The fact that
makes frequency of the Doppler effect more than just an interesting
curiosity is that it actually provides a method that is used to
measure the direction and speed of moving red blood cells. Clinically
we are most interested in measuring velocity since, as mentioned
above, it is altered in disease states.
 |
| Fig.1.7 |
A Doppler system then compares the transmitted waveform with
the received waveform for a change in frequency as shown in Figure
1.7. These are called "phase shifts" and they are automatically
determined within the Doppler instrument. If there is a higher
returning frequency (+AP) then the flow is called a "positive
Doppler shift" and represented as moving toward the transducer.
If there is a lower returning frequency (-AP) then the flow is
called a "negative Doppler shift" and represented as moving away
from the transducer. All components of the Doppler equation, except
velocity, are readily measured by the Doppler instrument.
 |
| Fig.1.8 |
The Doppler equation may be rearranged to solve for velocity
of blood movement as shown in Figure 1.8. The angle ? (the angle
the Doppler beam is incident to flow) may be measured or may be
assumed to be parallel depending upon orientation of the beam
by the system operator.
The Doppler device can be regarded as a complex speedometer designed
to detect red cell motion (i.e., blood flow) and measure its velocity.
What is important to recognize is that:
| FREQUENCY SHIFT >>> DOPPLER EQUATION >>> VELOCITY DATA |
2004