Ultrasound
Medical Author: Benjamin C. Wedro, MD, FAAEM
Medical Editor: William C. Shiel, Jr., MD, FACP, FACR
Introduction
While the patient's history and physical examination are the building blocks
of making a medical diagnosis, the ability to peer inside the body can be a
powerful tool. Ultrasound is an imaging technique that provides that
ability to medical practitioners.
What is an ultrasound?
Ultrasound produces sound waves that are beamed into the body causing return
echoes that are recorded to "visualize" structures beneath the skin. The ability
to measure different echoes reflected from a variety of tissues allows a shadow
picture to be constructed. The technology is especially accurate at seeing the
interface between solid and fluid filled spaces. These are actually the same
principles that allow SONAR on boats to see the bottom of the ocean.
What is ultrasonography?
Ultrasonography is body imaging using ultrasound in medical diagnosis. A
skilled ultrasound technician is able to see inside the body using
ultrasonography to answer questions that may be asked by the medical
practitioner caring for the patient. Usually, a radiologist will oversee the
ultrasound test and report on the results, but other types of physicians may use
ultrasound as a diagnostic tool. For example, obstetricians use ultrasound to
assess the fetus during
pregnancy.
Surgeons and emergency physicians use ultrasound at the bedside to assess
abdominal pain or other concerns.
A transducer, or probe, is used to project and receive the sound waves and the
return signals. A gel is wiped onto the patient's skin so that the sound waves are not
distorted as they cross through the skin. Using their understanding of
human anatomy
and the machine, the technician can evaluate specific structures and try to
answer the question asked by the patient's physician. This may take a fair
amount of time and require the probe to be repositioned and pointed in different
directions. As well, the technician may need to vary the amount of pressure used
to push the probe into the skin. The goal will be to "paint" a shadow picture of
the inner organ that the
health care practitioner has asked to be visualized.
The physics of sound can place limits on the test. The quality of the picture
depends on many factors.
- Sound waves cannot penetrate deeply, and an obese patient may be
imaged poorly.
- Ultrasound does poorly when gas is present between the probe and the
target organ. Should the intestine be distended with
bowel gas, organs
behind it may not be easily seen. Similarly, ultrasound works poorly in the
chest, where the lungs are filled with
air.
- Ultrasound does not penetrate bone easily.
- The accuracy of the test is very much operator dependent. This means
that the key to a good test is the ultrasound technician.
Ultrasound can be enhanced by using
Doppler technology which can
measure whether an object is moving towards or away from the probe. This can
allow the technician to measure blood flow in organs such as the heart or liver, or within
specific blood vessels.
Next: For what purposes are ultrasounds used? »
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