Ultrasound scanning is a technique that uses high-frequency sound waves to image internal structures or a fetus in the uterus.
Ultrasound scanning uses sound waves of a very high frequency, inaudible to the human ear. The sound waves travel through the body and their echoes are processed to create images of internal structures or of a fetus in the uterus. Ultrasound scanning is a versatile technique that can show movement as well as structure, and the range of examinations using this method is constantly expanding. For example, organs situated deep within body cavities, such as those in the pelvis, which were previously difficult to image, can now be examined using ultrasound scanning. Since ultrasound does not involve radiation, it is thought to be completely safe.
How does ultrasound scanning work?
The principle of ultrasound scanning is similar to that used in naval sonar, in which sound waves are bounced off objects deep in the ocean. In ultrasound scanning, a device called a transducer converts an electric current into highfrequency sound waves. The transducer is usually hand-held and used on the skin surface, but it is sometimes inserted on a probe into a natural opening such as the vagina or rectum.
The transducer may also be incorporated into an endoscope, a viewing tube, to image deeper inside the body. Sound waves emitted by the transducer are focused in a narrow beam that passes through different parts of the body as the transducer is moved back and forth. Sound waves pass readily through soft tissue and fluid and are reflected at a point where different densities meet, such as where fluid in the bladder meets the bladder wall.
In addition to sending out the sound waves, the ultrasound transducer acts as the receiver by converting the reflected echoes back into electrical signals. These signals are processed by a computer and displayed as a two-dimensional image on a monitor (see Having an ultrasound scan). The images are updated continuously, enabling scans to show movement, such as that of the fetus in the uterus (see Ultrasound scanning in pregnancy) or valves opening and closing within the heart (see Echocardiography). A specific type of ultrasound known as Doppler ultrasound scanning uses short pulses of ultrasound to look at the direction and the speed of blood flow. Blood that is flowing away from the Doppler probe appears blue on the scan, while blood that is flowing towards it appears red. A mixture of colours indicates turbulence. The speed of blood flow is measured by a computer.
What is ultrasound scanning used for?
Most pregnant women will have at least one ultrasound scan to examine the growing fetus. Ultrasound can also be used to image the brain of a newborn baby through the fontanelle (the soft spots in a baby's head between the bones of the skull). Using ultrasound in this way can identify bleeding into the brain from the surrounding blood vessels, a potential problem in premature babies.
Ultrasound scanning is commonly used to investigate internal organs because it produces good images of soft tissue, such as the liver, and fluid-filled structures, such as the gallbladder. It can also be used to examine the structure of the heart and its movement. Used endoscopically through the oesophagus, ultrasound can provide more detailed information on the heart or investigate organs deep within the body, such as the stomach and pancreas. The transducer can also be inserted into the vagina on a probe to examine the female reproductive organs. Ultrasound can be used on the eye to locate foreign bodies or to investigate disorders, such as retinal detachment. It can also be used to guide tissue sampling by pinpointing an abnormal area, such as in a prostate gland biopsy.
Doppler ultrasound scans are routinely used to investigate blood vessels in which blood flow may be reduced. For example, Doppler scans are often used to detect blood clots in the veins (see Deep vein thrombosis) and to detect thickened artery walls, particularly in the arteries in the neck (see Carotid Doppler scanning). In pregnant women with high blood pressure, Doppler ultrasound may be used to investigate the uterine artery, which carries blood to the uterus.
What are the risks of ultrasound scanning?
Ultrasound scanning is not thought to cause any adverse effects and can be repeated as often as necessary. It is the only imaging technique considered safe for routine screening of a fetus.
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