Abdominal ultrasound, also known as a sonogram, uses high-frequency sound waves to create pictures of the organs inside the body. As a painless and safe technology, doctors frequently use abdominal ultrasound to monitor pregnancy, diagnose the source of abdominal pain, evaluate blood flow through the abdomen or look for gallstones or kidney stones.
Procedure
During an ultrasound session, the technician bares the skin of the abdomen and applies a thin layer of gel to the area. The technician presses a hand-held, microphone-like tool called the transducer into the gel. As the transducer passes back and forth over the abdomen, it sends sound waves through the skin and picks up their echoes. The frequency of the sound waves is too high for the human ear to detect. The patient might feel some discomfort from the pressure of the transducer if the abdomen is tender, but otherwise the procedure is painless. Usually an abdominal ultrasound procedure takes half an hour or less.
Physics
RadiologyInfo.org outlines how ultrasound works along the same general principle as sonar. As the transducer passes back and forth over the abdomen, it emits high-frequency sound waves that penetrate the skin. When the sound signal encounters an organ, it bounces back and a microphone in the transducer amplifies the echo. The strength of the echo and the speed of its return give information about the density and distance of the objects the sound waves encounter. A computer linked to the transducer interprets the information from the echoes and transforms them into an image on the computer screen.
Doppler Ultrasound
A specialized application of ultrasound, Doppler ultrasound of the abdomen tracks the movement of blood cells, for example, to monitor blood flow through an abdominal aortic aneurysm. According to RadiologyInfo.org, the movement of blood cells changes the pitch of the echoes, which the computer can project as an image of blood flow.
Limitations
RadiologyInfo.org discusses the limitations of abdominal ultrasound. Bubbles of gas or air disrupt the transmission of sound and distort the relayed image. The gel used on the skin during the procedure minimizes the air between the transducer and the skin, but this limitation means that ultrasound is not useful to see the bowels or organs lying behind them. The strength of the ultrasound signal diminishes with distance, so layers of fat can impede the signal and produce weak images. The sound waves are not strong enough to penetrate bone, and can only show its outlines.
Risks
Unlike x-rays and computed tomography, or CT scans, ultrasound does not use ionizing radiation. According to the Food and Drug Administration, the low-energy sound waves used in ultrasound do not change the state of atoms or molecules in the body, so they do not damage tissues like ionizing radiation does. There are no known risks of exposure to the sound waves used in ultrasound, says MedlinePlus.
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