Doppler Flow Meter

Doppler Flow meter or Doppler ultrasound flow meter is a device for measuring the volumetric flow of liquid through a tube ( blood through a vessels). It works on the principle ultrasound and doppler phenomenon, it can only be used for liquids containing particles (erythrocytes) or bubbles.

Basic parts
A Doppler flowmeter consists of a probe and a computer.

The volume required to detect flow can only be determined from measurements in a pulsed Doppler system. The probes of the flowmeters are therefore equipped with one piezoelectric crystal typical for this system. The probe is connected to a computer that processes the signals. The output values ​​are read either from the device itself or from the connected screen. Probes with a frequency of 4 or 5 MHz are used to detect deeper-seated vessels, and usually 8 or 10 MHz to examine superficial vessels.

The principle of operation of the device
An electrical impulse is sent from the device's computer to the probe's crystal. The electric voltage based on the inverse piezoelectric effect deforms the crystal, which creates ultrasound. It passes from the probe into the body, reflects off the moving erythrocytes with a different frequency than the initial one (according to the doppler effect) and heads back to the probe. The crystal is mechanically deformed by this   ultrasonic wave and generates an electric voltage (piezoelectric phenomenon). The electrical impulse created in this way is compared in the computer with the original impulse.

Output values
The flow rate is calculated from the speed of blood that has flowed through the volume of a given vessel profile in a certain time.
 * The speed of moving particles (change in frequency) – is given by the average speed, which is determined from the speeds in the whole vessel (the speed is not the same everywhere. It is the slowest at the walls, the fastest in the center of the lumina). So calculated from the integral of the Doppler spectrum. To accurately determine the mean velocity, it is necessary to have the cursor correctly set so that it covers the entire diameter of the vessel.
 * The depth of the vessel – determined based on the time interval between the sending of the signal and the beginning of its reception.
 * The diameter of the vessels (from which we can determine the volume) - determined from the time interval between the beginning and end of receiving the signal..

Uses in medicine

 * Examination of the lower limbs of diabetic patients.
 * Determination of vessel lumen and blood flow velocity for the diagnosis of related problems (Aneurysm, strokes, thrombosis, porušené impaired organ functions,...).

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 * Doppler sonography