Basic Concepts

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Basic Concepts

• Of
• Medical Instrumentation

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Block diagram of a generalized instrumentation
system
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The Bourdon Gauge
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Block diagram of the pressure gauge based on
Bourdon tube
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A typical medical measurement system
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Feedback with and without clinician
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A patient monitors vital signs and notify a
clinician if abnormalities occur
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Detailed generalized medical measurement system
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Alternative operational modes

• Direct-indirect modes
• Sampling and continuous modes
• Generating and modulating sensors
• Analog and digital modes
• Real-time and delayed-time modes

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Example to sampled data
Laboratory test Typical value
Hemoglobin 13.5 to 18 g/dL
Hematocrit 40 to 54
Erythrocyte count 4.6 to 6.2 ? 106/ ?L
Leukocyte count 4500 to 11000/ ?L
Differential count Neutrophil 35 to 71 Band 0 to 6 Lymphocyte 1 to 10 Monocyte 1 to 10 Eosinophil 0 to 4 Basophil 0 to 2
Complete blood count for a male subject.
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Analog and digital signals
Analog signals can have any amplitude value
Digital signals have a limited number of
amplitude values
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Continuous and discrete-time signals
Continuous signals have values at every instant
of time
Discrete-time signals are sampled periodically
and do not provide values between these sampling
times
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Origins of common biological signal
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Medical measurement constraints
Measurement Range Frequency, Hz Method
Blood flow 1 to 300 mL/s 0 to 20 Electromagnetic or ultrasonic
Blood pressure 0 to 400 mmHg 0 to 50 Cuff or strain gage
Cardiac output 4 to 25 L/min 0 to 20 Fick, dye dilution
Electrocardiography 0.5 to 4 mV 0.05 to 150 Skin electrodes
Electroencephalography 5 to 300 ? V 0.5 to 150 Scalp electrodes
Electromyography 0.1 to 5 mV 0 to 10000 Needle electrodes
Electroretinography 0 to 900 ? V 0 to 50 Contact lens electrodes
pH 3 to 13 pH units 0 to 1 pH electrode
pCO2 40 to 100 mmHg 0 to 2 pCO2 electrode
pO2 30 to 100 mmHg 0 to 2 pO2 electrode
Pneumotachography 0 to 600 L/min 0 to 40 Pneumotachometer
Respiratory rate 2 to 50 breaths/min 0.1 to 10 Impedance
Temperature 32 to 40 C 0 to 0.1 Thermistor
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Setting sensor specifications
Specification Value
Pressure range 30 to 300 mmHg
Overpressure without damage 400 to 4000 mmHg
Maximum unbalance 75 mmHg
Linearity and hysteresis 2 of reading or 1 mmHg
Risk current at 120 V 10 ?A
Defibrillator withstand 360 J into 50 ?
Sensor specifications for a blood pressure sensor
are determined by a committee composed of
individuals from academia, industry, hospitals,
and government.
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Specifications for ECG
Specification Value
Input signal dynamic range 5 mV
Dc offset voltage 300 mV
Slew rate 320 mV/s
Frequency response 0.05 to 150 Hz
Input impedance at 10 Hz 2.5 M?
Dc lead current 0.1 ?A
Return time after lead switch 1 s
Overload voltage without damage 5000 V
Risk current at 120 V 10 ?A
Specification values for an electrocardiograph
are agreed upon by a committee.
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Classification of biomedical instruments

• Quantity sensed pressure, flow, temperature etc.
• Principle of transduction resistive, inductive,
  capacitive, ultrasonic or electrochemical
• Organ system studied cardiovascular, pulmonary,
  nervous, and endocrine systems.
• Clinical medical specialties pediatrics,
  obstetrics, cardiology, or radiology.

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Interfering and modifying inputs
An interfering input may shift the baseline
Original waveform
A modifying input may change the gain
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Simplified Electrocardiographic recording system
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Compensation Techniques

• Inherent insensitivity
• Negative feedback
• Signal filtering
• Opposing inputs

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Negative feedback
y
Gd

xd
-
Hf
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Signal filtering
Signals without noise are uncorrupted
Interference superimposed on signals causes
error. Frequency filters can be used to reduce
noise and interference
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Opposing inputs

• Differential amplifier v0 Gd(vA- vB)
• DC cancellation (bucking)

An input signal with dc offset
An input signal without dc offset
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Generalized Static Characteristics

• Accuracy
• Precision and reproducibility
• Resolution
• Statistical control
• Static sensitivity

• Zero drift
• Sensitivity drift
• Linearity
• Input ranges
• Input impedance

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Accuracy
Data points with
Accuracy closeness with which an instrument
reading approaches the true or accepted value of
the variable (quantity) being measured. It is
considered to be an indicator of the total error
in the measurement without looking into the
sources of errors.
low accuracy
Accuracy is often expressed in percentage
high accuracy
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Precision
Data points with

1. A measure of the reproducibility of the
   measurements i.e., given a fixed value of a
   variable, precision is a measure of the degree to
   which successive measurements differ from one
   another.

low precision

• Number of distinguishable alternatives. 2.434 V
  is more precise than 2.43 V.

high precision
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Resolution

• The smallest change in measured value to which
  the instrument will respond.

Statistical control random variations in
measured quantities are tolerable, Coulter
counter example
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Tolerance

• Maximum deviation allowed from the conventional
  true value.
• It is not possible to built a perfect system or
  make an exact measurement. All devices deviate
  from their ideal (design) characteristics and all
  measurements include uncertainties (doubts).
• Hence, all devices include tolerances in their
  specifications. If the instrument is used for
  high-precision applications, the design
  tolerances must be small.
• However, if a low degree of accuracy is
  acceptable, it is not economical to use expensive
  sensors and precise sensing components

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Static sensitivity
A low-sensitivity sensor has low gain
A high sensitivity sensor has high gain
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Static sensitivity constant over a limited range
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Zero and sensitivity drifts
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Linearity
A nonlinear system does not fit a straight line
A linear system fits the equation y mx b.
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Calibration for linearity
The one-point calibration may miss nonlinearity
The two-point calibration may also miss
nonlinearity
Measuring instruments should be calibrated
against a standard that has an accuracy 3 to 10
times better than the desired calibration
accuracy
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Hysteresis
A hysteresis loop. The output curve obtained when
increasing the measurand is different from the
output obtained when decreasing the measurand.
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Independent nonlinearity
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Input ranges
An input signal which exceeds the dynamic range
The resulting amplified signal is saturated at ?1
V
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Input impedance
System
Xd1 effort variable
Output
Xd2 flow variable