Doppler Ultrasound
A Doppler
ultrasound test
uses reflected sound waves to evaluate blood as it flows
through a blood vessel. It helps doctors evaluate blood
flow through the major arteries and veins 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).
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.
The four
types of Doppler ultrasound are:
-
“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 to provide a rapid 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
surrounding organs. In addition, a computer converts
the Doppler sounds into a graph that provides
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. In addition, 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.
Power Doppler is a newer ultrasound technique that
is up to five times more sensitive in detecting
blood flow than color Doppler. Power Doppler can
obtain some images that are difficult or impossible
to obtain using standard color Doppler. However,
power Doppler is most commonly used to evaluate
blood flow through vessels within solid organs.
Blood flow in individual blood vessels is most
commonly evaluated by combining color Doppler with
duplex Doppler. Together, they are able to provide
better information on the direction and speed of
blood flow than when these techniques are used
individually.
Why It Is Done
Doppler
ultrasound is done to:
-
Detect blood clots and blocked or narrowed blood
vessels in almost any part of the body, especially
in the neck, arms, and legs. Blocked or narrowed
arteries of the neck can cause dizziness, loss of
vision, paralysis, weakness, numbness, or other
symptoms of a stroke. Blood clots in the deep veins
of the leg can cause leg pain and swelling and can
increase a person's risk of pulmonary embolism.
-
Evaluate leg pain that may be caused by
intermittent claudication,
a condition caused by
atherosclerosis of
the lower extremities.
-
Evaluate blood flow after a stroke or other
condition that might be caused by a problem with
blood flow. Evaluation of a stroke can be done
through a technique called transcranial Doppler
(TCD) ultrasound.
-
Map veins that may be used for blood vessel grafts.
It also can evaluate the condition of grafts used to
bypass blockage in an arm or leg.
-
Determine the amount of blood flow to a transplanted
kidney or liver.
-
Monitor the flow of blood following blood vessel
surgery.
-
Determine the presence, amount, and location of
arterial
plaque. Plaque in
the carotid arteries can reduce blood flow to the
brain and may increase the risk of stroke.
A
transcranial Doppler ultrasound (TCD) may be used in
children with sickle cell disease to
evaluate their risk of stroke. In adults, it can be used
to evaluate blood flow and vessel spasms in the brain.
How To Prepare
You may be
asked to avoid products that contain nicotine
(cigarettes, chewing tobacco) for 30 minutes to 2 hours
before the test. Nicotine causes blood vessels to
constrict and may give false results.
How It Is Done
This test is
done by a doctor who specializes in performing and
interpreting imaging tests (radiologist)
or by an ultrasound technologist (sonographer). It is
done in an ultrasound room in a hospital or doctor's
office.
You will
need to remove any jewelry that might interfere with the
Doppler ultrasound scan. You may need to take off all or
most of your clothes, depending on which area is being
examined (you may be allowed to keep on your underwear
if it does not interfere with the test). You will be
given a cloth or paper covering to use during the test.
-
For abdominal scans, you will lie on your back.
-
For chest scans, you will lie on your back with your
neck slightly extended.
-
For head and neck scans, your head may be turned to
one side.
-
For an arm or leg scan, your head is slightly raised
and the exposed arm or leg is turned slightly
outward. Occasionally for a leg scan, you may be
asked to lie on your stomach.
Gel is
applied to the skin to promote the passage of the sound
waves. The transducer is placed in the gel and moved
along the skin. You need to lie very still during the
procedure. You may hear sounds that represent the flow
of blood through the blood vessels.
The test
usually takes 30 to 60 minutes.
Arteries in the arms and legs
This test is
often performed on both arms or both legs. Even if the
suspected blood flow problem is in only one limb, both
may be tested for comparison. If your arms are being
tested, they will be tested first while you are lying
down and then again while you are sitting.
Depending on
which blood vessels are being tested, a blood pressure
cuff may be wrapped around one or both limbs so that the
blood pressure can be taken at several different places.
When testing the legs, a blood pressure cuff may be
wrapped first around the calf and then around the thigh.
The test may be done at several locations on your leg.
When testing the arms, the pressure cuff may be wrapped
first around the forearm and then around the upper arm.
Veins in the arms and legs
For this
test, you will be asked to lie down and breathe
normally. You must lie very still. Any changes in blood
flow that occur as a response to your breathing patterns
are noted.
The test may
be repeated while the examiner presses on the veins
close to the surface of your skin to help detect a clot
in the vein (called a compression maneuver). The
examiner may do this with your legs or arms in different
positions to ensure that the blood supply is not blocked
in these positions. The examiner may also squeeze your
calf or forearm to help blood move more quickly through
the veins (called an augmentation maneuver). This is
done to evaluate blood flow toward your heart.
While your
legs are being tested, you may also be asked to try to
breathe out strongly with your nose pinched and your
mouth closed (called
Valsalva's maneuver).
This maneuver usually causes a sudden change in blood
flow through the veins.
Arteries in the neck
You will be
asked to lie down with a pillow underneath your head for
support. The test is performed on both sides of your
neck, and then the results are compared to standard
values to determine the amount of blockage or narrowing
of the arteries.
Transcranial ultrasound
For a
transcranial ultrasound, the transducer is passed
lightly over the skin at the base or side of your skull.
How It Feels
There is
normally no discomfort involved with having a Doppler
ultrasound test. The gel may feel cold when it is
applied to your skin unless it is first warmed to body
temperature. If your blood pressure is taken during the
test, you will feel pressure when the blood pressure
cuffs are inflated.
Risks
There are no
known risks associated with a Doppler ultrasound test.
Results
A Doppler
ultrasound test uses
reflected sound waves to evaluate blood as it flows
through a blood vessel.