Transcript Biomedical Sensors - ETSU

Biomedical Sensors
A key in knowing what
is wrong with you.
What is Measured and How?
• Measurand - a biomedical property that
can be measured.
• Transducible property - that property of a
measurand that allows it to be measured.
• Biosensor - converts a measurand into a
measurable signal, usually electrical.
Classification of Measurands
• Biopotentials, for example, ECG, EEG,
etc. That is, those that arise from an
electrical signal generated in the body.
• Nonbiopotentials, for example,
temperature, blood pressure, rate of blood
and air flow.
Classification of Sensors
• Diagnostic - temperature, blood pressure,
ECG
• Therapeutic- defibrillators, pacemakers
• Assistive - cochlear implants, functional
neuromuscular stimulation
Location of Sensor
• Noninvasive/noncontact - blood gases,
electrolytes, etc.
• Noninvasive/contact - ECG electrodes,
temperature, oximetry, etc.
• Invasive/short-term - EMG electrodes,
catheter, etc.
• Invasive/long-term - Pacemaker, FES
electrodes, etc.
Measuring a Physiological Potential
Voltage (V)
Measurable Physiological Potentials
0.1
1
10
100
1,000
Frequency (Hz)
10,000
Electrodes
• Metal conductor in an electrolyte solution
• Transduces an ion to an electron current
• Characterized by electrode potentials
Surface Electrodes
Foam
Tan Weave
Self Adhesive
Multi Use
Electrode Potential
Metal
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- - - - - - - - - - -- - -+ + + + + + + + +
+ + + + + +
- +
- + - + - +
- +
-
- - -
-
-
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+
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+
+
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+
+
+
- +
Skin
Ehc
- - - -
Electrolyte
+Gel
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+
- -
-
+
+
+ -
+
Half-cell
potential
Ag/AgCl Electrodes
• Very important clinically and in research.
• Cl is the primary charge carrier and is
present in tissue.
• Low noise electrodes.
PHYSICAL MEASUREMENTS
Force, acceleration, displacement
Temperature: systemic, thermography
Pressure: blood, pulmonary
Volume: blood, pulmonary
Flow: blood, air
Blood oxygen content (oximetry)
Linear Variable Differential Transformer
Ferrite Core
Differential Capacitive Transducer
Electromagnetic Flowmeter
Fm  qu  B
Fc  qE
B
A
E
u
B
Fc  Fm
qE  quB
V
E   uB
L
V
u
BL
V = potential diff
L = vessel width
A
Flowrate  Au 
V
BL
Electromagnetic Flowmeters
(a)
(b)
(a) Signals without noise are uncorrupted. (b) Interference superimposed on
signals causes error. Frequency filters can be used to reduce noise and
interference.
(a)
(b)
(c)
Figure 1.15 (a) Original waveform. (b) An interfering input may shift the baseline.
(c) A modifying input may change the gain.
(a)
(b)
Figure 1.16 Data points with (a) low precision and (b) high precision.
(a)
(b)
Figure 1.17 Data points with (a) low accuracy and (b) high accuracy.