Ultrasound Unit Health Article

Definition

An ultrasound unit is a noninvasive medical device used to produce images of body tissues and organs from differential reflections of ultrasonic sound waves. The technique of diagnostic imaging performed by ultrasound units is called ultrasonography. Ultrasonic waves are sound waves of a higher frequency than the human ear can detect. The frequency of a sound wave is the number of times per second that it cycles, and the number of cycles is measured in hertz (Hz). For example, one kilohertz (kHz) is one thousand cycles per second. Human hearing can detect sound in the range between 20 hertz to about 20 kilohertz (20kHz), or 20,000 cycles per second. Ultrasound images are generally produced using sound waves in the range between 1.6 to 10 million megahertz (MHz). Body tissues of different density reflect, or echo, sound waves differently, allowing the sonographer to distinguish between the structures.

Purpose

The first account of diagnostic ultrasound was published in 1942 by Dr. Karl Dussik, an Austrian psychiatrist. Dr. Dussik used ultrasound to locate brain tumors. Although ultrasound is better known as a technique of diagnostic imaging, it is also used at present in a variety of therapeutic applications.

Diagnostic applications

Ultrasonographic imaging can be used to visualize most soft-tissue organs. Dr. Dussik used ultrasound to visualize the cerebral ventricles in his pioneering use of the technique. Ultrasound is now used routinely to examine the kidneys or liver for the presence of tumors or cysts. The gall bladder can be checked for gallstones. Ultrasonography can also be used to examine blood vessels in the abdomen, extremities, or neck for evidence of swelling or blockage. One of the best-known diagnostic applications of ultrasound is its use during pregnancy to monitor the development, position, sex, and number of babies present in the mother's uterus.

Diagnostic ultrasound units are used to guide instruments during such invasive treatments as needle biopsies. Intraoperative sonography is used during many other procedures and even in combination with other medical imaging techniques. For example, intraoperative ultrasound is used during neurosurgery to detect brain tissue movement that can compromise the use of other more detailed imaging systems, such as computed tomography (CT) or magnetic resonance imaging (MRI).

Therapeutic applications

Ultrasonography also has therapeutic applications, although the frequencies used for therapy are usually in different frequency ranges than those used in diagnostic ultrasound.

BODY FLUID SAMPLING. A technique developed at MIT uses ultrasound to draw samples of tissue fluid through the skin withut the use of needles. Ultrasound waves disorganize the fatty layers in the outer layer of human skin, thus increasing the skin's permeability sufficiently to allow molecules of tissue fluid to travel through into a vacuum cylinder. The researchers apply ultrasound to the skin at 20 kHz frequency for two minutes. This frequency is much lower than that used to visualize fetuses in the womb. The technique shows promise as a noninvasive way for diabetics to monitor their blood sugar levels.

PHYSIOTHERAPY. Ultrasound waves produce heat as well as sound echoes. The low levels of heat produced by ultrasound appear to speed up wound healing, first by facilitating the release of histamine, a chemical that attracts white blood cells to the injured tissue. Second, ultrasound stimulates fibroblasts to secrete collagen, a fibrous protein found in connective tissue that increases the strength of the healing tissue. Ultrasound can also be applied to the area around the wound to provide mild heat in order to stimulate blood circulation in the area. A frequency of 3 MHz is used for most skin wounds, 1 MHz for deeper wounds or the area around the wound.

A British study indicates that ultrasound therapy applied to the wrist provides good short-term relief of mildly to moderately severe carpal tunnel syndrome. The beneficial effects of the treatment last for at least six months. It is thought that the ultrasound waves relieve the symptoms of the syndrome by relieving inflammation.

TUMOR DETECTION AND TREATMENT. Ultrasound can be used to scan for endometrial cancer in post-menopausal women without the need for a surgical biopsy. The ultrasound captures a detailed image of the lining of the uterus, which allows not only for immediate evaluation of the results, but is also more accurate than a biopsy.

Focused high-intensity ultrasound is being tested as a technique for destroying cancerous tumors within the body. The high-intensity beam, which is about 10,000 times as powerful as the ultrasound beams used to monitor pregnancies, appears to work by heating the cancer cells to nearly the temperature of boiling water. The cancer cells die within seconds. In 1999, the FDA granted approval for the use of focused ultrasound to treat enlarged prostate glands in men.