Ultrasound is a general soft tissue imaging technique
which uses high frequency sound that flows through body tissues.
The frequencies are way above the range of the human ear and the
beam carries very little energy, so that ultrasound has no known
biological or chemical effects. This is why ultrasound can be
used so frequently during pregnancy.
How
Does It Work?
Ultrasound
is performed by placing a small device called a transducer against
a body part.
The transducer acts as both a transmitter and receiver.
First the transducer sends out a pulse of sound, and then the
transducer listens to the returning echoes and forwards this information
to the electronics and computers in the ultrasound machine. The
ultrasound machine puts together an image on the television monitor
based on the returning echoes. The neat thing about this process
is that it so fast that the image appears and changes almost instantaneously
- at least to us slow-witted, non-electronic humans.
The sound beam has the shape of a rectangle or a wedge with little width, so that the ultrasound image on the monitor represents a rectangular or wedge shaped slice. The position and orientation of the slice depends on where and how the sonographer holds the transducer. As the sonographer moves the transducer, successive slices appear on the monitor. The overall process can be regarded as a type of sophisticated radar that uses sound to construct a virtual model of a portion of the body.
How
Is The Procedure Performed?
Ultrasound
exams are really easy. The sonographer applies a lotion to the
skin, actually unflavored gelatin, to provide a water contact
between the skin and transducer.
The
images are obtained in real time as the sonographer moves and
turns the transducer. Some minor discomfort can be experienced
if the sonographer has to press the transducer a little to establish
better skin contact or maintain a certain orientation to obtain
more diagnostic images. It's not so unusual for an elderly patient
to fall asleep during an ultrasound exam!
An exam usually takes less than 30 minutes and your results are usually available within minutes after the exam.
What Are Ultrasound's Limitations?
The
limitations of ultrasound depend on the physics of sound. The sound
beam won't propagate between tissue and air - the sound either bounces
back or scatters. For this reason ultrasound can't be used to evaluate
the lungs. The stomach and intestines also contain air so that images
of them produced with ultrasound contain a lot of noise and have
little reliable clinical value.
