Recent advances in fetal imaging have been the result of technological achievements in sonography and magnetic resonance imaging, with dramatic improvements in resolution and image display. Both 3- and 4-dimensional ultrasound imagings have continued to evolve, and Doppler applications have expanded. Magnetic resonance imaging has added immensely to the already profound impact of sonography on obstetrics. A sonographic examination performed with the exacting recommended standards of the American Institute of Ultrasound in Medicine (2007) offers vital information about fetal anatomy, physiology, growth, and well-being.
Since the first obstetrical application of sonographic imaging by Donald and co-workers (1958), this technique has become indispensable for fetal evaluation.
The real-time image on the ultrasound screen is produced by sound waves reflected back from organs, fluids, and tissue interfaces of the fetus within the uterus. Transducers made of piezoelectric crystals convert electrical energy into sound waves that are emitted in synchronized pulses, then “listen” for the returning echoes. Because air is a poor transmitter of high-frequency sound waves, soluble gel is applied to the skin to act 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. Dense tissue such as bone produces high-velocity reflected waves, which are displayed brightly on the screen. Conversely, fluid generates few reflected waves and appears dark or anechoic on the screen. The electrical pulses created by the echoes are converted into digital representations, the most common of which is the 2-dimensional real-time image. Depending on settings, these digital images can be generated at 50 to greater than 100 frames/second. Postprocessing techniques then smooth the combined images and yield the appearance of real-time imaging.
Higher-frequency transducers yield better image resolution, whereas lower frequencies penetrate tissue more effectively. Current transducers offer wide-bandwidth technology, which allows them to perform over a range of frequencies. In the second trimester, a 4- to 6-megahertz-bandwidth transducer is often in close enough proximity to the fetus to provide precise images. However, by the third trimester, a lower frequency 2- to 5-megahertz-bandwidth transducer may be needed for penetration, but can lead to compromised resolution. This explains why resolution is often poor when imaging obese patients and why low-frequency transducers are needed to reach the fetus through the maternal tissues (Dashe and colleagues, 2009). In early pregnancy, 4- to 9-megahertz-bandwidth vaginal transducers provide excellent resolution because the small embryo is close to the transducer.
Sonography should be performed only with a valid medical indication and with the lowest possible exposure setting to gain necessary information—the ALARA principle—As Low As Reasonably Achievable (American Institute of Ultrasound in Medicine, 2007, 2008). As discussed in Chapter 41, Sonography, recent studies have suggested that prolonged exposure to ultrasound affects the migration of brain cells in fetal ...