o_1977r8vv9vk1ts2ms0kd8pksa.pdf

10 D. Maulik
T = 1/f
λ
Amplitude
Fig. 2.2. Propagation of sound. Compression and rarefaction
of the molecules in a medium associated with the
propagation of a pressure wave related to sound or ultrasound
transmission. Horizontal axis represents the distance;
vertical axis represents the magnitude of pressure wave deflections
around the baseline. Upward deflection represents
the positive pressure changes and downward deflection
the negative pressure changes
Propagation Speed of Sound
The propagation speed of sound in a medium is the rate
of change of position of the sound wave in unit time in
that medium. It is called velocity when the direction of
motion is also specified. The speed of ultrasound propagation
in a medium is directly related to the bulk
modulus of elasticity and density of the medium. A
change in transmitting frequency within the range of
clinical usage does not alter the speed. Although speed
of sound is affected by temperature, no appreciable effects
are known in clinical applications. With diagnostic
ultrasonography, the propagation speed information
is used to compute the depth distance from the
echo return time. It is also a component of the Doppler
equation that allows determination of speed of the scatterer
from the Doppler frequency shift. The average
propagation speed of ultrasound in soft tissues is approximately
1,540 meters per second (m/s).
Wavelength, Frequency, Pulse
The wavelength of sound is comprised of one cycle of
compression and rarefaction. Therefore it is the distance
between a pair of consecutive peaks or troughs
of adjacent pressure waves (Fig. 2.3). The frequency of
sound is the number of such cycles occurring in 1 s.
One cycle is called a hertz (Hz). Wavelength and frequency
are inversely related:
k ˆ c=f
where k represents the wavelength, c the speed of
sound, and f the frequency. As the propagation speed
Distance
Fig. 2.3. Wavelength of sound. Horizontal axis represents
the distance and the vertical axis the magnitude of pressure
wave deflections around the baseline. The peak-topeak
distance (k) between the consecutive pressure waves
is one wavelength
of sound in tissue is known and is relatively constant,
one can determine the frequency from the wavelength
and vice versa (Table 2.1). The duration of one cycle,
or wavelength, of sound is called its period (Fig. 2.4).
Period is measured in seconds and microseconds. It
is inversely related to the frequency:
T ˆ 1=f
where T is the period, and f is the frequency of
sound.
Pulsed Doppler (both spectral and color flow mapping)
and pulse echo imaging systems transmit ultrasound
waves in pulses. The number of such pulses
transmitted per second is known as the pulse repetition
frequency (PRF) (Fig. 2.5). The length of one ultrasound
pulse is known as the spatial pulse length
(Fig. 2.6). Pulse length varies according to the mode
of ultrasound. With pulsed Doppler ultrasound, the
pulse length ranges from 5 to 30 cycles. In contrast,
the length is much shorter for pulsed echo imaging
system, as they generate two to three cycles per pulse.
Table 2.1. Frequency and corresponding wavelength of
commonly used Doppler transducers in obstetrics
Frequency (MHz) Wavelength (lm)
2.0 770
2.5 616
3.0 513
3.5 440
4.0 385
4.5 342
5.0 308
5.5 280
6.0 257
6.5 236
7.0 220
7.5 205