Blood Flow Detection > by the Doppler principle > by Electromagnetic principles © D. J. McMahon 2014 rev

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.)

F D = -2 V cos  ________ C FSFS F D = Doppler shift V = speed of blood flow C = speed of sound in blood F S = 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.

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. Electromagnetic flow detection -

Cross-section of a practical electromagnetic flow probe.

The Flowmeter Market: Transonic Medasonics Parks Koven Hokansen Huntleigh

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