Biomedical Engineering Reference
In-Depth Information
Fig. 2.3.
Drawings demonstrating the Doppler effect using reflected sound from
a target
within the beam. Thus, to overcome these problems, the pulsed-wave Dopp-
ler technique has been invented. By sending a short burst of ultrasound, the
frequency of that burst is distorted if the target from which it is reflected is
moving. Similarly to the continuous-wave Doppler technique, when the target
is moving, the frequency of the received burst of ultrasound alters, reflecting
the direction of the movement and the velocity. One may determine velo-
city just as with the continuous wave approach. The major disadvantage of
pulsed-wave Doppler is that the velocity that one can measure is limited.
The pulsed system inherently has a pulse repetition frequency (PRF). The
PRF determines how high a Doppler frequency the pulse system can detect
(the Nyquist limit). A combination of Doppler and two-dimensional echo was
developed for flow mapping (color Doppler). This approach utilizes multiple
Doppler gates, which register the Doppler signals rapidly. Because the flow
patterns are dramatically displayed using the flow mapping technique, this
approach is currently the most frequently used Doppler technique among the
three techniques in ultrasonography laboratory.
2.1.3
Recent Advances in Ultrasound Imaging
Ultrasound imaging has seen dramatic improvements in image quality over
recent years with the utilization of harmonic frequencies in the imaging of
both tissue and contrast agents. Although clinical validation of this new tech-
nology is ongoing, it is worthwhile to consider some of the physical principles
underlying its use. When sound traverses through the body, it is compo-
sed of a group of frequencies that define its spectral content. If the ultra-
sonic wave consists of a single frequency (the fundamental frequency), then
it forms a sine wave passing through the tissue. Harmonic frequencies are
those that occur at a multiple of the fundamental frequency, with the second
harmonic referring to twice the fundamental frequency. Ultrasonic energy
