Over the past several decades, a number of technical advances currently allow for superb imaging of female pelvic structures. Modalities include sonography, radiography, computed tomographic (CT) scanning, magnetic resonance (MR) imaging, and less commonly, positron emission tomographic (PET) imaging. Of these, the evolution of sonography has now led to its use in gynecology equivalent to that in obstetrics. Moreover, advances in three-dimensional (3-D) imaging techniques have added such tremendous value to sonographic examination that it rivals the use of CT scanning and MR imaging for evaluation of many gynecologic conditions. MR imaging has been expanded with MR-guided focused ultrasound surgery (MRgFUS) to be used as treatment for uterine leiomyomas.
In sonography, the picture displayed on a screen is produced by sound waves reflected back from an imaged structure. 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 or placed within the tip of the transvaginal probe's condom sheath acts as a coupling agent. Sound waves pass through layers of tissue, 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. Dense material, such as bone, or a synthetic material, such as an intrauterine device, produces high-velocity reflected waves, also termed echoes, which are displayed on a screen as white. Materials such as these are described as echogenic. Conversely, fluid is anechoic, generates few reflected waves, and appears black on a screen. Middle-density tissues 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—more than 40 frames/sec—that the picture on the screen appears to move in real-time (Cunningham, 2010d).
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 fluid within the cyst. 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). Conversely, with a calcified structure, sound passing through it is minimal and creates a band of reduced echoes beyond it, known as acoustic shadowing (Fig. 2-2) (Armstrong, 2001).
Transvaginal sonogram of a premenopausal ovary containing a follicular cyst. The cyst fluid appears black or anechoic. Note the white or hyperechoic area under the cyst, a sonographic feature called posterior acoustic enhancement or through-transmission.
Transvaginal sonogram of an ovarian teratoma demonstrating posterior acoustic shadowing (arrows...