All Doppler
systems have audio outputs and listening to this is very helpful
during a Doppler examination. The changing velocities (frequencies)
are converted into audible sounds and, after some processing, are
emitted from speakers placed within the machine.
High pitched sounds result from large Doppler shifts and indicate
the presence of high velocities, while low pitched sounds result
from lesser Doppler shifts. Flow direction information (relative
to the transducer) is provided by a stereophonic audio output in
which flow toward the transducer comes out of one speaker and flow
away from the transducer.
The audio output also allows the operator to easily differentiate
laminar from turbulent flow. Laminar flow produces a smooth, pleasant
tone because of the uniform velocities. Turbulent flow, because
of the presence of many different velocities, results in a commonly
high-pitched and whistling or harsh and raspy sound.
The audio output remains an indispensable guide to the machine operator
for achieving proper orientation of the ultrasound beam, even when
Doppler echocardiography is being used in conjunction with an ultrasound
imaging technique. The trained ear can readily appreciate minor
changes in spectral composition more readily than the eye, given
the same information displayed graphically. The major limitation
of audio Doppler outputs is the requirements for subjective interpretation
and the lack of a permanent objective record. The audio output from
a Doppler machine is not the same as that received by a stethoscope
or a phonocardiogram. The sounds detected with a stethoscope are
transmitted vibrations or pressure waves from the heart and great
vessels that are believed to be the result of rapid accelerations
and decelerations of blood. The Doppler audio output, in contrast,
is an audible display of the Doppler frequency shift spectrum produced
by red cells moving in the path of the ultrasound beam. It is a
sound produced by the Doppler machine that does not occur in nature
and, therefore, it does not originate in the heart.
All newer generations of Doppler echocardiography equipment contain
sophisticated sound frequency or velocity spectrum analyzers for
hard copy recording. Most commercially available Doppler systems
display a spectrum of the various velocities present at anytime
and are, therefore, called "spectral velocity recordings.
 |
| Fig.1.9 |
Flow velocity toward the transducer is displayed as a positive,
or upward, shift in velocities while flow velocity away from the
transducer is displayed as a negative, or downward shift in velocities
(Fig. 1.9). Time is on the horizontal axis.
 |
| Fig.1.10 |
The internal working of such systems are complex but the results
are rather simple. When flow is laminar and all the red cells are
accelerating and decelerating at approximately the same velocities,
a neat envelope of these similar velocities is recorded over time
(Fig. 1.10). When flow is turbulent, however, there are many different
velocities detected at any one time (a wide spectrum of velocities).
Such turbulence, produced by an obstruction to flow, results in
the spectral broadening (display of velocities that are low, mid
and high) and an increase in peak velocity as seen in disease states.
This display of the spectrum of the various velocities encountered
by the Doppler beam is accomplished by very sophisticated microcomputers
that are able to decode the returning complex Doppler signal and
process it into its various velocity components. There are two basic
methods for accomplishing this. The most popular is Fast Fourier
Transform (FFT) and the other is called Chirp-Z Transform. These
are simply ways for deciphering, analyzing and presenting vast amounts
of returning data.
 |
| Fig.1.11 |
A better understanding of the complex creation of a spectral
velocity recording helps one in performing and interpreting Doppler
studies. The spectral recording (Fig.
1.11) is really made up of a series of "bins" (vertical axis)
that are recorded over time (horizontal axis).
 |
| Fig.1.12 |
At any given point in time there is a differential speed of
movement of red cells with more red blood cells moving at the
velocity of the most intense bin than are moving at the other
velocities which are represented by the less intense bins (Fig.
1.12). The intensity of any bin refers to the "amplitude" or brightness.
Thus, the velocity spectral analysis is really a complex plot
of the various velocities over time.
 |
| Fig.1.13 |
Since this full spectral display is so highly processed there are
a variety of other outputs that can be displayed and they are electronically
derived from the spectral data (Fig. 1.13). These include mean velocity
and maximum velocity. A line drawn as an envelope around the spectrum
at the peak Doppler shift at any point during the cardiac cycle
is the peak velocity profile. Mean Doppler shift can be estimated
from a line drawn through the darkest part of the spectrum. The
brightness, or "amplitude," may also be displayed. For standard
clinical purposes the full spectrum is generally used.
2004