Generalized instrumentation system

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2/13/2014 BME 420 Department of Biomedical Systems and Informatics Engineering 1

Biomedical Instrumentation (BME420 )Chapter 1:Basic Concepts of Medical Instrumentation

John G. Webster4th Edition

Dr. Qasem Qananwah


Perceptible

outputOutput

display

Control

And

feedback

Signal

processing

Data

transmissionData

storage

Variable

Conversion

element

Sensor

Primary

Sensing

element

Measurand

Calibration

signal

Radiation,

electric current,

or other applied

energy

Power

source

Figure 1.1 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.

Generalized instrumentation system

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• Biopotential • Pressure• Flow• Dimensions (imaging)• Displacement (velocity, acceleration, force)• Impedance• Temperature• Chemical Concentration

Measurand: Physical quantity


Sensor and Transducer

• Transducer—Converts one form of energy to another

• Sensor—Converts a physical measurand to an

electrical output— Interface with living system—Minimize the energy extracted—Minimally invasive

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Signal Conditioning

• Amplification• Filtering• Impedance matching• Analog/Digital for signal processing• Signal form (time and frequency domains)


Output Display

• Numerical• Graphical• Discrete or continuous• Visual• Hearing

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Auxiliary Elements

• Calibration Signal• Control and Feedback (auto or

manual)— Adjust sensor and signal conditioning


1.3 Alternative Operational Modes

• Direct Mode: Measurand is readily accessible— Temperature— Heart Beat

• Indirect Mode: desired measurand is measured by measuring accessible measurand.

— Morphology of internal organ: X-ray shadows— Volume of blood pumped per minute by the heart:

respiration and blood gas concentration

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• Sampling and collecting data will depend on the following:— The rate of change in the measurand— Condition of the patient

• Generating and Modulating Sensors— Generating sensors produce their outputs from energy taken

from measurand (Photovoltaic cell)— Modulating Sensors uses the measurand to alter the flow of

energy from an external source (Photoconductive cell)• Analog and Digital Modes• Real-Time and Delayed-Time Modes

1.3 Alternative Operational Modes


1.4 Medical Measurement Constraints

• Magnitude and frequency range of medical measurand are very low

• Proper measurand-sensor interface cannot be obtained

• Medical variables are seldom deterministic• External energy must be minimized to avoid

any damage• Equipment must be reliable

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1.5 Classification of Medical Instrument

• Quantity that is sensed— pressure, flow, temp

• Principle of transduction— resistive, capacitive, electrochemical, ultrasound

• Organ system— cardiovascular, pulmonary, nervous

• Medicine specialties— pediatrics, cardiology, radiology


1.6 Interfering and Modifying InputsDesired Inputs: measurands that the instrument is designed to isolate.

Interfering Inputs: quantities that unintentionally affect the instrument as a consequence of the principles used to acquire and process the desired inputs.

Modifying Inputs: undesired quantities that indirectly affect the output by altering the performance of the instrument itself.

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1.6 Interfering and Modifying InputsElectrodes

50-Hz

ac magnetic

field

Displacement

currents

Differential

amplifier

+

-

+Vcc

Vcc

Z1

ZbodyZ2

vo

vecg

Desired input: Electrocardiographic voltage Vecg

Interfering input: voltage due to 50-Hz

Figure 1.2 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 electrode-skin interface impedances.


1.7 Compensation Techniques

To eliminate interfering and modifying input:1. Alter the design of essential instrument

components to be less sensitive to interference. (preferred)

2. Adding new components designed to offset the undesired inputs.

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1.7 Compensation Techniques• Inherent Insensitive• Negative Feedback to minimize Gd which is effected by the

modifying inputs

(xd – Hfy)Gd = y (1.1)

xdGd = y(1 + HfGd) (1.2)

(1.3)

• Signal Filtering (electric, mechanical, magnetic)• Opposing Inputs

d

df

d

1x

GH

Gy


Applications of Statistics to medical data- Design experiment- Clinical Study: summarize, explore, analyze- Draw inference from data: estimation,

hypothesis- Evaluate diagnostic procedures: assist clinical

decision making

1.8 Biostatistics

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Statistical MeasurementsMeasures of the mean and central tendency- Mean

- Median: Middle value- Mode: is the observation that occurs most frequently- Geometric Mean: used with data on a logarithmic

scale

n

XX

i

nnXXXXGM 321


Statistical MeasurementsMeasure of spread or dispersion of dataRange: Difference between the largest and smallest observationStandard deviation: is a measure of the spread of data about the mean

Coefficient of variation: standardize the variation to compare data measured in different scales.

1

2

-

-

n

XXs

i

%100

X

sCV

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Statistical Measurements

Percentile: gives the percentage of a distribution that is less than or equal to the percentile number.Standard error of the mean (SEM): Express the variability to be expected among the mean in future samples.Correlation Coefficient r: is a measure of a linear relationship between numerical variables x and y for paired observations

--

--

22

YYXX

YYXXr

ii

ii


Characteristics of Instrument Performance

Two classes of characteristics are used to evaluated and compare new instrument• Static Characteristics: describe the performance

for dc or very low frequency input.• Dynamic Characteristics: describe the

performance for ac and high frequency input.

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Parameters used to evaluate medical instrument:• Accuracy: The difference between the true value and the

measured value divided by the true value• Precision: is the degree to which repeated measurements

under unchanged conditions show the same results.• Resolution: The smallest increment quantity that can be

measured with certainty• Reproducibility: The ability to give the same output for

equal inputs applied over some period of time.

1.9 Generalized Static Characteristics


Parameters used to evaluate medical instrument:• Statistical Control: Systematic errors or bias are

tolerable or can be removed by calibration.• Statistical Sensitivity(DC gain): the ratio of the

incremental output quantity to the incremental input quantity, Gd.— To perform calibration between output and input — For linear calibration

1.9 Generalized Static Characteristics cont.

bmxy d

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-

-

2

d

2

d

dd

xxn

yxyxnm

-

-

2

d

2

d

dd

2

d

xxn

xyxxyb

bmxy d


Figure 1.3 (b) Static sensitivity: zero drift and sensitivity drift. Dotted lines indicate that zero drift and sensitivity drift can be negative.

Zero Drift: all output values increase or decrease by the same amount due to manufacturing misalignment, variation in ambient temperature, vibration,…. Sensitivity Drift: Output change in proportion to the magnitude of the input. Change in the slope of the calibration curve.

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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).

xd (Input)

B% of full scale

A% of reading

Overall tolerance band

Least-squares

straight line

(b)

Point at whichA% of reading = B% of full scale

y (Output)

(a)

x1 (x1 + x2)y1

x2 Kx1 Ky1y2Linearsystem

Linearsystem

Linearsystem

Linearsystem

and and

(y1 + y2)Linearity

Independent nonlinearity- A% deviation of the reading- B% deviation of the full scale

Input Ranges (I): Minimum resolvable input < I < normal linear operating range


variableflow

iableeffort var

d2

d1 X

XZ x

2

d2

2

d1

d2d1 XZZ

XXXP x

x

Input Impedance:• disturb the quantity being measured.

• Xd1 : desired input (voltage, force, pressure)

• Xd2 : implicit input (current, velocity, flow)

• P = Xd1.Xd2 :Power transferred across the tissue-sensor interface

• Generalized input impedance Zx

•Goal: Minimize P, when measuring effort variable Xd1, by

maximizing Zx which in return will minimize the flow variable Xd2.

•Loading effect is minimized when source impedance Zs is much

smaller then the Zx

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)()( 0101 txbdt

dxb

dt

xdbtya

dt

dya

dt

yda

m

m

mn

n

n

)()( 0101 txbDbDbtyaDaDa m

m

n

n

01

01

)(

)(

aDaDa

bDbDb

Dx

Dyn

n

m

m

1.10 Generalized Dynamic CharacteristicsMost medical instrument process signals that are functions of time.

The input x(t) is related to the output y(t) by

ai and bi depend on the physical and electrical parameters

of the system.

Transfer FunctionsThe output can be predicted for any input (transient, periodic, or

random)


01

01

)(

)(

aDaDa

bDbDb

Dx

Dyn

n

m

m

01

01

)()(

)()(

)(

)()(

ajωajωa

bjωbjωb

jωX

jωYjH

n

n

m

m

Frequency Transfer FunctionCan be found by replacing D by j

Example:If x(t) = Ax sin ( t)

then y(t) = |H()| Ax sin ( t + /_H())

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Figure 1.5 (a) A linear potentiometer, an example of a zero-order system. (b) Linear static characteristic for this system. (c) Step response is proportional to input. (d) Sinusoidal frequency response is constant with zero phase shift.

Zero-Order Instrument

a0 y(t) = b0 x(t)

Ka

b

jX

jωY

Dx

Dy

0

0

)(

)(

)(

)(

K: static sensitivity


First-Order Instrument)()(

)(001 txbtya

dt

tdya

)()(1τD tKxty

τD

K

Dx

Dy

1)(

)(

1τ/arctan1

τ1

22ω

τω

K

jωX

jωY

jω

K

jωX

jωY

-

0

0

0

1

a

bK

a

a Where is the time constant

/1 teKty --

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t

1

(c)

(a)

C

+

-

+

-

y(t)

Output y(t)

Input x(t)

Slope = K = 1

(b)

Y (j

X (j

Log

scale

1.0

0.707

Log scale

(d)

0°

- 45°

-90°

Log scale

t

1

0.63

LS

L

S

SL

L

S

x(t)

x(t)

y(t)

R

First-Order Instrument

τD

K

Dx

Dy

1)(

)(

/1 teKty --

)()()(

txtydt

tdyRC

1)(1 txKRC


Second-Order Instrument

txbtya

dt

tdya

dt

tyda 0012

2

2 tKxtyω

ζD

ω

D

nn

1

22

2

unitsinput by defined unitsoutput y,sensitivit static0

0 a

bK

rad/s frequency, natural undamped 2

0 a

aωn

essdimensionl ratio, damping 2

ζ20

1 aa

a

12

2

2

nnω

ζD

ω

D

K

Dx

DyOperational Transfer Function

ωωωω

ζ

ωωζωω

K

jωX

jωY

ωζjωωjω

K

jωX

jωY

nnnn

nn

//

2arctan

/4/1

1/2/

22222

2

-

-

Frequency Transfer Function

Many medical instrument are 2nd order or higher

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Figure 1.7 (a) Force-measuring spring scale, an example of a second-order instrument. (b) Static sensitivity.(c) Step response for overdamped case = 2, critically damped case = 1, underdamped case = 0.5. (d) Sinusoidal steady-state frequency response, = 2, = 1, = 0.5.

Output y(t)

(b)

(d)(c)

1

Ks

x(t)

y(t)yn yn + 1

Resonance

2

Logscale

1

2

-90°

0.51

2 -180°

1

0.5

0.5

Log scale

Log scale

K1

t

t

Input x(t)

Slope K =1

Ks

0°

n

n

Y (j

X (j

Output

displacement

(a)

Input

Force x(t)0

y(t)

2nd order mechanical force-measuring Instrument

Ks = spring constant


Design Criteria and Process

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Commercial Medical Instrumentation

Development Process

•Prototype development

•Testing on animals or human subjects

•Final design review (test results for, specifications, subject feedback,

cost)

•Production (packaging, manual and documents)

•Technical support

Generalized instrumentation system

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