There
are many advantages to imaging the body with ultrasound.
Most
importantly, there is no ionizing radiation as with X-rays, so that ultrasound
is used extensively during pregnancy.
Furthermore, soft tissues, such as
the liver, spleen, kidneys and pancreas can be imaged directly without the
injection of any sort of radio-opaque substances or isotopes to make them
visible. In addition, the entire abdomen and pelvis can be rapidly scanned
while the patient is lying on the table and photographs can be made of the area
in question.
The
sonographer asks the patient to take a deep breath and hold it. This brings the
top of the liver out from under the ribs. Then the transducer is either moved
in a straight line in a longitudinal or transverse
direction, or else rocked back and forth in one place. Either procedure will
produce a two-dimensional scan on the screen. The sonographer scans the rest of
the liver and takes pictures of areas of interest.
What
are the drawbacks to ultrasound?
Probably
the most serious is the fact that sound is not able to travel through certain
organs; their surfaces reflect almost 100% of sound waves, so that the interiors
of these organs and those lying directly beneath them cannot be imaged. Organs
filled with air such as the lungs, stomach and intestines are opaque to sound
as are hard tissues such as bone.
However,
an organ like the liver is ideal for the sonographer to work with. It is a very
large homogeneous soft organ that permits sound pulses to move through with
only small amounts of reflection so that the spleen, pancreas or kidney which
lie beneath the liver can be imaged as well. For
this reason, the liver is called an acoustic
window.
Formerly
it was almost impossible to view the cervix and lower uterus because they lay
under the air-filled intestines which reflected most of the sound. Then doctors
discovered that they could create an ideal acoustic window if they had the
patient drink 32 ounces of fluid one hour before an ultrasound exam. The
distended bladder pushed the air-filled intestines out of the way
and
permitted sound to reach the reproductive organs in the lower pelvis.
Another
barrier to sound is the air layer between the patient's skin and the
transducer. In order to
overcome
the reflections at this level, the sonographer liberally smears mineral oil (or
another lubricant) on the patient's skin before she begins the scanning. If
this sounds like a messy exam consider the set-up of ultrasound machines in the
1950's where the patient had to be immersed in
a
water bath.
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