Diagnostic ultrasound imaging uses high-frequency sound waves from 1 to 20 MHz to produce cross-sectional images of the body. A small handheld transducer connected to a larger ultrasound unit is used to scan the body part of interest (e.g., the abdomen), and the resultant images are displayed on a monitor in real time (Fig. 11.10). Images may be stored digitally or printed later on film. Various body tissues reflect the sound waves in characteristic fashion, and diagnostic ultrasound is similar to the sonar imaging used by aquatic mammals and submarines. Differing tissues (e.g., the liver and the right kidney) have different acoustic properties. At the interface between tissues with different acoustic properties, the sound waves are reflected back to the transducer and are displayed on the screen, showing the tissues as well as the tissue interface. Unfortunately, gas-containing structures (lungs, bowel) as well as bones either absorb or scatter most of the sound waves and are not usually imaged satisfactorily by ultrasound. Solid organs (liver, spleen, kidneys), organs containing fluid (heart, gallbladder, urinary bladder), blood vessels, fatty tissue, and muscles, however, are usually amenable to diagnostic sonographic imaging.
Ultrasound finds widespread application in diagnostic radiology, obstetrics, gynecology, and cardiology because it produces diagnostic results comparable or even superior in some cases to other imaging techniques that require the use of ionizing radiation. Ultrasound machines are often portable and may be taken to the patient's bedside. Moreover, diagnostic ultrasound has no known harmful effects.
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