After discovery of the Roentgen ray and the demonstration of the various uses to which it might be put, it was thought possible that it might also afford a valuable method of investigating the shape and size of the pelvis.
—J. Whitridge Williams (1903)
X-ray techniques were just on the horizon when the first edition of this textbook was published. The first application focused on the maternal pelvis without attention to the fetus. Thus, congenital abnormalities were routinely not discovered until birth. Subsequent radiographic efforts to evaluate the fetus were later replaced by ultrasonography and more recently by magnetic resonance (MR) imaging, techniques which have become increasingly sophisticated. The subspecialty of fetal medicine has developed only because of these advances, and today’s practitioner can hardly imagine obstetrical care without them.
Prenatal sonography can be used to accurately assess gestational age, fetal number, viability, and placental location, and it can aid diagnosis of many fetal abnormalities. With improvements in resolution and image display, anomalies are increasingly detected in the first trimester, and Doppler is used to manage pregnancies complicated by growth impairment or anemia. The American College of Obstetricians and Gynecologists (2016) recommends that prenatal sonography be performed in all pregnancies and considers it an important part of obstetrical care in the United States.
The real-time image on the ultrasound screen is produced by sound waves that are reflected back from fluid and tissue interfaces of the fetus, amnionic fluid, and placenta. Sector array transducers contain groups of piezoelectric crystals working simultaneously in arrays. These crystals convert electrical energy into sound waves, which are emitted in synchronized pulses. Sound waves pass through tissue layers and are reflected back to the transducer when they encounter an interface between tissues of different densities. Dense tissue such as bone produces high-velocity reflected waves, which are displayed as bright echoes on the screen. Conversely, fluid generates few reflected waves and appears dark. Digital images generated at 50 to more than 100 frames per second undergo postprocessing that yields the appearance of real-time imaging.
Ultrasound refers to sound waves traveling at a frequency above 20,000 hertz (cycles per second). Higher-frequency transducers yield better image resolution, whereas lower frequencies penetrate tissue more effectively. Transducers use wide-bandwidth technology to perform within a range of frequencies. In early pregnancy, a 5- to 10-megahertz (MHz) transvaginal transducer usually provides excellent resolution, because the early fetus is close to the transducer. And, in the first and second trimesters, a 4- to 6-MHz transabdominal transducer is similarly close enough to the fetus to yield precise images. By the third trimester, however, a lower frequency 2- to 5-MHz transducer may be needed for tissue penetration—particularly in obese patients—and this can lead to compromised resolution.