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Introduction to Biosignal Detection Chapter 5. Direct Flow
Biomedical Information Technology Lab 1
5. Direct Flow
• Hemodynamic properties• Electromagnetic flowmeter• Ultrasonic flowmeter
– Transit-time flowmeter– Ultrasonic Doppler flowmeter
• Laser Doppler flowmeter• Dilution method• Thermal flowmeter• Red blood cell speedometer
Object Quantities
Amount of flow• Volume flow rate
– volume crosses a surface within a unit time
– SI system m3/s
– physiological measurement L/s, L/min, mL/min
• Mass flow rate– mass crosses a surface within a unit time
– kg/s
Blood Flow in Vessels
Capillary AortaArteriole
Carotid arteryFlow rate and velocity
vary in different size of blood vessels
No one measurement method is available to the whole range of flow rates or velocity.
Blood Flow in Tissues
Tissue blood flow varies significantly for different tissues and physiological conditions
Velocity Profile in a Vesselparabolic velocity profile
Backward flow
Velocity
Flow rate
1 /
p
large artery
a long straight tube having a circular cross section
Steady and laminar flow
Pulsatile flow
If the magnetic field and the velocity are uniform, then:
Electromagnetic Flowmeter
V: potential difference between two electrodes
d: the distance between two electrodes
U: flow velocityB: magnetic flux density
Flow rate
BUqF Electromotive force
Principle = when a fluid containing electric charges q flows in a magnetic field B with velocity U, an electromotive force F will be exerted on the particle.
BUdV
B
dVUdQ
44
2
Introduction to Biosignal Detection Chapter 5. Direct Flow
Biomedical Information Technology Lab 2
Three Types of Electromagnetic Flowmeter
A) Cannular:vessel is cut transversely, and its ends are attached to the rigid inflow and outflow tubes of the probe.
B) Perivascular:a rigid cuff that contains magnet and electrodes and can be slipped around the vessel.
C) Intravascular:magnet coils and sensing electrodes are fitted to tip of catheter, and the catheter is inserted into the vessel.
vessel
Cannular Electromagnetic Flowmeter Probe
Perivascular Electromagnetic Flowmeter Probe
Electromagnetic Velocity Catheter
Ultrasonic Blood Flowmeter
• A sound wave propagating in a moving medium is
affected by the velocity of the medium
• Sound scattered by a moving object is affected by
the velocity of the scattering object
• Both phenomena can be used to measure blood
flow velocity or flow rate in a blood vessel
• Transit time flowmeter and Doppler flowmeter
Propagation of Ultrasound
• Ultrasound frequency > 20kHz
• Sound velocity c, frequency f and wave length λ
• When sound pressure p and sound particle velocityU (or medium velocity), characteristic impedancebecomes Z=p/U=ρc (ρ medium density)
• Sound wave will be partially reflected by differentcharacteristic impedance on boundary betweentwo different media
fc
ir pZZ
ZZp
21
21
incident density of sound wavereflection density
of sound wave
Introduction to Biosignal Detection Chapter 5. Direct Flow
Biomedical Information Technology Lab 3
Typical Values of Various TissuesUltrasonic Transit Time Method
c»U
transceiver
transceiverD: distance between transmitting
and receiving elementsU: fluid velocityθ: beam angle with respect to the
axis of the conduitc: sound velocity±: downstream(+) or upstream(-)
Transit Time
Difference in Transit Time
Difference in Phase
T ω: angular frequency of
the sound wave
U
t
c
QtSGA cos
cossin
2Received signal:
A: signal amplitude, G: coupling constant, S: crystal’s surface area, : beam angle, Q: total flow
Wide-beam Technique
Ultrasonic beam is wider than the vessel diameter
Transit-time Ultrasonic Flowmeter
End view
Side view
1 ∼ 32 mm
fsDoppler Shift
fs
If θ=φ,
f2
f1 sfc
Ucf
cos1
12 cosf
Uc
cf
cosUc
Doppler shift in the scattered wave due to the moving red blood cells in the blood stream Normal femoral arterial flow
Spectrum of Doppler Signal
Introduction to Biosignal Detection Chapter 5. Direct Flow
Biomedical Information Technology Lab 4
Methods for Specific Site
Range discriminating Doppler flowmeter aims to select special site
Pulse DopplerRandom signal Doppler
Pulse Doppler Method
By gating the received signal at a specific time interval, the signal reflected by the object in a specific range can be obtained selectively
Random-signal Doppler System
Blood vessel
The transmitted wave is modulated by a noise wave, and correlation is taken between the received wave and the delayed noise wave with a definite delay time.
Among the received waves, only the reflected wave components from an object in a definite range, at which the transit time is just the delay time, can have a strong correlation.
Perivascular Doppler Flowmeterimplantable use in experimental animals and clinical use during surgical operation.
filled with hydrophilic polymer
Pulse Doppler Catheter
mm
Circular crystal
Pulse Doppler Catheter
flexible steerable guide wire
flexible steerable guide wire
Introduction to Biosignal Detection Chapter 5. Direct Flow
Biomedical Information Technology Lab 5
Indicator Dilution Method
indicators = color dyes, radio isotopes, electrolytes, or heat
upstream downstream
dttcQI
0
amount of injected indicator
The process of making weaker or less concentrated
Concentration of the indicator at time t
Indicator Dilution Curve
dttcIQ
0
Initial circulationcomponent
flow rate
flow
If the flow rate is constant
Empirical Approximation ofInitial Circulation Component
Two-point method Forward triangle method
Half width method
Thermodilution Catheter
Swan-Gantz
Heat temperature change
Cold fluid (saline or isotonic dextrose solution, 0oC, 10ml)
Cardiac Output Monitoring
cold saline or dextrose solution
mixing
temperature decrease
Continuous Thermodilution Catheter
upstream
downstream
injected fluid temperature measurement
Introduction to Biosignal Detection Chapter 5. Direct Flow
Biomedical Information Technology Lab 6
mbUaH 5.0m
2RIH H is heat dissipation
a and b are constants and determined by calibration,U is flow velocity
At a constant temperature operation
Heat Dissipation MethodThe rate of heat dissipation from a heated element placed in the blood stream depends on the flow velocity.
Thermistor Velocity ProbeTwo thermistors are commonly used in the flow probe. One measures the fluid temperature, and the other is heated to a higher temperature, and the temperature difference between the heated thermistor and the fluid is kept constant.
a fine wire for heating is wound around the sensing element
Hot-film Velocity Probe
The thin metal film is maintained at a constant temperature of about 5℃above the flowing fluid temperature by employing the feedback bridge
Hot-film Velocity Probe
Pyrex glass
Fick Method for Cardiac Output (CO) Measurement
Balance for the volume of indicator -Fick’s principle
Blood ejected from the right ventricle travels along the pulmonary arteries and in the capillaries of the lungs uptakes oxygen from the alveoli, then through the pulmonary veins gets the left atrium.
indicator
TT
Ti dtFcdtFcQ
dt
dV0 220 11
Vi is the volume of the indicatorF1 and F2 are flow ratesc1 and c2 are indicator concentrationsQT is the amount of indicator added through breath
Tcc
Q
T
dtF
T
dtFCO T
TT
12
0 20 1
assuming the overall variation of Vi negligible
oxygen uptakemt
U
Red Blood Cell Velocity
0
Projection microscope screen
Introduction to Biosignal Detection Chapter 5. Direct Flow
Biomedical Information Technology Lab 7
mtU
Red Blood Cell Velocity
dttftf 21
the average transit time for a red blood cell to cross a fixed distance is obtained by seeking the maximum cross-correlation function.
mU
Red Blood Cell Velocity
time interval is fixed, and the transit distance in this time interval is determined by searching maximum spatial cross correlation
Red Blood Cell Velocity
f=U/d
f: frequency component in light signalU: velocityd: grating constant
When a red blood cell moves across the grating with a velocity U, light signals have a frequency component of f = U/d
Discrimination of flow direction
Mechanical Flowmeters
tUdt
tdU P
dt
dQLF
2
conductive
nonconductive
The flowing fluid has kinetic energy, and may cause a force or displacement in the sensing elementwhen it is disturbed by a mechanical element, corresponding to the flow rate or flow velocity.
(a-c) pressure difference
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