UNIT - 1
HUMAN PHYSIOLOGICAL SYSTEMS
A medical instrument performs a specific function on a biological system. The function
may be the measurement of physiological parameters like temperature, blood pressure,
velocity of blood flow, pH of blood or action potential of heart muscle etc.
DESIGN OF MEDICAL INSTRUMENTS
Factors to be considered when designing a medical instrument:
1. Accuracy
2. Frequency response
3. Hysteresis
4. Isolation
5. Linearity
6. Sensitivity
7. Signal to noise ratio
8. Simplicity
9. Stability
10. Precision
ACCURACY
It is the closeness with which an instrument reading approaches the true value of a
variable being measured.
FREQUENCY RESPONSE
It is the response of an instrument for various frequency components present in a
physiological signal.
HYSTERESIS
The lagging behind of the movement of the measuring needle with respect to the
changes in the measured variable.
ISOLATION
Electrical isolation between the system on which measurements are made and the
ground.
LINEARITY
The degree to which variations in the output of an instrument follow input variations.
SENSITIVITY
It is the ability of an instrument to detect even a very small change takes place in the
input. The sensitivity is also expressed in terms of resolution of the instrument. Resolution is the
minimum variation that can be accurately measured.
SIGNAL TO NOISE RATIO
It is the ratio between the unwanted signal and the true signal. Since the magnitude of the bio
signals are very low, the signal to noise ratio should be very high to get reliable information
about the input.
SIMPLICITY
Simplicity of an instrument is one of the essential factors to eliminate the human
error.Complicated devices may result some confusion when we are operating them.
STABILITY
Stability of an instrument is the ability of that instrument to produce constant output
for a given input.
PRECISION
Precision is the measure of reproducibility of the measurement.
COMPONENTS OF THE BIO-MEDICAL INSTRUMENT SYSTEM:
Biological
system
Transducer
Signal
Processor
Measurement
Monitoring
Feedback
Transform
Diagnosis
Survey
Surgical Tool
Therapy
The block diagram of a generalized bio-medical instrument system is shown in Fig.1.
It consists of a transducer, signal processor, measuring or monitoring units, feedback system
and surgical tools.
A transducer is a device capable of converting one form of energy or signal to its corresponding
electrical signal.
Thus, a transducer transforms the physiological signal like temperature, pressure or bio-potential
into a form that can be read by the signal processor. Signal processor is an important part of the
instrument system which amplifies, modifies (or) changes the electrical output of the transducer
in a suitable manner to run the recording (or) display devices. The type of signal processing
depends upon the function of the instrument system.
In the figure 1, each switch position connects an instrument for measurements, for monitoring,
for diagnosis, for survey (or) for therapy with the signal processor.
ELECTRODES:
A device used to pickup a biological signal is called an electrode.
Electrodes are devices employed to pick up the electrical signals of the human body.
Generally, a pair of electrodes plays the role of a transducer. The electrodes transfer the
bioelectric event to the input of an amplifier. Therefore, the amplifier should be designed such
that, it accommodates the characteristics of electrodes.
The type of electrode to be used depends upon the anatomical location of bio electric event
and the dimension of the electric generation. The electrode should made of the material relatively
inert to the body chemicals.
Types of electrodes:
i.
ii.
iii.
Micro electrodes
Depth and needle electrodes
Surface electrodes
Electrodes
Micro electrodes
Metal micro
Electrodes
Non-metal
microelectrodes
Depth and needle electrodes
Depth
electrodes
needle
electrodes
Surface
Electrodes
1. Metal
2. suction
3. Adhesive
4. multipoint
5. floating
Micro electrodes:
Microelectrodes are used to measure the bioelectrical potential near or within a single cell. These
are also called intra cellular electrodes.
Depth & needle electrodes:
These electrodes are used to measure the bioelectrical potential of brain and specific group of
muscles.
Surface electrodes:
Surface electrodes are used to measure the bioelectrical signal available from surface of
skin. These are also used for sensing the potential from heart, brain of nerves.
Metal microelectrodes:
Metal microelectrodes are formed by electrolytically etching the tip of the tungsten or
stainless steel wire to a fine point. The figure(1) shows the positioning of the microelectrodes.
The measurement of bioelelectric potential requires two electrodes. Therefore, the
voltage measured is the difference between the instantaneous potential of the microelectrodes
and the reference electrodes.
Non-metal microelectrodes (or) micro piplet:
It consists of a glass micropipette whose tip’s diameter is about 1 micrometer. The micropipette
is filled with an electrolyte (usually KCL) which is compatible with the cellular fluids (fig 2).
Depth and Needle electrolyte:
Depth: These are used to study the electrical activity of the neurons in superficial layer of the
brain . The electrodes consist of a bundle of Teflon insulated platinum (90%) iridium (10%)
alloy wires. It is also used to measure oxygen tension.
Needle: These are generally used to record the peripheral nerves action potential (electro neuro
graphy). The needle electrode resembles a medicine dropper.
Surface electrodes:
Generally, larger area surface electrodes are used to sense ECG potential and smaller area
surface electrodes are used to sense EEG and EMG potential.
Surface electrodes are classified as
i.
ii.
iii.
iv.
v.
Metal plate electrodes
Suction cup electrodes
Adhesive tape electrodes
Multipoint electrodes
Floating electrodes
TRANSDUCERS:
A Transducer is a device which detects or senses the bio signal & it converts it into electrical
signal for signal processing.
Transducer can be classified into different types based on energy conversion, application etc.,
Active and Passive Transducer:
Transducers which give their output without the use of an excitation voltage or modulation of
a carrier signal are called active transducers.
Transducers which give their output using excitation voltage or modulation of a carrier signal
are called passive transducers.
Active Transducer:
There are various types of active transducers. They are,
i.
ii.
iii.
iv.
Magnetic induction transducer
Piezo electric transducer
Photo voltaic transducer
Thermo electric transducer
Magnetic induction transducer:
When an electric conductor is moved in a magnetic fieldin such a way that the magnetic flux
through the conductor is changed, a voltage is induced. This voltage is proportional to the rate
change of flux.
(i.e.)
Where,
𝑒=
−𝑑∅
𝑑𝑡
= −𝐵𝑙𝑣
Φ - magnetic flux
B – magnetic induction
l – length of the conductor
v – velocity of moving conductor
Piezo electric type transducer:
When certain crystals are subjected to compression or tension, charge separation occurs in their
crystals and thereby an electrical voltage is developed. This is called piezo electric effect. Thus
the piezo electric type transducers convert displacement or pressure into an electrical variable.
If we apply a dynamical pressure along the mechanical axis (y-axis) of the transducer, then an
a.c electrical voltage is induced along the electrical axis (x axis) of the transducer.
Quartz, Barium titanate (BaTiO3), lithium niobate (LiNaO2) are commercial piezo electric
transducer materials.
Photo voltaic type transducer:
Direct conversion of light into electricity is called photo voltaic effect.
Photovoltaic transducers are active transducers which generate electrical voltage proportional to
the gradient energy incident on it.
Silicon photo voltaic cell can be used as pulse sensor.
To determine sodium & potassium ion concentration in blood, light absorption techniques are
used. Under, suitable conditions the voltage produced in the silicon photo voltaic cell is directly
proportional to the amount of light transmitted through the sample.
Thermo electric type transducer:
These transducers are based on Sebeck effect. When two junctions of a thermocouple are
maintained at different temperatures, an emf is developed and is proportional to the difference
between the junctions of the thermocouple. The transducer based on seeback effect is used to
measure physiological temperature.
The reverse of see ack effect is called Peltier effect in which the flow of current causes one
junction to heat and the other junction to cool. These transducers are used in doctors cold box
which is used to store and carry antibiotic to distant places.
Passive transducer:
Passive transducer converts the physiological parameters like blood pressure and temperature
into an electrical output using a d.c or a.c excitation voltage. Resistors, capacitors and inductors
are the important passive elements used to make passive transducers.Passive transducers cannot
be operated in the reverse direction like active transducer.
Resistive transducers:
Strain guage, photo resistor, photo diode, photo transistor, thermistor and metallic resistance
wire are resistive transducers. These devices are operating on the principle that the measured
parameter causes a small change in resistance of the transducer. A Wheatstone bridge is
commonly used to measure the resistive change in the transducer.
Strain gauge:
Strain guage is an electrical device which is used to measure stress or pressure in terms of
strain using the principle of change or resistivity due to mechanical stress (piezo-resistive effect).
Strain gauge type pressure transducers are used for intra arterial and intra venous, pressure
measurements. To measure blood volume flowing through the arm or leg, the strain guages are
wrapped around them. Due to the flow of blood the diameters of strain guage are changed which
inturn causes a resistance change in the strain gauge. This fact is used to measure blood volumes
and changes in blood volume. The instrument used to measure blood volumes is called
plethysmograph.
For good strain guage transducer:
1. The resistance of the strain guage should be more.
2. The strain guage resistance wires should have low temperature co-efficient of
resistance.
3. The resistance wire would not have any mechanical hysteresis.
4. There must be linear reaction between strain and resistance of the wire.