Data collected from Doppler velocity recordings using both spectral and color Doppler mode confirm the fundamental aspects of the fetoplacental circulation. Changes in placental vascular resistance, cardiac contractibility, vessel compliance, and blood viscosity alter the normal dynamics of fetal cerebral circulation.
Reference values have been established for the main cerebral vessels. During the last trimester of normal pregnancies, the values of Doppler waveform indices decrease in all main cerebral vessels. After birth, vascular resistance decreases, and later stabilizes. Cerebral autoregulation persists from fetal to postnatal life, with low waveform indices in cerebral vessels of growth-retarded fetuses and neonates. The low indices indicate decreased cerebrovascular resistance, representing the redistribution of flow, or the brain-sparing effect.
Combined parameters recorded from different vascular beds may provide support for the diagnosis of significant hemodynamic changes and are of prognostic value in predicting fetal outcome such as fetal IUFGR and Rh disease.
As far as imaging of the vascularization (arterial and venous) of the prenatal brain, it should be clear that no brain scan should be considered complete without looking at its main vessels or at least at the pericallosal artery on a median section. Should such a protocol be too complicated to achieve in all anomaly scans, it is important to scrutinize brain vascularization in each and every case of suspected brain anomaly.
Three-dimensional power Doppler US technology, especially combined with high-frequency TVS, as described in this and other chapters, is the most preferable noninvasive and reproducible imaging method to be used. The accurate information about deviant brain vascularity may help to render proper obstetric, neurologic, and neurosurgical management.
Recent sonographic advances have contributed to accurate and reliable visualization of intrauterine vascularity. The first breakthrough in the assessment of fetal circulation was the Doppler system. Technical developments in Doppler ultrasound (US) equipment, especially highly sensitive color Doppler, power Doppler, and bidirectional power Doppler imaging techniques, have made it possible to study the fetal circulatory system, including cerebral vascularization. Early fetal circulation has been demonstrated by conventional two-dimensional (2D) color Doppler since the 1990s.1 Fine vessels inside the choroid plexus were depicted and assessed in 1994.2,3 Two-dimensional color/power Doppler combined with transvaginal sonography (TVS) became a powerful tool to demonstrate early fetal vascularization.4,5 Embryonal/fetal development is amazingly rapid, and the structure of the brain changes throughout pregnancy. Introduction of three-dimensional (3D) power Doppler technology in the late 1990s enabled visualization of intracranial vessels. This assessment, combined with TVS, has provided 3D sonoangiographic images, and adds useful information in the prenatal evaluation of normal brain development, vascular malformation, and tumoral vascularity.6,7
DEVELOPMENT OF EMBRYONAL CEREBRAL CIRCULATION
Vascular endothelial cells cover the entire inner surface of blood vessels and are influenced by vascular endothelial growth factor (VEGF), which is a potent angiogenic factor working as an endothelial cell-specific mitogen and exerting a trophic effect on neurons ...