Conventional
two-dimensional echocardiographic systems emit high frequency bursts
of sound (ultrasound) into the tissues. In standard echocardiographic
imaging a given pulse of ultrasound is transmitted into the body
and then reflected back from the various tissues. Since the speed
of sound in tissue is known (approximately 1540 m/sec), a standard
ultrasound imaging system can wait for a given time for the transmitted
pulse to travel to a target (time X) and then back (time 2X) and
the given target will be received and recorded. In complex two-dimensional
imaging systems this alternating process is repeated in a variety
of directions thousands of times each second. The best ultrasound
images are made when the target is perpendicular (or specular) to
the sound waves.
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| Fig.1.4 |
Frequency is a fundamental characteristic of any wave phenomenon,
including sound, and refers to the number of waves that pass a given
point in one second (Fig. 1.4). It is usually described in units
of cycles per second or Hertz (Hz). Thus, the top of the illustration
in Figure 1.4 shows an example of a waveform of 10 Hz while the
one below is 5 Hz. Ultrasound is emitted in waveforms of a known
frequency.
Doppler echocardiography, on the other hand, depends entirely on
measurement of the relative change in the returned ultrasound frequency
when compared to the transmitted frequency. Depending on the relative
changes of the returning frequencies, Doppler echocardiographic
systems measure these characteristics of disturbed flow: direction,
velocity and turbulence. This enables examiners to differentiate
between normal and abnormal flow patterns and, in some cases, to
quantitate those characteristics that are helpful in determining
the severity of abnormal flow states.
Most readers understand frequencies in relationship to the pitch
of audible sound. The relationship between pitch and frequency is
simple: the pitch of any given sound is proportional to its frequency.
As sound wave frequency increases, pitch gets higher; and as frequency
decreases, pitch declines.
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| Fig.1.5 |
Doppler systems are totally dependent on the changes in the frequency
of the transmitted ultrasound that result from the encounter of
the wavefront with moving red blood cells. Figure 1.5 shows a transducer
on the left that is emitting a given frequency of ultrasound toward
the right and into the tissues. The transmitted sound waves encounter
a group of red cells moving toward the transducer and are reflected
back at a frequency higher than that at which they were sent producing
a positive Doppler shift. The opposite effect occurs when a given
frequency sent into the tissues encounters red cells moving away.
The result is the return of a frequency lower than that transmitted,
and the Doppler shift is negative.
2004