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FIG 6.7 Volume rendering of a normal 23-week fetal heart examined with micro-computed tomography Cutaway view shows right atrium, left ventricle, interventricular septum, and right ventricular outflow tract with opposed pulmonary valve leaflets (From Hutchinson JC, Arthurs OJ, Ashworth MT, et al Clinical utility of postmortem microcomputed tomography of the fetal heart: diagnostic imaging vs macroscopic dissection Ultrasound Obstet Gynecol 2016;47[1]:58–64.) Magnetic Resonance Imaging High-field MRI at 9.4T was found diagnostically superior to conventional 1.5T less than 22 gestational weeks, but the imaging times may be as long as 18 hours to obtain resolutions comparable to those achieved by micro-CT and HREM.49 This is currently the main barrier to its use in postmortem diagnostic imaging of the fetus Physiology of the Fetal Circulation in Health and Disease The human fetoplacental circulation shows adaptive changes that can be measured noninvasively using Doppler ultrasound and has allowed comparison with the previously reported animal studies.51,52 Initial experimental work in fetal sheep demonstrated a redistribution of flow in response to hypoxemia.53 With the availability of noninvasive Doppler techniques, similar information on the altered Doppler waveforms associated with abnormalities of pregnancy has been gathered Initial studies in the human used blind continuous wave ultrasound of the umbilical cord revealed (e.g., low diastolic flow in the umbilical artery in association with uteroplacental insufficiency).54 While on the fetal side of the placenta, an increased resistance to flow in growth-restricted pregnancies was described.55 Technical improvements, including newer color Doppler modalities such as energy and directional power, have enabled the visualization and interrogation of smaller vessels in regional circulations, and indicators of fetal well-being have been derived.56 A comparison of Doppler waveforms in the carotid, aortic, and umbilical arteries and in the middle cerebral artery has provided evidence of redistribution of flow in the growthrestricted human fetus.57,58 Animal work has supported the concept that, in the presence of uteroplacental insufficiency, the cerebral circulation becomes the vascular bed with the lowest impedance in the fetoplacental circulation As systemic impedance rises, flow is directed retrogradely through the arch toward the cerebral circulation.59 Increased flow to the brain results in a decreased pulsatility index recorded in the middle cerebral artery (Fig 6.8) FIG 6.8 (A) Doppler panel illustrating normal pulsatility index of the middle cerebral artery (MCA PI) (B) Waveform of redistribution of flow toward the fetal brain characterized by increased systolic and diastolic flow velocities in this example, lowering the pulsatility index Umbilical Cord Flows The pulsatility index was derived in the 1970s to quantify waveforms in the umbilical cord and assess fetal compromise Abnormalities of flow in the cord are characterized first by a reduction, and then a reversal, of diastolic velocities, thus increasing the pulsatility index A meta-analysis of subsequent studies has confirmed umbilical artery velocity measurements to be useful measures of fetal
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