Blood Flow Detection
> by the Doppler principle
> by Electromagnetic principles
© D. J. McMahon 2014
rev 141114
Classic Doppler Shift example
If the signal source is approaching, the frequency detected
by the observer continuously shifts higher:
If the signal source is receding, the frequency detected
by the observer continuously shifts lower.
Doppler Flow Detection:
Signals in the range of 4 to 12 MHz are sent thru a
vessel at an acute angle. Formed elements in the
blood (red cells, etc) reflect most of the signal, and
shift the frequency because of their motion
(Doppler Shift).
The reflected signal is detected, and the difference
between the transmitted and reflected signals
(which is in the audible frequency range) is filtered,
amplified, and sent to a speaker.
The greater the blood flow, the higher the
Doppler shift and the audio frequency.
(Human hearing range is ~20 Hz to ~20,000 Hz.)
V cosq
FD = -2 ________ FS
C
FD
V
C
FS
= Doppler shift
= speed of blood flow
= speed of sound in blood
= speed of transmitted signal
Typical portable Doppler flow detector:
The more peripheral (shallow) the vessel being
studied, the higher the frequency of the Doppler.
The deeper the vessel, the lower the frequency
needed.
9 MHz – 10 MHz is usually used for arteries < 1cm deep.
5 MHz for vessels ~ 2 cm deep.
2 MHz – 4 MHz for larger, deeper vessels and fetal hearts.
The size of the probe is inversely proportional
to its frequency –
A 10 MHz probe is < 1 cm in diameter.
A 2 MHz probe is ~4 cm in diameter.
Transmit / receive crystals in a 10 MHz Doppler probe:
Multi-probe Doppler unit:
Uses plug-in modules for
several probe sizes.
Single-probe unit for
peripheral vessels only.
Doppler probes (and any ultrasound probe) must
be coupled to the skin with a water-based gel. The
slightest air gap or bubble will significantly
attenuate the signal.
Doppler for fetal heartbeat detection.
(NOT the same as ultrasound imaging.)
Some units have bi-directional probes, to confirm the
direction of blood flow.
Model UW7
Doppler flow detectors can also be used to take
blood pressures manually:
The first audible pulse is more detectable with Doppler
shift, especially in patients with compromised vessels.
Doppler probes are also used for direct
measurement of flow in a vessel, usually during
vascular or cardiac surgery, to verify successful
vessel repair.
Transonic vascular flow meters:
Implanted disposable Doppler probe for post-op
monitoring of vessel patency.
Electromagnetic Flow Detection:
When passing thru a magnetic flux (B), blood flow (u) creates
voltage (e) between points at right angles to the flow. That
voltage is directly proportional to flow.
Electromagnetic flow detection -
Magnet is AC driven, and the generator creates a feedback
voltage to balance-out transformer voltage on the voltage
created by flow. That voltage is amplified, filtered, and
displayed as flow.
Cross-section of a practical electromagnetic flow probe.
The Flowmeter Market:
Transonic
Medasonics
Parks
Koven
Hokansen
Huntleigh
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