Continuous-Wave Doppler Echocardiography
CW Doppler uses two separate crystals: one to continuously emit
ultrasound and one to continuously receive it. CW Doppler is basically PW Doppler
with an infinite pulse repetition frequency that eliminates the problem of aliasing
( Fig. 33-5
). However, this
infinite pulse repetition rate allows insufficient time for the first pulse to return
to the transducer before the next is emitted. Consequently, the ultrasonograph cannot
determine which pulse of sound was frequency shifted and therefore cannot precisely
define the location of the moving target. Nonetheless, the maximum velocity can
prove to be vital information: by simplification of the Bernoulli equation, Holen
and colleagues and Hatle and coworkers have proven that
ΔP = 4V2
where ΔP is the peak gradient across a stenosis and V is the maximum velocity
determined by CW Doppler in meters per second.[10]
[11]
Thus, CW Doppler defines higher blood flow
velocities than PW does, but unlike the latter, CW Doppler cannot precisely define
the location of the velocities.
 |
