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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 area to ensure good sound transmission, and 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 anus to image the prostate gland or into the vagina to better image the uterus and ovaries. 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, usual activities can usually be resumed 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. Because sound waves are blocked by gas (for example, in the lungs or intestine) and by bone, using ultrasonography to image structures deeper in the body is more difficult. It can be done only when there is no gas or bone between the transducer and the area being evaluated.
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)
Gallbladder and biliary tract: For example, to detect gallstones and blockages in the bile ducts
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
Female reproductive organs: For example, to detect tumors and inflammation in the ovaries, fallopian tubes, or uterus
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)
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.
This procedure uses changes that occur in the frequency of sound waves when they are reflected from a moving object (called the Doppler effect). In this case, the moving objects are red blood cells in the blood. Thus, Doppler ultrasonography can be used to evaluate blood flow—how fast it flows, which direction it flows in, and whether blood is flowing through blood vessels. It can detect blocked blood vessels, especially in leg veins, and 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 indicates flow toward and blue indicates flow away from the transducer.
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.
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