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ELE451 Chapter 1

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ELE-451: INTRODUCTION TO
BIOMEDICAL ENGINEERING
Dr. Atila YILMAZ
Text Book: Webster J.G.
Medical Instrumentation, Application and Design
Contents:
1.
Introduction
2.
Bioelectricity
3.
Medical Sensors and Their Principles
4.
Biopotential Electrodes and Amplifiers
5.
Electric Activities: ECG, EEG, EMG
6.
Other Applications
Currently recognized interdisciplinary fields that associate physics
and engineering with medicine and biology:
BEN = bioengineering, MPH = medical physics,
MEN = medical engineering, BPH = biophysics,
MEL = medical electronics, BEM = bioelectromagnetism,
The coordinate origin represents the more
theoretical sciences, such as biology and
physics. As one moves away from the origin,
the sciences become increasingly applied.
Combining a pair of sciences from medical and
technical fields yields interdisciplinary sciences
such as medical engineering. It must be
understood that the disciplines are actually
multidimensional, and thus their twodimensional description is only suggestive.
we have a special interest in those disciplines that combine
engineering and physics with biology and medicine. These
disciplines are briefly defined as follows
I
I
I
Biophysics and Biochemistry: The science that is
concerned with the solution of biological problems in
terms of the concepts of physics and chemistry.
Bioengineering (Biomechanics and Bioelectronics): The
application of engineering to the development of health
care devices, analysis of biological systems, and
manufacturing of products based on advances in this
technology. This term is also frequently used to
encompass both biomedical engineering and biochemical
engineering (biotechnology).
Biotechnology: The study of microbiological process
technology. The main fields of application of biotechnology
are agriculture, and food and drug production.
Biomedical Engineers apply electrical, mechanical, chemical, optical
and other engineering principles to understand, modify, or control
biological systems, as well as design and manifucture products that can
monitor physiologic functions and assist in the diognosis, and treatment
of patients.
I
I
I
Medical electronics: A division of biomedical engineering
concerned with electronic devices and methods in
medicine.
Medical physics: A science based upon physical
problems in clinical medicine.
Biomedical engineering: An engineering discipline
concerned with the application of science and technology
(devices and methods) to biology and medicine.
I ) Measurements of fields
(A) Bioelectricity
(B) Bioelectromagnetism (C) Biomagnetism
(Biomagnetism)
Neural cells
electroencephalography (EEG)
electroneurography (ENG)
electroretinography (ERG)
magnetoencephalography (MEG)
magnetoneurography (MNG)
magnetoretinography (MRG)
Muscle cells
electrocardiography (ECG)
electromyography (EMG)
magnetocardiography (MCG)
magnetomyography (MMG)
Other tissue
electro-oculography (EOG)
magneto-oculography (MOG)
electronystagmography (ENG) magnetonystagmography (MNG)
magnetopneumogram
magnetohepatogram
Some Events in Biomedical Engineering
I
I
I
I
I
In 1903, Willem Einthoven devised the first electrocardiography to
measure the electric activityof the heart
The most significant innovation for clinical medicine was the development
of X-rays in 1895, Roentgen,W.K)
The English school of neurophysiology began when Richard Caton (British;
1842-1926) became interested in the recording technique of du BoisReymond and applied it to the measurement of the electric activity of the
brains of rabbits and monkeys. The first report of his experiments,
published in 1875 (Caton, 1875), is believed to constitute the discovery of
the electroencephalogram (EEG).
The founder of membrane theory was Julius Bernstein (German; 1839-1917),
a pupil of Hermann von Helmholtz. Bernstein stated that the potential
difference across the membrane was maintained by the difference in
concentration of potassium ions on opposite sides of the membrane.
In the early 1970s a major innovation was introduced into diagnostic
imaging by Hounsfield. This innovation, x-ray computed tomography (CT),
is recognized today as the most significant single event in medical
imaging since the discovery of x rays.
Stimulation experiment of Luigi Galvani (1786).
He found that when the nerve and the muscle of a frog were
simultaneously touched with a bimetallic arch of copper and zinc, a
contraction of the muscle was produced
d'Arsonval's great solenoid. (d'Arsonval, 1893). (Heating the tissues)
The first experiment on magnetic stimulation of the nervous system. It
was caused by the stimulating effect of the time varying magnetic field to
the retina. He called this phenomenon "magnetophosphenes."
1.2. Physical Systems
I Musculosceletal System
I Respiratory System
I Gastrointestinal System
I Nervous System
I Endocrine System
I The Circulatory System
I The Body as a Control System
The development of medical devices benefited greatly
from the following rapid surge of ‘technical finds’:
Advances in Solid State Electronics
I New Prosthetic Devices
I Nuclear Medicine
I Ultrasound on Sonar Technology
I Material ‘Spare Part’ Tecnology
I Advances in Materials
I Computers
I
1.3. Development of Biomedical Instrumentation
In general, medicine is a multistep procedure performed by
a physician, group of physician, or an institution and
repeated until the symptoms dissapear
Diognosis
ICollecting data
IAnalysis of data
IDecision making
Treatment
IInstituted from the decision
IRepeat
Special features of medical instrumentation systems
More complicated systems than non-medical
systems
I Living tissue and its function is involved
I Accessibility of the information is limited
I The measured signal is not always directly
available and may be corrupted by noise
I Physiological phenomena are seldom
deterministic
• intra- and intervidual variability
• pathophysiological processes are not known
I
The basic objectives of any instrumentation system
fall into one of the following major cathegories:
measurements for
Information gathering
I Diognosis (detection, measuring, correction)
I Evaluation (determining the ability of system)
I Monitoring (obtain information about the state of sys.)
I Control (automatically regulating the system)
I
Figure 1.3.1 (The Scientific Method) In the scientific method, a hypothesis
is tested by experiment to determine its validity.
Problem
statement
Review
prior work
State
hypothesis
Perform
experiments
Design further
experiments
More
experiments
necessary
Analyze
data
Final
conclusions
Problem
solved
Figure 1.3.2 (Clinical Diagnosis) The physician obtains the history,
examines the patient, performs tests to determine the diagnosis and
prescribes treatment.
Chief
complaint
Obtain
history
List the
differential
diagnosis
Examination
and tests
Treatment
and
evaluation
Select further
tests
Use data
to narrow the
diagnosis
Final
diagnosis
More than
one likely
Only one
likely
Figure 1.3.3 (Feedback in Mesurement System) A typical measurement
system uses sensors to measure the variable, has signal processing and
display, and may provide feedback.
Outputs
Measurand
Sensor
Signal
conditioning
Feedback
Effector
Signal
processing
Data
storage
Data
displays
Data
communication
Functional components of the system
• Object: measurand -- >signal--> information
I • Sensor element
I • Signal conditioner or processing
I • Output recorder and display
I • Control, Stimulus, Feedback
I
Generalized Medical Instrumentation System
The sensor converts energy or information from the measurand to
another form (usually electric). This signal is the processed and
displayed so that humans can perceive the information. Elements and
connections shown by dashed lines are optional for some applications
Control
And
feedback
Power
source
Sensor
Measurand
Primary
Sensing
element
Calibration
signal
Radiation,
electric current,
or other applied
energy
Variable
Conversion
element
Signal
processing
Output
display
Data
storage
Data
transmission
Perceptible
output
Measurand
object that the system measures
I physical quantity
I electric voltage (ECG, EEG, ...), pressure, ...
I property
I expanse, severity, …
I condition
I heart rate, weakness, pregnancy…
Measurand
• generation of the measurand
I generated by the physiological system
spontaneously
- EEG, ECG, blood pressure
I produced by the subject voluntarily
- muscle activity, deep ventilation
I externally stimulated or activated
- evoked potentials, paced potentials
Measurand
Accessibility of the measurand
important issue because it determines the methods and
devices to be used to record that quantity:
- internal origin of the signals
deep recordings, catheter recordings (EPS)
- internal origin but appears on body surface
electric potential on the skin
- signal emanates from the body
magnetic field (MCG, MEG)
- information derived from a tissue sample
blood sample
- transmission or emission of applied external energy
electric current (bio-impedance)
Measurand
Some medically important measurands
biopotentials: EEG, ECG, ...
I pressure: blood, intracranial, respiratory
I flow: velocity and volumes, blood, respiratory gas
I dimensions: (imaging) vessel diameters
I displacement: velocity, acceleration, force
I impedance: electric, mechanical airway
resistance
I temperature: body, internal
I chemical concentration: pH, blood gases,
glucose
I
Stimulator:
in a wide meaning, methods and devices
to feed external energy to the living system
I to get a desired response (measured
quantity) to the application of energy
I
Stimulator
Stimulation of a living function
electrical current
- nerve conduction velocity
- cardiac electrophysiological testing
magnetic field
- stimulus of the central nervous system
light
- visual evoked potentials
mechanical oscillation
- auditory evoked potentials
Stimulation
In some physiological measurements and medical
imaging applications, external energy or quantity
is needed to produce the desired information from
the body:
I physiological measurements
bioimpedance measurements
I medical imaging
ionizing radiation (x-ray, gamma)
excitation of nuclear magnets (MRI)
ultrasound energy (Doppler)
laser (imaging)
Examples
1.3.2. Medical Measurement Constraints
The general properties of medical parameters and condition of subject
limit the practical choices available to designers
I
I
I
I
I
I
I
I
I
The measurement ranges of the parameters are quite low
Audio frequency range or below (may contain dc and low frequencies)
Safe level of the various type of energy is difficult to establish
The heating of the tissue must be limited
Equipment must be reliable, simple to operate, and capable of
withstanding physical abuse
Exposure to corrosive chemicals
Electric shock hazard should be minimized
Measuring in anyway endanger the life
It should not require the subject to endure undue pain, disconfort or
any other undesirable conditions
Classification of the biomedical instruments
1. quantity that is sensed
• pressure, flow, temperature ...
• easy to compare devices used in measuring any quantity
2. principle of transducer
• resistive, inductive, capacitive, ultrasonic,…
• engineering approach
3. physiological system
• cardiovascular, pulmonary, nervous, ...
• medical approach, overlapping
4. clinical medicine specialities
• pediatrics, cardiology, radiology, patient monitoring ...
• valuable for medical personnel who are interested in specialized
instruments
Measurement
Range
Frequency
Hz
Method
Blood flow
1 to 300 mL/s
0 to 20
Electromagnetic or
ultrasonic
Blood pressure
0 to 400 mmHg
0 to 50
Cuff or strain gage
Cardiac output
4 to 25 L/min
0 to 20
Fick, dye dilution
Electrocardiography
0.5 to 4 mV
0.05 to 150
Skin electrodes
Electroencephalography 5 to 300 µ V
0.5 to 150
Scalp electrodes
Electromyography
0.1 to 5 mV
0 to 10000
Needle electrodes
Electroretinography
0 to 900 µ V
0 to 50
Contact lens electrodes
pH
3 to 13 pH units
0 to 1
pH electrode
pCO2
40 to 100
mmHg
0 to 2
pCO2 electrode
pO2
30 to 100
mmHg
0 to 2
pO2 electrode
Pneumotachography
0 to 600 L/min
0 to 40
Pneumotachometer
Respiratory rate
2 to 50
breaths/min
0.1 to 10
Impedance
Temperature
32 to 40 °C
0 to 0.1
Thermistor
Table 1. Sensor specifications for a blood pressure sensor
are determined by a committee composed of individuals
from academia, industry, hospitals, and government.
Specification
Value
Pressure range
–30 to +300 mmHg
Overpressure without damage
–400 to +4000 mmHg
Maximum unbalance
±75 mmHg
Linearity and hysteresis
± 2% of reading or ± 1 mmHg
Risk current at 120 V
10 µA
Defibrillator withstand
360 J into 50 Ω
Table 2. Specification values for an electrocardiograph are agreed upon
by a committee.
Specification
Value
Input signal dynamic range
±5 mV
Dc offset voltage
±300 mV
Slew rate
320 mV/s
Frequency response
0.05 to 150 Hz
Input impedance at 10 Hz
2.5 MΩ
Dc lead current
0.1 µΑ
Return time after lead switch
1s
Overload voltage without damage
5000 V
Risk current at 120 V
10 µΑ
Simplified electrocardiographic recording system
Two possible interfering inputs are stray magnetic fields and capacitively
coupled noise. Orientation of patient cables and changes in electrode-skin
impedance are two possible modifying inputs. Z1 and Z2 represent the electrodeskin interface impedances.
Electrodes
vecg
Z1
Z2
Zbody
+Vcc
60-Hz
ac magnetic
field
+
Differential
amplifier
−
Displacement
currents
Vcc
vo
desired input signal
- the potential difference on the skin (Vecg)
interfering inputs - line-frequency interference,
- imbalance in electrode impedances
- amplifier noise
• modifying inputs
- effect of orientation of the patients cables,
- time-dependent change in electrode impedance
- time-varying changes in amplifier
1.3.3. Some of the Characteristics of Medical
Instruments
It demonstrates the quality of the measurements; The following factors
should be considered:
Range: All the levels of imput amplitude and frequency (Exceeding the range)
Amplitude
5 mV
Time
-5 mV
Amplitude
1V
(b)
Time
-1 V
Dynamic
Range
Frequency Response: It is its variation in sensitivity over the
frequency range of the measurement
1.0
Amplitude
0.1
0.05 Hz
150 Hz
Frequency
Figure. 1. Frequency response of the electrocardiograph.
Figure 2.(a) Static-sensitivity curve that relates desired input xd to output y.
Static sensitivity may be constant for only a limited range of inputs. (b) Static
sensitivity: zero drift and sensitivity drift. Dotted lines indicate that zero drift and
sensitivity drift can be negative.
Figure (a) An input signal without dc offset.
(b) An input signal with dc offset.
Amplitude
Time
Amplitude
(b)
Dc offset
Time
Sensitivity:
It determines how small a variation of a variable or
parameter can reliably be measured (directly related to the resolution of
the device
Resolution: the minimum variation that can accurately be read
X
=
∑
X
y =
i
n
GM = n X 1 X 2 X 3 ⋅ ⋅ ⋅ X n
⎛ s
CV = ⎜
⎝X
y = mxd + b
∑ (X
s=
⎞
⎟(100 % )
⎠
b=
Gd
1 + H hG
∑ (X
i
d
2
d
d
2
d
i
−X
)
)(
− X Yi − Y
− X
(∑ y )(∑ x ) − (∑ x y )(∑ x )
n ∑ x − (∑ x )
2
d
d
2
n −1
∑ (X
r =
i
xd
) ∑ (Y
2
m=
i
)
−Y
)
2
n ∑ x d y − (∑ x d )(∑ y )
n ∑ x d2 − (∑ x d )
2
Linearity: The sensitivity would be the same for all
absolute levels of input
x1
Linear
system
y1
(x1 + y2)
and
x2
Linear
system
(y1 + y2)
Linear
system
and
y2
Kx1
Ky1
Linear
system
Least-squares
straight line
(a)
y (Output)
B% of full scale
Figure 1.4 (a) Basic definition of linearity
for a system or element. The same linear
system or element is shown four times
for different inputs. (b) A graphical
illustration of independent nonlinearity
equals ±A% of the reading, or ±B% of full
scale, whichever is greater (that is,
whichever permits the larger error).
A% of reading
Overall tolerance band
xd (Input)
Point at which
A% of reading = B% of full scale
(b)
Figure (a) Original waveform. (b) An interfering input may shift the
baseline. (c) A modifying input may change the gain.
(a)
(c)
(b)
Accuracy: It is a measure of systemic error and calculated by
accuracy=the true value-the measured value/the true value
Precision: The precision of a measurement expresses the
number of distinquishable alternatives from which a given
result is selected
Signal to Noise Ratio
Stability
Isolation
Electrical Safety
Figure Data points with (a) low precision, good accuracy and (b) high
precision, low accuracy.
(a)
(b)
Figure Data points with (a) low accuracy, high precision and (b) high
accuracy, high precision.
(a)
(b)
Figure 1.8 Design process for medical instruments Choice
and design of instruments are affected by signal factors, and
also by environmental, medical, and economic factors.
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