Transcript Lec 2,3 Medical Instrumtation System

BIOMEDICAL
INSTRUMENTATION SYSTEM
By: Engr. Hinesh Kumar
Outline
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Generalized Medical Instrumentation Systems
Components of Medical Instrumentation Systems
PC Based Medical Instruments Systems
Operational Modes
Medical Measurement Constraints
Classifications of Biomedical Instruments
Measurements Input Source
Characteristics of Instrument Performance
System Static & Dynamic Characteristics
General Design Criteria & Process of Medical Instruments
Commercial Medical Instrumentation Development Process
Basic Instrumentation System
Generalized Medical Instrumentation System
* Elements and connections shown by dashed lines are optional for some applications.
Generalized Medical Instrumentation System
Components of Medical Instrumentation System
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Measurand
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Sensor / Transducer
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Signal Conditioning
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Output Display
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Auxiliary Components
Measurand
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The physical quantity, property, or condition that the
system measures is called measurand.
The accessibility of the measurand is important
because it may be:
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Internal (Blood Pressure)
On the Body Surface (Electrocardiogram)
Emanate from the body (Infrared Radiation)
Derived from Tissue Sample (such as Blood or a Biopsy)
Cont…
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Most medically important measurands can be
grouped in the following groups:
 Biopotential,
Pressure,
 Flow,
 Dimensions (Imaging),
 Displacement (Velocity, Acceleration, And Force),
 Impedance,
 Temperature, And
 Chemical Concentrations
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The measurand may be localized to a specific organ
or anatomical structure.
Sensor
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The transducer is defined as a device that converts
one form of 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, to the exclusion of
all others.
The sensor should non invasive and minimally
invasive.
Signal Conditioning
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Simple signal conditioners may only amplify and filter
the signal or merely match the impedance of the sensor
to the display.
Often sensor outputs are converted to digital form and
then processed by specialized digital circuits or a
microcomputer.
For example, signal filtering may reduce undesirable
sensor signals.
It may also average repetitive signals to reduce noise,
or it may convert information from the time domain to
the frequency domain.
Output Display
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The results of the measurement process must be
displayed in a form that the human operator can
perceive.
The best form for the display may be:
 Numerical
 Graphical,
 Discrete
or Continuous,
 Permanent or Temporary
 Visual / Hearing
Auxiliary Components
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A calibration signal with the properties of the
measurand should be applied to the sensor input or
as early in the signal-processing chain as possible.
Many forms of control and feedback may be
required to elicit the measurand, to adjust the sensor
and signal conditioner, and to direct the flow of
output for display, storage or transmission.
The control and feedback may be automatic or
manual.
Cont…
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Data may be stored briefly to meet requirements of
signal conditioning or to enable operator to examine
the data that precede alarm conditions. Or data may
be stored before signal conditioning, so that different
processing schemes can be utilized.
Conventional principles of communication can often
be used to transmit data to remote displays at
nurses’ stations, medical centers, or medical dataprocessing facilities.
PC Based Medical Instruments
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Personal computer are popular in medical field and
also software is largely commercially available and
the users can purchase and use it.
Computer are widely accepted in the medical field
for data collection, manipulation, processing and a
complete workstations for a variety of applications.
A personal computer becomes a workstation with
the simple installation of one or more “instrumentson-a-board” in its accessory slots.
PC Based Medical Instruments
Typical configuration of PC based Medical Instruments
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Fig illustrates the typical configuration of a PC based
workstation. System is highly flexible and can
accommodate a variety of inputs, which can be
connected to PC for analysis, graphics and control.
Basic elements in the system include sensors or
transducers that convert physical phenomena into a
measurable signal, a data acquisition system, an
acquisition/analysis software package or programme
and computing platform.
The systems works totally under the control of software.
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PC medical instruments are gaining in popularity for
several reasons including price, programmability
and performance specifications.
Software development, rather than hardware
development, increasingly dominates new product
design cycles.
This includes operating systems, devices drivers,
libraries, languages and debugging tools.
Instruments Operational Modes
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Direct / Indirect Mode
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Sampling and Continuous Modes
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Generating and Modulating Modes
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Analog and Digital Modes
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Real time and Delayed Time Modes
Direct / Indirect Modes
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Direct Mode
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Desired measurand can be interfaced directly to a sensor because
the measurand is readily accessible
If the sensor is invasive, direct contact with the measurand is
possible but expensive, risky and least acceptable.
Temperature
Heartbeat
Indirect Mode
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Desired measurand can not be interfaced directly and not
accessible
Morphology of internal organ: X-ray shadows
Volume of blood pumped per minute by the heart: respiration and
blood gas concentration
Pulmonary volumes: variation in thoracic impedance
plethysmography
Sampling Mode and Continuous Mode
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Sampling Mode
Sampling can change so slowly that they may be sampled
infrequently.
 Body Temperature
 Ion Concentration
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Continuous Mode
Frequent or constant monitoring of measurand
 Electrocardiogram
 Respiratory Gas Flow
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Generating and Modulating Modes
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Generating Mode
Generating sensors produce their signal output from energy
taken directly from the measurand.
 Also known as self-powered modes.
 Example: Photovoltaic cell is a generating sensor because it
provides an output voltage related to its irradiation, without
any additional external energy source.
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Modulating Mode
 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.
 Example: Photoconductive cell is a modulating sensor; to
measure its change in resistance with irradiation, we must
apply external energy to the sensor.
Analog and Digital Modes
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Analog Modes
Analogue or Continuous signal is able to take any value
within a dynamic range.
 Most currently available sensors operate in the analog
mode.
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Digital Modes
Digital or Discrete signal is able to take on only a finite
number of values.
 The advantages of the digital mode of operation include
greater accuracy, repeatability, reliability, and immunity to
noise.
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Real Time and Delayed Time mode
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Real Time Mode
 Sensor
acquire the signal in real-time mode
 The result are displayed immediately
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Delayed Time Mode
 Display
results are delayed due to image processing
such as averaging and transformations.
General Constraint In Design of
Medical Instrumentations Systems
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Medical equipment are primarily used for making
measurements of physiological parameters of the human
body and also in some cases as stimulus or some kind of
energy is applied to the human body for diagnosis and
treatment.
Some of important factors, which determine the design of a
medical measuring instrument, are:
Measurement Range: Generally the ranges are quite low
compared with non-medical parameters. Most signals are in
microvolt range.
Frequency Range: Most of the biomedical signals are in the
audio frequency range or below and many signal contain
dc and very low frequency components
Cont….
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The signal to be measured imposes constraints on how it
should be acquired and processed.
Many measurand in living systems are inaccessible.
Placement of sensor(s) in/on the body plays a key role
in medical instrumentation design.
Magnitude and frequency range of medical measurand
are very low.
Interference and cross-talk artifacts.
Proper sensor interface with measurand cannot be
obtained.
Medical variables are seldom deterministic (varying
with time).
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Many medical measurements vary widely among
normal patients, even when conditions are similar.
Safety of patient and medical personnel also must
be considered.
Safe levels of stimulation or applied energy are
difficult to establish,
External energy must be minimized to avoid any
damage.
Equipment must be reliable, easy to operate, and
durable.
Government regulations.
Common Medical Measurands
Classifications Of Medical Instruments
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Medical Instruments can be classified in the four
categories
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Quantity that Sensed
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Principle of Transduction
3.
Organ System
4.
Clinical Medicine Specialties
Classifications
1.
Quantity That Sensed
Advantage of this classifications is that it makes different
methods for measuring any quantity easy to compare.
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Pressure
Flow
Temperature
Principle of Transduction
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Resistive
Inductive
Capacitive
Ultrasonic (Sound waves)
Electrochemical (pH probe, Hydrogen Sensor)
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Organ System
Isolates all important measurements for specialists
who need to know only about a specific area
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Cardiovascular Systems
Pulmonary System
Nervous System
Endocrine System
Clinical Medicine Specialties
This approach is valuable for medical personnel who
are interested in specialized instruments.
 Pediatrics
 Obstetrics
 Cardiology
 Radiology.
Measurement Input Sources
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Desired Inputs: Measurands that the instrument
is designed to isolate.
Interfering Inputs: Quantities that accidentally
affect the instrument as a consequence of the
principles used to acquire and process the desired
inputs.
3.
Modifying Inputs: Quantities that cause a
change in the input –output relations of the
instrument.
Example: ECG Signal Measurement
Desired Input: ECG voltage (Vecg)
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Interfering Input: 60/50 Hz noise voltage, displacement currents
2.
Modifying Input: – orientation of the patient cables when the plane of the cable is
perpendicular to the magnetic field the magnetic interference is maximal
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Figure: Simplified electrocardiographic recording system Two possible interfering
inputs are stray magnetic fields and capacitive coupled noise. Orientation of
patient cables and changes in electrode–skin impedance are two possible
modifying inputs. Z1 and Z2 represent the electrode–skin interface impedances.
Characteristics of Instrument Performance
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Evaluate new instrument designs, quantitative criteria for
the performance of instruments.
These criteria must clearly specify how well an instrument
measures the desired input and how much the output
depends on interfering and modifying inputs.
Characteristics of instrument performance are usually
subdivided into two classes on the basis of the frequency
of the input signals.
Static Characteristics
Dynamic Characteristics
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Static
Characteristics
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The output for a wide range of constant inputs
describe the performance
instruments for dc or very low frequency inputs.
of
demonstrate the quality of the measurement, including
nonlinear and statistical effects.
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Dynamic Characteristics require the use of differential
and/or integral equations to describe the quality of the
measurements.
Generalized Static Characteristics
Following are the parameters used to evaluate medical instrument:
• Accuracy: refers to the degree of conformity between the
measurand and the standard. It can be calculated using the
difference between the true value and the measured value
divided by the true value.
• Precision: refers to the exactness of successive measurements,
also sometimes considered the degree of refinement of
measurement.
Good Accuracy,
Good Precision
Good Accuracy,
Poor Precision
Poor Accuracy,
Good Precision
Poor Accuracy, Poor
Precision
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Resolution: It refers to the degree to which the measurand
can be broken into identifiable adjacent parts.
Reproducibility: The ability of an instrument to give the
same output for equal inputs applied over some period of
time.
Statistical Control: It ensures ensures that random
variations in measured quantities that result from all factors
that influence the measurement process is tolerable.
Static Sensitivity: Static Sensitivity of instrument or system is
the ratio of the incremental output quantity to the
incremental input quantity.
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Zero Drift: It occurs when all the output values increase or
decrease by the same absolute amount.
Factors can cause zero drift: manufacturing misalignment,
variations in ambient temperature, hysteresis, vibration,
shock, and sensitivity to forces from undesired directions.
Linearity: A system or element is linear if it has properties
such that if y1=x1 and y2=x2, then y1+y2 is the response
to x1+x2, and Ky1=Kx1. They are clearly satisfied for an
instrument with a calibration
curve that is a straight line.
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Input Ranges: The normal linear operating range
specifies the maximal or near maximal inputs that give
linear outputs.
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Input Impedance: It is the ratio of the phasor
equivalent of a steady-state sinusoidal effort input
variable (voltage, force, pressure) to the phasor
equivalent of a steady-state sinusoidal flow input
variable (current, velocity, flow).
Generalized Dynamic Characteristics
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Most of the biological signals time-varying in nature and
therefore we should make sure that the instrument is a
time-invariant system for an accurate measurement.
The dynamic characteristics of an instrument include its
transfer function, its frequency response, and its phase
or time delay.
Dynamic characteristics require the use of differential or
integral equations to describe the quality of the
measurements.
Transfer functions are used to predict the stability of a
system.
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Zero Order Instruments:
It has ideal dynamic performance, because the output is
proportional to the input for all frequencies and there is
no amplitude or phase distortion.
 Example: Linear Potentiometer
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First Order Instruments:
It contains a single energy storage element.
 Example: Low-pass RC filter.
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Second Order Instruments:
 Instruments
that requires second-order differential
equation is required to describe its dynamic response.
 Many medical instruments are second order or higher,
and low pass.
 Example: Force-measuring Spring Scale
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Time Delay:
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elements that give an output that is exactly
the same as the input, except that it is delayed in time.
 Example:
Equipment that require significant signal
processing schemes.
General Design Criteria and Process of
Medical Instrument
Figure: Design process for medical
instruments Choice and design of
instruments are affected by signal
factors, and also by environmental,
medical, and economic factors.
Regulations of Medical Devices
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The medical instrumentation industry in general and
hospitals in particular are required to be most regulated
industries.
This is because when instruments are made on human beings
and by the human beings, the equipment should not only be
safe to operate but must give intended performance so that
the patients could be properly diagnosed and treated.
To minimize the problems various countries have introduced
a large numbers of codes, standards and regulations for
different types of equipment and facilities.
It is therefore, essential that engineers understand their
significance and be aware of the issues that are brought
about by technological and economical relaities.
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Regulations: A regulation is an organization’s way of
specifying that some particular standard must be adhered
to. These are rules normally promulgate by the
government.
Codes: A systems of principles or regulations or a
systematized body of law or an accumulation of a
system of regulations and standards. In general, a code
is compilation of standards relating to providing health
care to the state population.
Specification: Documents used to control the procurement
of equipment by laying down the performance and other
associated criteria. These documents usually cover design
criteria, system performance, materials and technical
data.
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Standards: A standard is a multi-party agreement for
establishment of an arbitrary criterion for reference.
Alternatively standard is prescribed set of rules,
conditions or requirements concerned with the definition
of terms, classification of components, delineation of
procedures, specifications of materials, performance,
design or operations, measurements of quality and
quality in describing materials, products, systems,
services or practice. Standards exist that address
systems (protection of the electrical power distribution
systems from faults), individuals (measure to reduce
potential electric shock hazards) and protection of the
environment (disposal of medical waste).
Types of Standards
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There are in general three type of standards for medical
devices:
Voluntary Standards: Developed through a consensus
process where manufactures, users, consumers and
government agencies participate.
Mandatory Standards: Required to be followed under
law. They are incumbent on those to whom the standard
is addressed and enforceable by the authority having
jurisdiction
Proprietary Standards: Developed either by a manufacturer
for its own internal use or by a trade association for use by
its members.
Regulatory Requirements
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In 1976 the United States Congress passed what are
known as the Medical Device Amendments (Public Law
94-295) to the Federal Food, Drug, and Cosmetics Act
that dates back to the 1930s.
The primary purpose was to ensure the safety and
efficacy of new medical devices prior to marketing of
the device.
Medical devices were classified in two ways.
First, the division of such devices into Class I, II, and
III was based on the principle that devices that pose
greater potential hazards should be subject to more
regulatory requirements.
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Second, seven categories were established:
preamendment,
postamendment,
substantially
equivalent, implant, custom, investigational, and
transitional.
The seven categories into which medical devices are
divided are described in Table, which also includes
classification rules and examples.
Software used in medical devices has become an
area of increasing concern.
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Class I General Controls: Manufacturers are required to
perform registration, premarketing notification, record
keeping, labeling, reporting of adverse experiences, and
good manufacturing practices. These controls apply to all
three classes.
Class II Performance Standards: Apply to devices for
reasonable assurance of safety and efficacy, and for
which existing information is sufficient to establish a
performance standard. However, until performance
standards are developed by regulation, only general
control apply.
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Class III Premarketing Approval: Such approval is
required for devices used in supporting or sustaining
human life and preventing impairment of human
health.
The FDA has extensively regulated these devices by
requiring manufacturers to prove their safety and
effectiveness prior to market release.