Biomedical Engineering Reference
In-Depth Information
Fig. 2.2.
A spatially oriented M-mode examination for two-dimensional imaging
together (
C
). In combination, these dots reveal the shape and size of the
object being examined (
D
).
2.1.2
The Doppler Technique
The Doppler technique is playing an increasingly important role in ultrasound
examination, particularly of the heart. There have been significant advances
in Doppler instrumentation and in our understanding of how this information
can be clinically useful. The Doppler examination is based on the Doppler
effect first described by Christian Johann Doppler in 1842. If a source of sound
is stationary, then the wavelength and frequency of the sound emanating from
that source are constant. If, however, the source of the sound is moving toward
one's ear, then the wavelength is decreasing and the frequency is increasing.
If the source of sound moves away from the ear, then the wavelength is
increasing and the frequency decreasing. Figure 2.3 demonstrates how one
can use reflected sound to determine the motion of a target that reflects the
ultrasound. Briefly speaking, the Doppler shift, or frequency, represents the
difference between the received and the transmitted frequencies. The relation
between the reflected frequency (
f
r
) and transmitted frequency (
f
t
)differs
depending on the direction of the movement of the target. The faster the
movement, the larger the difference is between
f
r
and
f
t
.
There are at least three kinds of Doppler ultrasound: continuous-wave
Doppler, pulsed-wave Doppler, and color Doppler techniques. Figure 2.3 il-
lustrates the principle of continuous-wave Doppler ultrasound. One uses two
transducers, a transmitter and a receiver, or a transducer with both trans-
mitter and receiver elements. Because the ultrasonic beam is constant, all the
moving targets within the beam produce Doppler signals. There is no way
knowing where the individual target might be with relation to the transdu-
cer. In addition, one cannot determine how many moving targets there are
