Biomedical Engineering Reference
In-Depth Information
user interface
communications/
input/output
system
displays
keyboard
display
beamformer/
transmitter
master
clock
cpu
xy
z
switch
amplifiers/beamformer
display
formation
scanner
ultrasound
imaging
system
transducer
back end
FIGURE 16.24
Block diagram of a digital ultrasound imaging system.
perfect focusing at each depth point), and signal processing (often in terms of filtering) pro-
vides the signals needed for the particular selected mode. For ordinary B-mode imaging,
the envelopes of the pulse echoes are extracted, and lines containing the echoes are scan
converted, the process of interpolating the lines containing pulse-echo envelopes into a
filled-out, grayscale image that can be displayed on conventional PC or TV screens such
as the images in Figures 16.15 through 16.17. Additional steps such as log compression
are used to improve the range of pulse-echo amplitudes visible on the screen. The images
in Figures 16.15 and 16.16 are of the most common type: B-mode or brightness mode.
16.2.8 Imaging and Other Modes
Other modes can supply additional information, especially about blood flow. The Doppler
effect takes advantage of the apparent change in the ultrasound frequency caused by the veloc-
ity
the transducer makes with the vessel, as seen in Figure 16.25.
The shift in frequency from the transmitted one,
v
of the blood and the angle
y
f
0
, or the classic Doppler shift frequency,
f
D
, can be expressed as
f
D
¼½
2
ð
v
=
c
Þ
cos
y
f
ð
16
:
47
Þ
0
0
where
c
0
is the sound speed of the intervening medium. Scattering from blood is mainly
from groups of red blood cells. In a vessel there is a distribution of velocities, so what is dis-
played is a Doppler spectrum containing frequencies corresponding to the range of sound
speeds insonified. This scattering is usually not visible by ordinary B-mode imaging but
is detectable by sensitive ultrasound Doppler instrumentation. Doppler shifts are either
detected along the length of an acoustic line selected to intersect a vessel of interest as con-
tinuous wave (CW) Doppler, or a small time interval is placed over the region of interest by
pulsed wave (PW) Doppler as shown in the image of Figure 16.17.
A more global view of blood flow, especially for the chambers of the heart and larger
blood vessels, can be obtained by color flow imaging (CFI) mode. While lacking the
