ultrasound test uses reflected sound waves to see how
blood flows through a blood vessel. It helps doctors evaluate blood flow
through major arteries and veins, such as those of the arms, legs, and neck. It can show
blocked or reduced blood flow through narrowing in the major arteries of the
neck that could cause a
stroke. It also can reveal blood clots in leg veins
(deep vein thrombosis, or DVT) that could break loose
and block blood flow to the lungs (pulmonary embolism). See pictures of a
stroke and an
embolus . During pregnancy, Doppler ultrasound may be
used to look at blood flow in an unborn baby (fetus) to check
the health of the fetus.
During Doppler ultrasound, a handheld
instrument (transducer) is passed lightly over the skin above a blood vessel.
The transducer sends and receives sound waves that are amplified through a
microphone. The sound waves bounce off solid objects, including blood cells.
The movement of blood cells causes a change in pitch of the reflected sound
waves (called the Doppler effect). If there is no blood flow, the pitch does
not change. Information from the reflected sound waves can be processed by a
computer to provide graphs or pictures that represent the flow of blood through
the blood vessels. These graphs or pictures can be saved for future review or
evaluation. See a picture of a
Doppler ultrasound .
The four basic types of Doppler ultrasound
"Bedside" or continuous wave Doppler. This type
uses the change in pitch of the sound waves to provide information about blood
flow through a blood vessel. The doctor listens to the sounds produced by the
transducer to evaluate the blood flow through an area that may be blocked or
narrowed. This type of ultrasound can be done at the bedside in the hospital
with a portable machine to provide a fast estimate of the extent of blood
vessel damage or disease.
Duplex Doppler. Duplex Doppler ultrasound uses
standard ultrasound methods to produce a picture of a blood vessel and the
surrounding organs. Also, a computer converts the Doppler sounds into a graph
that gives information about the speed and direction of blood flow through the
blood vessel being evaluated.
Color Doppler. Color Doppler uses standard
ultrasound methods to produce a picture of a blood vessel. Also, a computer
converts the Doppler sounds into colors that are overlaid on the image of the
blood vessel and that represent the speed and direction of blood flow through
the vessel. Power Doppler is a special type of color Doppler. Power Doppler can get some images that are hard or impossible to
get using standard color Doppler. Power Doppler is most commonly used to
evaluate blood flow through vessels within solid organs.