Doppler ultrasound is a useful tool in measuring blood flow through stenosed or regurgitant valves.
The principle is simple - in unobstructed valves, the blood flows through in an orderly laminar fashion,
with red blood cells aligned in a clearly defined column. The red cells all flow at the same velocity.
However, when there is narrowing, the flow of blood becomes turbulent, resulting in a disarray of red
cells with different red cells demonstrating different velocities. The figure below demonstrates this:
The principle is simple - in unobstructed valves, the blood flows through in an orderly laminar fashion,
with red blood cells aligned in a clearly defined column. The red cells all flow at the same velocity.
However, when there is narrowing, the flow of blood becomes turbulent, resulting in a disarray of red
cells with different red cells demonstrating different velocities. The figure below demonstrates this:
Right, that was the easy bit. Now we get to some real physics!
Imagine a probe being placed directly onto the aorta. The probe emits ultrasound waves, which reflect
off the flowing blood cells and are received by the probe. This is translated as velocity. The velocity of
flow ultimately depends on 3 main parameters
off the flowing blood cells and are received by the probe. This is translated as velocity. The velocity of
flow ultimately depends on 3 main parameters
- Velocity of sound in blood (constant) - ‘c’ (or moving object)
- Angle at which ultrasound is emitted relative to flowing column of blood (or moving object) - Ø
- Doppler shift - this is a change in the frequency of the transmitted ultrasound wave (F0) following reflection off the circulating blood column
V = (c / 2F0 cosØ) x Fd
Since cos 90 = 0 and cos 0 = 1 (remember trigonometry?!?), the closer the angle of the probe to 90
degrees, the lower the value of velocity. In other words, if the angle of probe placement is 90 degrees,
the value of V will 0. If the angle of the probe is parallel to the flow of blood, the values will be
accurate or ‘true’. However, it is should be noted that at angles > 30 degrees, a significant error can
occur with velocity measurements.
Continuous wave doppler measures velocities accurately across a valve, in a continuous fashion.
This means a constant signal is being sent out by one crystal, while the reflected signal is received
by another crystal. On the other hand, pulse wave doppler is particularly useful at measuring velocities
at a particular point along the ultrasound beam. This is called the sample volume. The ultrasound wave
is emitted by one crystal, and the reflected wave is received by the same crystal.
This can be seen in the figure below -
degrees, the lower the value of velocity. In other words, if the angle of probe placement is 90 degrees,
the value of V will 0. If the angle of the probe is parallel to the flow of blood, the values will be
accurate or ‘true’. However, it is should be noted that at angles > 30 degrees, a significant error can
occur with velocity measurements.
Continuous wave doppler measures velocities accurately across a valve, in a continuous fashion.
This means a constant signal is being sent out by one crystal, while the reflected signal is received
by another crystal. On the other hand, pulse wave doppler is particularly useful at measuring velocities
at a particular point along the ultrasound beam. This is called the sample volume. The ultrasound wave
is emitted by one crystal, and the reflected wave is received by the same crystal.
This can be seen in the figure below -
Pulse wave dopplers are not accurate at velocities > 2 m/s as aliasing occurs.
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