Instrumentation &
Measurement in Biomedical
SEBB 3043
Chapter 1:
Introduction to Biomedical
Instrumentation and Measurement
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Outline
• Terminology
• Biomedical Signals
• Medical Instrumentation Systems,
Modes and Constrains
• Medical Instrumentation Classification
• Biomedical Inputs & Compensation
Techniques
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Terminology
• Biology-Deals with broad spectrum of life
sciences (study of all living organisms i.e.
plants, animals, insects)
• Medical sciences-Study of only human being
• Medical Engineering-Study Engineering
principles in medical science.
• Bio- Medical Engineering- Cover more
animals on the earth
• Engineering or Instrumentation is defined as
science of using measurements.
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The science of structure of the body is known as
Anatomy and it is classified according to the following
basis:
• Gross anatomy deals with the study of the structure
of the organs as seen by the naked eye on
dissection. It describes the shape, size, component
and appearance of the organ under study.
• Topographical Anatomy deals with the position of
the organs in relation to each other, as they are seen
in section through the body in different planes.
• Microscopic anatomy (Histology) is the study of
the minute structure of the organs by means of
microscopy.
• Cytology is a special field of histology in which the
structure, function and development of the cell are
studied.
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Physiology, related to the normal function of the organs
of the body
– Can be classified in different ways. For example:
• Cell physiology is the study of the function of the
cell.
• Pathophysiology relates to the pathological (study
or symptoms of disease) function of the organs.
– In addition, classification into various sub-areas
dealing with different organs can be made. E.g:
• Circulatory physiology is the study of the blood
circulation relating to functioning of the heart.
• Respiratory physiology deals with the functioning
of breathing organs.
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Physiological Systems of the Body
• Human body consists of many systems
– Respiratory system
– Endocrine system
– Heart and Circulatory system (Cardiovascular)
– Nervous system
– Digestive system
– Reproduction system
– Etc
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Biomedical Signals
– Bioelectric Signals: Generated by nerve cells
and muscle cells. Their basic source is the cell
membrane potential which under certain
conditions may be excited to generate an
action potential. The electric field generated
by the action of many cells constitutes the
bio-electric signal.
• Eg:) signals.
– Bioacoustic Signals: The measurement of
acoustic signals created by many biomedical
phenomena provides information about the
underlying phenomena.
• Eg:
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– Biomechanical Signals: These signals originate
from some mechanical function of the biological
system. They include all types of motion and
displacement signals, pressure and flow signals etc.
• Eg::
– Biochemical Signals: The signals which are
obtained as a result of chemical measurement from
the living tissue or from sample analyzed in the
laboratory.
• Eg::
– Biomagnetic Signals: Extremely weak magnetic
fields are produced by various organs such as the
brain, heart, and lungs. The measurement of these
signals provides information which is not available
in other types of bio-signals such as bio-electric
signals.
• Eg:
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– Bio-Optical Signals: These signals are
generated as result of optical function of the
biological systems, occurring either naturally or
induced by the measurement process.
• Eg:
– Bio-impedance Signals: The impedance of the
tissue is a source of important information
concerning its composition, blood distribution
and blood volume etc. The measurement of
galvanic skin resistance is a typical example of
this type of signals. The bio-impedance signal is
also obtained by injecting sinusoidal current in
the tissue and measuring the voltage drop
generated by the tissue impedance.
• Eg:
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Medical Instrumentation Systems
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• Measurand: The physical quantity or condition that the
system measures is called the measurand. The measurand
may be internal (blood pressure), on the body surface
(electrocardiogram potential), emanating from the body
(infrared radiation) or from a tissue sample (biopsy).
• Sensor/Transducer: A transducer is a device that converts
one form of the energy to another. A sensor converts a
physical measurand to an electric output. The sensor
should respond only to the form of energy present in the
measurand.
Eg: a piezo-electric crystal converts mechanical vibrations
into an electrical signal
Sensor: Primary sensing element (diaphragm: converts
pressure to displacement) and a variable-conversion
element (strain gage: converts displacement to electric
voltage).
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Medical Instrumentation Systems
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• Signal Conditioner: Converts the output of the transducer into an
electrical quantity suitable for operation of the display or recording
system. Simple signal conditioners may only amplify and filter the
signal or merely match the impedance of the sensor to the display.
Eg: Signal filtering may reduce undesirable sensor signals or average
repetitive signals to reduce noise or may convert information from time
domain to frequency domain.
• Display System: The result of the measurement process must be
displayed in a form that the human operator can perceive:
numerical or graphical, discrete or continuous, permanent or
temporary.
– Most displays are in a visual form, but audible signals forms like alarms or
Doppler ultrasonic signals can also be found.
• Auxiliary Elements: Calibration, Control and feedback, Data
Storage
– Alarm System - with upper and lower adjustable thresholds to indicate when
the measurand goes beyond present limits.
– Data transmission- information obtained may be carried to other parts of an
integrated system or to transmit it from one location to another.
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Alternative Operational Modes
• Direct – Indirect Modes: Measurand can interface
directly to the sensor or needs another measurand to
interact between the original measurand and the sensor.
Eg: Cardiac output (vol blood/min pumped by heart) by
measurement of respiration and blood gas concentration or
dyes; Morphology of internal organs by x-ray shadows.
• Sampling and Continuous Modes: Body temperature &
ion concentration change so slowly that may be sampled
infrequently. Electrocardiogram & respiratory gas flow
require continuous sampling.
– Type of measurand, objective of measurement, condition of
patients… influence the frequency of sampling.
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• Generating and Modulating Sensors
Generating sensors produce signal output from energy taken
directly from measurand - photovoltaic cell (produces output
voltage related to its radiation, without external energy source)
Modulating sensors use the measurand to alter the flow of energy
from an external source in a way that affects the output of the
sensor – photoconductive cell (external energy must be applied to
measure its change in resistance with irradiation)
• Analog and Digital Modes
Analog signals are continuous and able to take on any value within
dynamic range. Digital signals are discrete and only able to take a
finite number of different values.
• Real-Time and Delayed-Time Modes: Sensor must acquire signals
in real time as the signal actually occur. The output of the system
may not display the results immediately due to types of signal
processing. Often such short delays are acceptable unless urgent
feedback required.
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Medical Measurement Constrains
• Parameters measurement ranges are quite low (µvolts)
• Inaccessible crucial variables (damage syst) – indirect
measurements
• Quantities vary with time
• Medical measurements vary widely among normal
patients even at similar conditions
• To cope with this variability is to assume empirical
statistics and deterministic distribution functions
• Safe levels of various types of energy (x-ray,
ultrasound…) are difficult to establish because
mechanism of tissue damage are not well understood
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• Equipment must be reliable, easy to operate, and capable
of withstanding physical abuse and exposure to corrosive
chemicals.
• Electronic equipment must be designed to minimize
electric-shock hazards.
• Safety of patients and medical personnel must be
considered in all phases of the design and testing of the
instrument.
• The Medical Device Amendments (1976) & Safety
Medical Devices Act (1990) amend the Federal Food,
Drug and Cosmetics Act to provide for the safety and
effectiveness of medical devices intended for human use.
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Classification of Biomedical Instruments
Quantity Sensed
Principle of
Transduction
Organ System
Clinical
Medicine
Specialty
Pressure
Resistive
Cardiovascular
Pediatrics
Flow
Inductive
Pulmonary
Obstetrics
Temperature
Capacitive
Nervous
Cardiology
Current
Ultrasound
Endocrine
Radiology
Concentration
Electrochemical
Gastrointestinal
Neurology
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BLOOD INSTRUMENTS
HEART INSTRUMENT
Blood Pressure meter
ECG
Blood PH meter
Pace Maker
Blood flow meter
Defibrillator
Blood cell counter
Heart Lung Machine
Calorimeter
Bed side monitor
Spectra – Photometer
Plethysmograph
Flame photometer
Electronic stethoscope
Digital BP meter
Phonocardiograph
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BRAIN INSTRUMENTS
MUSCLE INSTRUMENTS
EEG
EMG
Tomograph
Muscle Stimulator
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KIDNEY INSTRUMENTS
EAR INSTRUMENTS
Dialysis Instrument
Audiometer
Lithotripsy
Hearing aid
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EYE INSTRUMENTS
LUNG INSTRUMENTS
Occulometer
Spirometer
Aid for blind
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BODY INSTRUMENTS
PHYSIOTHERAPY
INSTRUMENTS
Ultra Sonography
Diathermy, Short wave
Thermograph
Electrosleeper
Radiograph
Vibrator (Massage type)
EPF
U.V. Lamp
Endoscope
Microwave diathermy
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Interfering & Modifying Inputs
Desired input – measurand that the instrument is designed to isolate
Interfering input – quantities that inadvertently affect the instrument
Modifying input – undesired quantities that indirectly affect output
and performance of instrument
Desired input: Vecg
Interfering input: 60 Hz
noise ac magnetic field
(in series with desired)
Modifying input Plane of patients cable:
If parallel to magnetic
field – inteference = 0
If perpendicular = max
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Compensation Techniques
Effects of interfering or modifying inputs can be reduced or
eliminated by altering the design or adding components to the
original instrument.
• Inherent Insensitivity: All instrument components have
inherently sensitivity ONLY to desired input
• Negative Feedback: Makes the output less dependent on the
transfer function. Takes a portion of output and feeds it back to
the input. The output dependent signal is subtracted from input
and the difference is the effective input.
• Signal Filtering: input filters block interfering and modifying
inputs without altering the desired signal. Output filters are more
difficult because the desired & undesired output signals are
superimposed
• Opposing Inputs: Additional interfering inputs can be used to
cancel undesired output components.
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BASIC OBJECTIVES INSTRUMENTATION
1.
2.
3.
4.
5.
Information Gathering: measure variable for knowledge,
characteristic of the measurements may not be known in
advance.
Diagnosis: measurements are made to help in the detection &
the correction of some malfunction of the system being
measured
Evaluation: measurements are used to determine the ability of a
system to meet its functional requirements.
Monitoring: measure of process in order to obtain continuous
or periodic information about the state of the system being
measured.
Control the operation of a system based on changes in one or
more of the internal.
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• Significant Figures
Metric prefix
What’s the current on a
circuit with 10V and
a resistance of 3 Ω?
tera
• Scientific Notation
n.ij x 10x
• Units and physical
constants (SI)
Multiplying
factor
1012
Symbol
T
giga*
G
mega
M
kilo
k
hecto
h
deka
da
deci
d
centi
c
CGS - centimeter
grams seconds
milli
m
micro
µ
MKS - meters
kilograms second
nano
n
pic
p
femto
f
atto
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Physical constants and units Constant
Value
Boltzmann’s constant
Capacitance-Faraday
Electric charge-Coulomb
Conductance-Siemens
Conductivity-Siemens/meter
Current- ampere
Energy-Joule
Frequency-Hertz
Power-watt
Resistance-ohm
…
Electric charge (e-)
Electron (volt)
Electron (mass)
eV
9.12x10-31 Kg
8.85x10-12 F/m
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q
1.6x10-19 J
Permittivity of free
space
Pi
K
1.6x10-19 C
4πx10-7 H/m
Velocity of
electromagnetic waves
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1.38x10-23 J/K
Permeability of free
space
Planck’s constant
Symbol
km
U0
εo
6.3626x10-34 J-s
h
3x10-8 m/s
c
3.141592654
π
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Logarithmic representation of signal levels: decibel
Decibel: means of logarithmically expressing the ratio between two
signal levels
• Converting between dB to gain notation
Voltage gain = Voutput / Vinput= 6V/0.5V=12
dB= 20 log (Vo/Vin) = 20 log (12)= 21.6
Current - dB= 20 log (Io/Iin)
Power - dB= 100 log (Po/Pin)*
• Converting dB to voltage
Vo =Vin 10dB/20
* Power is proportional to the square of the voltage current
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Summary
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