depends upon time. Some ex-
amples that we are familiar with are mass and volume.
There is scientific evidence that suggests that the
property can neither be created nor destroyed. An
extensive property that satisfies this requirement is
called a conserved property.
There are many experiments reported in the
literature that support the idea that charge, linear
momentum and angular momentum are conserved.
Mass and energy, on the other hand, are conserved
under some restrictions:
a. If moving, the speed of the system is
significantly less than the speed of light.
b. The time interval is long when compared to
the time intervals of quantum mechanics.
c. There are no nuclear reactions.
It is very unlikely that conditions a and b will be
violated by any biomedical system that will be
studied. In the time scale of biology, the shortest
event is on the order of 10 8 s which is still longer than
nuclear events. However, emission tomography and
nuclear medicine are systems where nuclear reactions
do occur and one will have to be careful about
assuming that mass or energy is conserved in these
systems.
2.1a.3 Constructing engineering
models
Practicing engineers are asked to solve problems based on
physical relationships between something of interest and
the surrounding environment. Because there is quite
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