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Several technical advances in recent decades currently allow superb imaging of female pelvic structures. As a result, use of sonography in gynecology now equals that in obstetrics. Enhancements to traditional sonography continue to fill important clinical gaps. For example, technical refinements now allow three-dimensional (3-D) imaging to rival the roles of computed tomography (CT) and magnetic resonance (MR) imaging for many conditions. In addition, application of MR imaging now includes MR-guided high-intensity focused-ultrasound therapy, used for uterine leiomyomas.
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In sonography, the picture displayed on a screen is produced by sound waves reflected back from an imaged structure. To begin, alternating current is applied to a transducer containing piezoelectric crystals, which convert electric energy to high-frequency sound waves. A water-soluble gel applied to the skin acts as a coupling agent. Sound waves then pass through tissue layers, encounter an interface between tissues of different densities, and are reflected back to the transducer. Converted back into electric energy, they are displayed on a screen.
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Dense material, such as bone, or a synthetic material, such as an intrauterine device (IUD), produces high-velocity reflected waves, also termed echoes, which are displayed on a screen as white. These are described as echogenic. Conversely, fluid is anechoic, generates few reflected waves, and appears black on a screen. Middle-density tissues variably reflect waves to create various shades of gray, and images are described as hypoechoic or hyperechoic relative to tissues immediately adjacent to them. Images are generated so quickly—50 to 100 frames per second—that the picture on the screen appears to move in real time.
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Sound reflection is greatest when the difference between the acoustic impedance of two structures is large. This explains why cysts are so well demonstrated with sonography. Strong echoes are produced from the cyst walls, but no echoes arise from the cyst fluid. As more sound traverses the cyst, more echoes are received from the area behind the cyst, a feature known as through transmission or acoustic enhancement (Fig. 2-1). In contrast, with a dense structure, the sound passing through it is diminished, which creates a band of reduced echoes beyond it, known as acoustic shadowing (Fig. 2-2).
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The frequency of emitted ultrasound waves is expressed in megahertz (MHz), which means million vibrations per second. The frequency is inversely related to ...