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Biomedical InstrumentationWebster Chapter -1

Introduction

Medical InstrumentInvasive, noninvasive; external, implantedDiagnostic, therapeuticdetect biochemical, bioelectrical, or biophysical parametersreproduce the physiologic time response of these parametersprovide a safe interface with biological materials

ExamplesEndocardial catheter; ECG electrodes and monitorExternal & implanted pacemakers, defibrillatorsEEG based neurological monitor; deep brain stimulator for Parkinson’s disease

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General Medical Instrumentation System Control

Andfeedback

SensorPowersource

PerceptibleoutputOutput

displaySignalprocessing

Datatransmission

Datastorage

VariableConversionelement

Sensor

PrimarySensingelement

Measurand

Calibrationsignal

source

Figure 1.1 Generalized 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.

Radiation,electric current,or other appliedenergy

Characteristics of Physiological Signals

Parameter Measurement Range

Frequency Range

Measurement Method

Electrocardiogram (ECG)

0.5 – 4mV 0.01 – 250Hz Skin electrodes

Electroencephalo-gram (EEG)

5 – 300μV DC – 150Hz Scalp electrodes

Electromyogram (EMG)

0.1 – 5mV DC – 10KHz Needle/skin electrodes

Electrooculogram (EOG)

50 – 3500μV DC – 50Hz Contact electrodes( )Blood flow 1 – 300mL/sec DC – 20Hz Ultrasonic flowmeter

Respiratory Rate 2 – 50 breaths/min 0.1 – 10Hz Strain guage

Body Temperature 32 – 40 oC DC – 0.1Hz Thermistor/thermocouple

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Examples of Medical InstrumentsImplanted pacemaker

External ECG monitor

nerve stimulatornerve stimulator

Laser for coronary angioplasty

X ray or imaging devices

Electrosurgical instrument

Pulse oximeter

Defibrillation electrode

Infrared thermometer

Automatic blood pressure

System Block DiagramInterference“Induced”

Transfer Function

Signal source

InputOutput

Internal interference

(added)

Any measurement includes signal+noiseSignal sources: ECG. EEG, blood

InstrumentSystem

(added) Signal sources: ECG. EEG, blood pressure, temperature…Noise sources

External: 60 Hz, radio frequency (RF), magnetic…Internal: muscle noise, motion artifact, eye blink artifact

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General instrument static characteristics of• Accuracy (True (from NIST)-measured)

• Precision (No of significant digits)

Static System Properties

digits)

•Resolution (smallest measurable qty)

•Reproducibility (give same output)

•Statistical control (variation of meas red q antities in tolerablemeasured quantities in tolerable limits)

•Static sensitivity – slope, zero-drift (ambient surroundings), sensitivity drift (ex power supply fluctuations)

More next slide

Static system properties cont.Linearity (not all instruments have a perfect linear response; deviation from linear fit line is necessary nonlinearityInput Dynamic Range ratio Linearx1 y1

LINEARITY

Input Dynamic Range - ratio between the maximum undistorted signal (i.e., maximum input signal satisfying the linearity specification for the sensor) and the minimum detectable signal for a given set of operating conditions expressed in dBInput Impedance the

System

LinearSystem

x2 y2

LinearSystem

x1 + x2 y1 + y2

Input Impedance the instantaneous rate at which energy is transferred by a system

System

LinearSystem

Kx1 Ky1

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Generalized Dynamic CharacteristicsMost biomedical instruments must process signals that change with time. The dynamics of the measurement system, therefore, must be chosen to properly reproduce the dynamics of the physiologic variables the system is sensing. Mostly, we will deal with consider linear, time invariant systems unless otherwise explicitly noted. For such systems the dynamics can be fully described by simple differentialFor such systems, the dynamics can be fully described by simple differential equations of the form:

where x(t) is the input signal (usually the physiologic parameter of interest), y(t) is the output signal (usually the electronic signal), and the a’s and b’s are constants determined by the physical characteristics of the sensor system.

Most practical sensor front-ends are described by differential equations of zero, first or second order (i e n=0 1 2) and derivatives of the input are usually absent so

)(...)(... 0101 txbdtdxb

dtxdbtya

dtdya

dtyda m

m

mn

n

n +++=+++

or second order (i.e., n 0,1,2), and derivatives of the input are usually absent, so m=0.Linear, time invariant systems are simply characterized by their response to sinusoidal inputs of the form x(t) = A sin(wt), where the output is a sinusoid at precisely the same frequency of the form y(t) = B(w) sin(wt + f(w)).

01

01

)(...)()(...)(

)()((

ajajabjbjb

jXjYjH n

n

mm

++++++

==)ωωωω

ωωω

Zero-Order SystemExpression of the input-output relationship

Time-domain R l ti hiRelationship

Transfer Function

Example

)()( 00 txbtya =

0

0)(abjH =ω

Linear potentiometer

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First-Order SystemSystem contains a single energy-storage elementTime-domain relationship

)()( txbtyadya =+

Transfer Function

)()( 001 txbtyadt

a =+

01

0

)()(

ajabjH+

=ω

ωor

ExampleRC Low-pass or High-pass Filters

RC circuit and response

C

+

−

+

−

y(t)

Output y(t)

Input x(t)

Slope = K = 1x(t)

R

Figure 1.6 (a) A low-pass RC filter, an example of a first-order instrument. (b) Static sensitivity for constant inputs. (c) Step response for larger time constants (τL) and small time constants (τS). (d) Sinusoidal frequency response for large and small time constants.

t

1

(c)

(a)p ( )

(b)

Y (jω)X (jω)

Logscale

1.00.707

Log scale ω(d)

ωSωL

φ

τL

τS

x(t)

y(t)

0°

− 45°

−90°

Log scale ω

t

1

0.63

τLτS

τL

τS

φy(t)

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Second-Order SystemSecond-order system can approximate higher-order systemsTime-domain Relationship

)()(2

txbtyadyayda =++

Transfer Function

)()( 00122 txbtyadt

adt

a =++

12 20

1 ==aa

aξ12 20

1 >=aa

aξ 12 20

1 <=aa

aξ

Overdamped Underdamped Critically Damped

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Food and Drug Administration (FDA)

Government body entrusted with the responsibility to l t d di l d i d tregulated medical devices, drugs, etc.

Primary task: certify safety and efficacy

FDA regulates through FDA Instrumentation Categories

Design Control Class I

Process Control Class II

Good Manufacturing Practices Class III

FDA Device Regulations

Class I – General ControlsRequired to perform registration labeling and goodRequired to perform registration, labeling, and good manufacturing practices and to report adverse effects

Class II – Performance StandardsRequired to prove “substantial equivalence” via the 510(k) process

Class III – Pre-market Approval (PMA)Requires extensive testing and expert scrutinyPMA is necessary for devices used in supporting or sustaining human life