Introduction to Acoustics

a)
b)
c)
100
0
V
mV
10
0
a = 13
a = 3
Off
d) mV
100
0
200 µs
15 cm depth PRF = 5kHz
200 µs 200 µs 200 µs 200 µs
Time gain = 2 a
Fig. 21.22a–d Time gain compensation (TGC). (a) Anew
ultrasound pulse is transmitted each pulse repetition period
(200 µs in this case). (b) Echoes from shallow and deep
depths return at known times because of the speed of ultrasound
and the expected round-trip travel time. The later
echoes received from deeper tissues are weaker than the
echoes received from shallow tissues because the echoes
from deeper tissues have passed through more-attenuating
tissue. (c) An amplifier programmed to increase the gain
for deep echoes more than for shallow echoes is used to
compensate for attenuation. This depth gain compensation
(DGC) or time gain compensation (TGC) amplifier is under
operator control. (d) The echoes, after TGC are ready
for demodulation by analog or digital methods
sitive indicator of displacement of the tissue reflecting
the ultrasound.
This simple model places constraints on the ultrasound
examination. First, at any depth, the strongest
echoes from a bone or air interface perpendicular to the
beam are 10 6 (60 dB) stronger than the weakest echoes
from cells in liquid (blood). So, a 10 bit digital echoprocessing
system is the minimum to handle the range
of echo strengths expected in an image. Second, because
the speed of ultrasound is about 1.5mm/µs in most tissues,
echoes returning from 15 cm deep arrive 200 µs
after ultrasound pulse transmission. A new pulse cannot
be sent into the tissue along the same line or along
Medical Acoustics 21.4 Methods of Medical Ultrasound Examination 861
100
0
V
15 cm depth PRF = 5kHz
200 µs 200 µs 200 µs 200 µs 200 µs
100 V
2 µs
Imaging
(broad band)
Doppler
(narrow band)
Transcranial doppler
(high energy, low MI)
Fig. 21.23 Different ultrasound transmit bursts for imaging
and Doppler applications. Short broadband transmit bursts
are used for B-mode imaging; long narrow-band transmit
bursts are used for Doppler applications. A longer transmit
burst contains more energy if the maximum acoustic
negative pressure (mechanical index) are limiting factors.
A short transmit burst allows better depth resolution and
also spreads the diffraction side-lobes of the beam pattern,
suppressing lateral ambiguity. A longer transmit burst
is preferable for Doppler applications where echoes from
blood are weak and coherence length is important for
demodulation
another line until all of the echoes from the first pulse
have returned. So, the maximum pulse repetition frequency
(PRF) for an examination to a depth of 15 cm
is 5 kHz (1/200 µs). Ultrasound energy is attenuated to
1/4 of the incident energy in most body tissues after
passing through a depth of 80 wavelengths. So after
passing through a depth of 400 wavelengths and returning
through 400 wavelengths, the echo strength is only
0.25 10 or 10 −6 of the strength of a similar echo from the
shallowest tissue. To compensate for this attenuation,
an amplifier must amplify the deep echoes 60 dB more
than the shallow echoes (Fig. 21.22). An alternative is
to digitize the echoes with a 20 bit digitizer, 10 bits for
the dynamic range of the echoes from different tissues
and 10 bits to account for attenuation. In either case,
the maximum depth of an examination is limited to 400
wavelengths of ultrasound. An ultrasound depth resolution
cell (a pixel) is equal to about twice the wavelength
of ultrasound. So, an ultrasound image can contain 200
pixels in the depth direction (Table 21.5). These con-
Part F 21.4