The real-time image on the ultrasound screen is produced by sound waves that are relected 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 diferent densities. Dense tissue such as bone produces high-velocity reflected waves, which are displayed as bright echoes on the screen. Conversely, luid generates few relected 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 efectively. 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.
cation, using the lowest possible exposure setting to gain neces sary information-the AAA principle-as low as reasonably 4chievable. Examinations are performed only by those trained pathology, using techniques to avoid ultrasound exposure beyond what is considered safe for the fetus (American College of Obstetricians and Gynecologists, 2016; American Institute of Ultrasound in Medicine, 2013b). No causal relationship has ognized adverse efect in human pregnancy. The International
Society of Ultrasound in Obstetrics and Gynecology (2016) further concludes that there is no scientifically proven associa tion between ultrasound exposure in the first or second trimes ters and autism spectrum disorder or its severity.
All sonography machines are required to display two indices: the thermal index and the mechanical index. The thermal index is a measure of the relative probability that the examination may raise the temperature, potentially high enough to induce injury. That said, fetal damage resulting from commercially available ultrasound equipment in routine practice is extremely unlikely. The potential for temperature elevation is higher with longer examination time and is greater near bone than in soft tissue. heoretical risks are higher during organogenesis than later in gestation. The thermal index for soft tissue, Tis, is used before 10 weeks' gestation, and that for bone, ib, is used at or beyond 10 weeks' (American Institute of Ultrasound in Medicine, 2013b). The thermal index is higher with pulsed Doppler applications than with routine B-mode scanning (p. 213). In the first trimester, if pulsed Doppler is clinically indicated, the thermal index should be ;0.7, and the exposure time should be as brief as possible (American Institute of Ultrasound in Medicine, 2016) . To document the embryonic or fetal heart rate, motion-mode (M-mode) imaging is used instead of pulsed Doppler imaging.
The mechanical index is a measure of the likelihood of adverse efects related to rarefactional pressure, such as cavi tationwhich is relevant only in tissues that contain air. Microbubble ultrasound contrast agents are not used in pregnancy for this reason. In mammalian tissues that do not contain gas bodies, no adverse efects have been reported over the range of diagnostically relevant exposures. Because fetuses cannot contain gas bodies, they are not considered at risk.