Doppler Flow

The Doppler principle of flow is expressed by the following relationship: V=cDf

2ftCosq where V is blood flow velocity

Df is the frequency shift between reflected and emitted signals ft is the frequency of the emitted ultrasound q is the angle between the ultrasound beam and the direction of blood flow c is the velocity of sound (1450 m/sec in soft tissues)

The echocardiography machine can display both continuous wave and pulsed wave Doppler representations of blood flow across valves or through vessels.

Continuous wave Doppler (CwD) employs continuous pulses of sound waves, and thus the exact depth of flow cannot be determined. However, blood flows at high velocities can be measured accurately.

Pulsed wave Doppler (PwD) uses short bursts of sound which permits precise location of flow. However, blood flows at high velocities cannot be determined accurately. Instead, at high velocities a phenomenon called aliasing, or wraparound, occurs with PwD.

For CwD and PwD flow above the baseline indicates flow towards the transducer whereas flow below the baseline indicates flow away from the transducer. However, for PwD, high velocity flow will be displayed on both sides of the baseline (aliasing or wrap-around), regardless of the real direction of flow.

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