he ductus venosus is imaged as it branches from the umbilical vein at approximately the level of the diaphragm. Fetal position poses more of a challenge in imaging the ductus venosus than it does with either the umbilical artery or the middle cerebral artery. he waveform is biphasic and normally has forward flow throughout the cardiac cycle. The irst peak reflects ventricular systole, and the second is ventricular diastolic illing. These are followed by a nadir during atrial contraction-termed the a-wave.
It is believed that Doppler indings in preterm fetuses with growth restriction show a progression in which umbilical artery Doppler abnormlities are followed by ones in the middle cerebral artery and then in the ductus venosus. However, manifestations of these abnormalities vary widely (Berkley, 2012). With severe fetal-growth restriction, cardiac dysfunction may lead to low in the a-wave that is decreased, absent, and eventually reversed, along with pulsatile low in the umbilical vein (Fig. 10-53).
Ductus venosus abnormalities have potential to identiy preterm growth-restricted fetuses that are at greatest risk for adverse outcomes (Baschat, 2003, 2004; Bilardo, 2004; Figueras, 2009). As noted by the Society for Maternal-Fetal Medicine, however, they have not been suiciently evaluated in randomized trials (Berkley, 2012). In sum, Doppler assessment of vessels other than the umbilical artery has not been shown to improve perinatal outcome, and thus their role in clinical practice remains uncertain (American College of Obstetricians and Gynecologists, 2015).
Image resolution with MR is oten superior to that with sonography because it is not as hindered by bony interfaces, maternal obesity, oligohydramnios, or an engaged fetal head. Thus, it can serve as an adjunct to sonography in evaluating suspected fetal abnormalities. Examples include complex abnormalities of the fetal CNS, thorax, gastrointestinal system, genitourinary system, and musculoskeletal system. MR has also been used in the evaluation of maternal pelvic masses and placental invasion. MR imaging, however, is not portable, it is time-consuming, and its use is generally limited to referral centers with expertise in fetal imaging.
To guide clinical use, the American College of Radiology and Society for Pediatric Radiology (2015) have developed
FIGURE 10-53 Venous Doppler abnormalities. A. Reversal of a-wave flow in the ductus venosus. Arrows depict a-waves below the baseline. This finding may be identified with cardiac dysfunction in the setting of severe fetal-growth restriction. B. Pulsatile flow in the umbilical vein. The undulating umbilical venous waveform below the baseline indicates tricuspid regurgitation. Above the baseline is the umbilical artery waveform, in which there is no visible end-diastolic flow. Because the venous waveform is below the baseline in this image, it is not possible to determine whether the umbilical artery end-diastolic flow is reversed.
a practice guideline for fetal MR imaging. This document acknowledges primacy of sonography as the preferred screening modality. Moreover, it recommends that fetal MR imaging be used for problem solving to ideally contribute to prenatal diagnosis, counseling, treatment, and delivery planning. Specific indications for fetal MR imaging are listed in Table 10-11 and are discussed subsequently.
MR imaging uses no ionizing radiation, but theoretical concerns include the efects of luctuating electromagnetic ields and high sound-intensity levels. The strength of the magnetic
TABLE 10-11. Fetal Conditions for Which Magnetic Resonance Imaging May Be Indicateda
Agenesis of the corpus callosurn
Solid or cyJtic masses Vascular malformations Hydranencephaly Infarction. Hemorrhage