Normal embryonic brain anatomy, as visualized using intrauterine sonography with a 20-MHz flexible catheter-based, high-resolution, real-time miniature transducer, and three-dimensional sonographic visualization of the normal embryo are presented in this special article.
The central nervous system (CNS) has a three-dimensional (3D) outline, and changes its appearance dramatically during pregnancy. Although the three-dimensional assessment of the CNS is commonly performed using magnetic resonance imaging (MRI) and computed tomography (CT) in postnatal life, the fetal CNS has still been evaluated using axial images through the maternal abdominal wall. The practical application of the transvaginal approach for the sonographic assessment of fetal brain structures was introduced at the beginning of the 1990s. Transvaginal observation of the fetal brain can produce sagittal and coronal views of the brain from the fetal parietal direction, and sonographic images through the fontanelles or cranial suture, provide detailed structure. This method has contributed to the establishment of a new field of neurosonography. Recent advanced 3D ultrasound scanning has remarkably improved not only surface rendering, but also the multiplanar analysis of internal structure and 3D angiography. A combination of both 3D ultrasound and the transvaginal approach to the fetal brain produces images of brain morphology and circulation in multiple sections from any direction. Automatic scans require only a few seconds for data acquisition from the whole brain without having to shift the transducer. This new method provides us with intelligible information about the fetal brain, despite the short scan time required. Furthermore, saving and loading of the volume data sets are easy, and the ability to perform off-line analysis allows for a more objective and accurate diagnosis.
The recent advent of high-resolution ultrasound equipment and the introduction of transvaginal sonography have provided further information about normal and abnormal fetal brain structure. The transvaginal approach through the fetal anterior fontanelle has enabled us to visualize both coronal and sagittal sections of the fetal brain, and this has revealed a large number of structural anomalies which can be detected and diagnosed accurately. However, none of the other fetal organs and/or organ systems, including the heart and the abdominal organs, undergo such extensive structural changes, or show such a range of sonographic images during fetal life as the brain. To differentiate normal anatomy from pathology, it is important that we are able to recognize and understand the structural anatomy and embryology of the fetal brain.
Echoencephalography is very useful for the examination of the fetus and newborns, because it is easy to perform repeatedly at the beside, it is noninvasive, and it represents a real-time examination. A fetus is usually observed through the mother's body. To evaluate the neonatal brain, the transducer is routinely placed over the fontanelle. In this review article, the anatomy of the brain as visualized using echoencephalography, and the standard echoencephalography planes used are explained, and then some abnormal cases are shown. Finally, the detection of the brain circulation is discussed.
Doppler sonography is a convenient and useful procedure for evaluating intracranial lesions and hemodynamics, especially in the fetus and neonate. Initially B-mode ultrasonic images were used as the main procedure for investigating intracranial lesions. However, two-dimensional Doppler sonography, so-called color Doppler (CD) sonography, has superseded classical echosonography. It is possible with CD to visualize the intracranial arteries and veins in real time. In addition, the pulsed Doppler system (PD) in combination with CD, can be used to measure selectively the flow velocity at any point in the CD-visualized vessels. PD combined with CD could represent the selective flow condition at the intracranial main vessels, the anterior cerebral artery, basilar artery, middle cerebral artery and internal cerebral vein. But the flow conditions in these main arteries may not reflect the peripheral hemodynamics. Recently we used power flow Doppler imaging (PF) to show vessels that have low flow and small caliber. Now we are able to visualize the lenticulostriate artery (LSA), which perforates the branches of the middle cerebral artery, and have demonstrated the steady flow conditions of intracranial peripheral circulation. Three-dimensional reconstruction of PF images may provide a new quantitative and qualitative method of evaluating intracranial circulation. Selective echoangiography should clarify the mystery that surrounds brain circulation in perinatal period.
Cerebral blood flow is regulated by neurogenic, myogenic, and metabolic mechanisms. The sympathetic nervous system is responsible for the neurogenic mechanism of cerebrovascular autoregulation, but the effects of sympathetic activation on cerebral vessels have not yet been fully clarified. We performed transcranial Doppler (TCD) monitoring of the time-averaged maximum velocity (Vm) and pulsatility index (PI) in the middle cerebral artery during a sympathetic activation utilizing the cold pressor test (CPT) in two groups of eight healthy volunteers and ten elderly hypertensives. A ratio of mean arterial blood pressure (MABP) to Vm (MABP/Vm) was also calculated as an index of the resistance of the cerebrovascular bed. MABP significantly increased in both groups during CPT. Vm did not significantly change in either group. PI significantly decreased during CPT only in the healthy volunteers. During CPT, MABP/Vm increased to a greater extent in the group of elderly hypertensives than in the group of healthy volunteers. Despite increases in the MABP/Vm ratio, the PI decreased during CPT in healthy volunteers. Although PI has been used as an index of cerebrovascular resistance, a recent study has indicated that PI is correlated to the compliance of the proximal insonated artery. Our results suggest that the decrease in the PI observed during CPT in healthy volunteers reflects the reactivity of the proximal arteries, which regulate vascular tone.
Since its introduction in the early eighties, TCD has established itself as a valuable non-invasive tool for assessing the bloodflow in the major arteries of the brain. As such, it is important to either help prevent, or, alternatively, to help assess the damage of Stroke. Today, the emphasis is slowly changing from a purely diagnostic tool to a (longer-term) monitoring tool, that helps protect the brain during and after procedures with increased risk to the patient's brain. The development of easy-to-use and reliable emboli detection will further this goal.