Pulsed wave (PW) Doppler systems use a transducer that alternates
transmission and reception of ultrasound in a way similar to the
M-mode transducer (Fig.
1.18). One main advantage of pulsed Doppler is its ability
to provide Doppler shift data selectively from a small segment
along the ultrasound beam, referred to as the "sample volume".
The location of the sample volume is operator controlled. An ultrasound
pulse is transmitted into the tissues travels for a given time
(time X) until it is reflected back by a moving red cell. It then
returns to the transducer over the same time interval but at a
shifted frequency. The total transit time to and from the area
is 2X. Since the speed of ultrasound in the tissues is constant,
there is a simple relationship between roundtrip travel time and
the location of the sample volume relative to the transducer face
(i.e., distance to sample volume equals ultrasound speed divided
by round trip travel time). This process is alternately repeated
through many transmit-receive cycles each second.
This range gating is therefore dependent on a timing mechanism
that only samples the returning Doppler shift data from a given
region. It is calibrated so that as the operator chooses a particular
location for the sample volume, the range gate circuit will permit
only Doppler shift data from inside that area to be displayed
as output. All other returning ultrasound information is essentially
Another main advantage of PW Doppler is the fact that some imaging
may be carried on alternately with the Doppler and thus the sample
volume may be shown on the actual two-dimensional display for
guidance. PW Doppler capability is possible in combination with
imaging from a mechanical or phased array imaging system. It is
also generally steerable through the two-dimensional field of
view, although not all systems have this capability.
In reality, since the speed of sound in body tissues is constant,
it is not possible to simultaneously carry on both imaging and
Doppler functions at full capability in the same ultrasound system.
In mechanical systems, the cursor and sample volume are positioned
during real-time imaging, and the two-dimensional image is then
frozen when the Doppler is activated. With most phased array imaging
systems the Doppler is variably programmed to allow periodic update
of a single frame two-dimensional image every few beats (Fig.
1.19). In other phased arrays, two-dimensional frame rate
and line density are significantly decreased to allow enough time
for the PW Doppler to sample effectively. This latter arrangement
gives the appearance of near simultaneity.
The sample volume is really a three-dimensional, teardrop shaped
portion of the ultrasound beam (Fig.
1.20). Its volume varies with different Doppler machines,
different size and frequency transducers and different depths
into the tissue. Its width is determined by the width of the ultrasound
beam at the selected depth. Its length is determined by the length
of each transmitted ultrasound pulse.
Therefore, the farther into the heart the sample volume is moved,
the larger it effectively becomes. This happens because the ultrasound
beam diverges as it gets farther away from the transducer.
The main disadvantage of PW Doppler is its inability to accurately
measure high blood flow velocities, such as may be encountered
in certain types of valvular and congenital heart disease. This
limitation is technically known as "aliasing" and results in an
inability of pulsed Doppler to faithfully record velocities above
1.5 to 2 m/sec when the sample volume is located at standard ranges
in the heart (Fig.
1.21). Aliasing is represented on the spectral trace as a
cut-off of a given velocity with placement of the cut section
in the opposite channel or reverse flow direction. Because aliasing
is so common in disease states, it will be considered in more
detail in the next section.
The spectral outputs from PW and CW appear differently
1.22). When there is no turbulence, PW will generally show
a laminar (narrow band) spectral output. CW, on the other hand,
rarely displays such a neat narrow band of flow velocities even
with laminar flow because all the various velocities encountered
by the ultrasound beams are detected by CW.
It can usually be said that when an operator wants to know where
a specific area of abnormal flow is located that pulsed wave Doppler
is indicated. When accurate measurement of elevated flow velocity
is required, then CW Doppler should be used. The various differences
between pulsed and continuous wave Doppler are summarized in Figure