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Ultrasonography uses high-frequency sound (ultrasound) waves to produce images of internal organs and other tissues. A device called a transducer converts electrical current into sound waves, which are sent into the body’s tissues. Sound waves bounce off structures in the body and are reflected back to the transducer, which converts the waves into electrical signals. A computer converts the pattern of electrical signals into an image, which is displayed on a monitor and recorded on film, on videotape, or as a digital computer image. No x-rays are used.
Ultrasonography is painless, relatively inexpensive, and considered very safe, even during pregnancy.
If the abdomen is being examined, people may be asked to refrain from eating and drinking for several hours before the test.
Usually, the examiner places thick gel on the skin over the area to be examined to ensure good sound transmission. A handheld transducer is placed on the skin and moved over the area to be evaluated.
To evaluate some body parts, the examiner inserts the transducer into the body—for example, into the vagina to better image the uterus and ovaries or into the anus to image the prostate gland.
To evaluate the heart, the examiner sometimes attaches the transducer to a viewing tube called an endoscope and passes it down the throat into the esophagus. This procedure is called transesophageal echocardiography.
After the test, most people can resume their usual activities immediately.
Ultrasound images are acquired rapidly enough to show the motion of organs and structures in the body in real time (as in a movie). For example, the motion of the beating heart can be seen, even in a fetus.
Ultrasonography is effectively used to check for growths and foreign objects that are close to the body’s surface, such as those in the thyroid gland, breasts, testes, and limbs, as well as some lymph nodes.
Ultrasonography is effectively used to image internal organs in the abdomen, pelvis, and chest. However, because sound waves are blocked by gas (for example, in the lungs or intestine) and by bone, ultrasonography of internal organs requires special skills. People who have been specifically trained to do ultrasound examinations are called sonographers.
Ultrasonography is commonly used to evaluate the following:
Heart: For example, to detect abnormalities in the way the heart beats, structural abnormalities such as defective heart valves, and abnormal enlargement of the heart’s chambers or walls (ultrasonography of the heart is called echocardiography)
Blood vessels: For example, to detect dilated and narrowed blood vessels
Gallbladder and biliary tract: For example, to detect gallstones and blockages in the bile ducts (see Imaging Tests of the Liver and Gallbladder : Ultrasonography)
Liver, spleen, and pancreas: For example, to detect tumors and other disorders (see Imaging Tests of the Liver and Gallbladder : Ultrasonography)
Urinary tract: For example, to distinguish benign cysts from solid masses (which may be cancer) in the kidneys or to detect blockages such as stones or other structural abnormalities in the kidneys, ureters, or bladder (see Ultrasonography)
Female reproductive organs: For example, to detect tumors and inflammation in the ovaries, fallopian tubes, or uterus (see Diagnostic Procedures : Ultrasonography)
Pregnancy: For example, to evaluate the growth and development of the fetus and to detect abnormalities of the placenta (such as a misplaced placenta, called placenta previa—see Ultrasonography)
Ultrasonography can also be used to guide doctors when they remove a sample of tissue for a biopsy. Ultrasonography can show the position of the biopsy instrument, as well as the area to be biopsied (such as a mass). Thus, doctors can see where to insert the instrument and can guide it directly to its target.
Ultrasound information can be displayed in several ways:
A-mode: As spikes on a graph (used to scan the eye)
B-mode: As a 2-dimensional anatomic images (used during pregnancy to evaluate the developing fetus or to evaluate internal organs)
M-mode: As waves displayed continuously to show moving structures (used to evaluate the fetus's heartbeat or to evaluate heart valve disorders)
B-mode ultrasonography is most commonly done.
Doppler ultrasonography uses changes that occur in the frequency of sound waves when they are reflected from a moving object (called the Doppler effect). In medical imaging, the moving objects are red blood cells in the blood. Thus, Doppler ultrasonography can be used to evaluate
Doppler ultrasonography is used
To evaluate how well the heart is functioning (as part of echocardiography)
To detect blocked blood vessels, especially in leg veins, as in deep vein thrombosis, when veins are blocked by a blood clot
To detect narrowed arteries, especially the carotid arteries in the neck, which carry blood to the brain
For this test, color is superimposed on the shades-of-gray image of blood flow produced by Doppler ultrasonography. The color indicates direction of blood flow. Red may be used to indicate flow toward the transducer, and blue may be used to indicate flow away from the transducer. The brightness of the color indicates how fast the blood is flowing.
Color Doppler ultrasonography can help assess the risk of stroke because it helps doctors identify and evaluate narrowing or blockage of arteries in the neck and head. The procedure is useful for evaluating people who have had a transient ischemic attack or stroke and people who have risk factors for atherosclerosis but no symptoms. Color Doppler ultrasonography is also used to assess blood flow to internal organs and tumors.
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