 |
| Fig.1.24 |
The aliasing phenomenon occurs when the abnormal velocity exceeds
the rate at which the pulsed wave system can record it properly.
PW Doppler spectral tracing in Figure
1.24 from an individual with aortic insufficiency with the
transducer positioned at the apex. In this situation, abnormal
diastolic flow is detected toward the transducer and recorded
in a positive, or upwards direction. The system first detects
a pulsed (and aliased) spectral profile. After the fourth beat,
the system is switched into CW and the full profile is recognized.
The aliased portion in the first three beats is cut off the top
of the velocity spectrum and replaced at in the reverse channel,
or below the baseline (open arrow).
 |
| Fig.1.25 |
The phenomenon of aliasing is best explained using a simple example
(Fig.
1.25). A mark is placed on a turning wheel and the wheel rotates
in a clockwise fashion at a speed of one turn every four seconds.
If the sample rate (or pulse repetition frequency) is one sample
per second the mark is recorded at each progressive 90 degree
position. The final recording would then show the proper clockwise
direction of motion of the wheel (Fig.
1.25 left column).
If the sample rate (or pulse repetition frequency) is slowed
to only one sample every three seconds a strange phenomenon occurs
(Fig.
1.25 right column). Note that the mark is moving 180 degrees
between sampling times and that while actually turning clockwise
the recording makes the wheel appear to be moving in the opposite,
or counter-clockwise, direction. This is also the reason why propellers
and wagon wheels appear to go backwards in movies as the film
frame rate is too slow to accurately keep up with these rapidly
moving structures.
2004