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Fig. 21 Abdominal CT images. a Portable ultrasound scanner. b Ultrasound scanning probe
(source and receiver), and c conducting gel to facilitate sound wave transmission from the probe
into the body
physical characteristics material through which the sound waves are travelling, and
this property is used to form images in the ultrasound examination. To perform an
ultrasound examination, an ultrasound probe which acts as a source of high-fre-
quency sound waves is put in contact with the body surface (Fig. 21 ). Air gaps
intervening between the probe tip and the body surface are relatively poor con-
ductors of ultrasound waves, so a sound conducting gel between the probe tip
(source/receiver) and the skin to facilitate transmission of the sound waves into the
body tissue. As the sound waves propagating through the tissues encounter transition
points, or boundaries, between different internal body structures, some of the sound
is re
ected back to the probe, and some continues to propagate. The high-frequency
sound waves that are re
fl
ected back are then detected by the same ultrasound probe
which acted as the source. The ultrasound scanner uses the amplitude and return time
of the waves which were re
fl
ected to construct a rendering of the body anatomy
encountered by the sound waves as they propagated through within the body. Real-
time visualization of movement of anatomic structures is also possible with ultra-
sound, as well as the movement of
fl
fluid. Using the Doppler effect, ultrasound can
measure fluid movement direction and speed, including variation with time, through
cine sequences [ 20 ]. At the current time, ultrasound is primarily used as a targeted
modality, with each image used to look at speci
fl
c tissue structures with a relatively
small
field of view (as opposed to CT, which can be used visualize the entire cross-
section of the body on each image) Bene
ts of ultrasound are that it does not expose
the patient to ionizing radiation, it is portable, can be used for noninvasive visual-
ization of dynamic internal processes of the body. However, drawbacks (at the
current time) can be signi
cant dependence of image quality
on the operator performing the study, image resolution which is relatively low, and
for our purposes in visualization of spinal structures, limited penetration of osseous
cant, and include signi
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