two main types of Doppler echocardiographic systems in common use
today, continuous wave and pulsed wave. They differ in transducer
design and operating features, signal processing procedures and
in the types of information provided. Each has important advantages
and disadvantages and, in our opinion, the current practice of Doppler
echocardiography requires some capability for both forms.
Continuous Wave Doppler
Continuous wave (CW) Doppler is the older and electronically more
simple of the two kinds. As the name implies, CW Doppler involves
continuous generation of ultrasound waves coupled with continuous
ultrasound reception. A two crystal transducer accomplishes this
dual function with one crystal devoted to each function (Fig. 1.17).
The main advantage of CW Doppler is its ability to measure high
blood velocities accurately. Indeed, CW Doppler can accurately record
the highest velocities in any valvular and congenital heart disease.
Since velocities exceeding 1.5 m/sec are frequently seen in such
disorders, accurate high velocity measurement is of particular importance
for allowing the recognition of the full abnormal flow profile.
It is also of considerable importance for the quantitative evaluation
of abnormal flows, as will be seen later.
The main disadvantage of CW Doppler is its lack of selectivity or
depth discrimination. Since CW Doppler is constantly transmitting
and receiving from two different transducer heads (crystals) there
is no provision for imaging or range gating to allow selective placing
of a given Doppler sample volume in space. As a consequence, the
output from a CW examination contains Doppler shift data from every
red cell reflecting ultrasound back to the transducer along the
course of the ultrasound beam.
Thus, true CW Doppler is functionally a stand-alone technique whether
or not the capability is housed within a two-dimensional imaging
transducer. The absence of anatomic information during CW examination
may lead to interpretive difficulties, particularly if more than
one heart chamber or blood vessel lies in the path of the ultrasound
It is possible , however, to program a phased array system to perform
both two-dimensional and CW Doppler functions almost simultaneously.
The quasi-simultaneous CW-imaging uses a time sharing arrangement
in which the transducer rapidly switches back and forth from one
type of examination to the other. Because this switching is done
at very high speeds, the operator gets the impression that both
studies are being done continuously and in real-time. During the
imaging period, no Doppler data is being collected, so an estimate
is generated, usually from the preceding data. During the Doppler
collection period, previously stored image data is displayed. This
arrangement usually degrades the quality of both the image and Doppler