Ultralydsymposium - NORSK FORENING FOR ULTRALYD ...

fourth ventricle are ‘pulled’ to the lower border of the cerebellar hemispheres, the fourth ventricle isfound covered by the cerebellum.SECOND TRIMESTERAPPROXIMATELY 14 TO 27 WEEKS, BPD ≈ 30–74 MMBy scanning in different planes, it is possible to evaluate the shape of the brain and its ventricles.Recently, guidelines for the ‘basic examination’ of the central nervous system and for the ‘fetalneurosonogram’ have been published that show how horizontal, coronal and sagittal sections can beused to examine the CNS systematically (ISUOG Guidelines 2007).Many biometric parameters and ratios have been proposed to evaluate the fetal brain and itsventricular system. For the clinical practice simple measurements are preferable. The atrial width ofthe lateral ventricle has shown to have a constant value throughout the second and third trimesters asshown by Cardoza and coworkers (7.6 ± 0.6 mm) and Pilu and co-workers (6.9 ± 1.3 mm) (Cardoza,Goldstein et al. 1988; Pilu, Reece et al. 1989). Therefore, atrial diameters above 10 mm should raisethe suspicion of ventriculomegaly. The surface of the cortices can be evaluated by tangential(oblique) sections, which is not easy, or by sections perpendicular to the surface of the brain. Thethird ventricle is relatively well imaged in the early second trimester, but it narrows as gestationprogresses, and develops into a virtual space between the thalami (Timor-Tritsch and Monteagudo1996). The corpus callosum and the cavum septi pellucidi are easily depicted in the sagittal section.The cerebral aqueduct of Sylvius develops into a narrow tube during the early second trimester. Thelower cerebellar vermis may still be small until 16–18 weeks (Babcook, Chong et al. 1996 ). Thereare no normative data on the size of the fetal fourth ventricle at 12–24 weeks of gestation.Bronshtein and colleagues noted that the lateral diameter of the fourth ventricle comprises less thanhalf of the lateral cerebellar width, while the postero-anterior diameter of the fourth ventricle is lessthan two-thirds of the cerebellar postero-anterior diameter (Bronshtein, Zimmer et al. 1998).Measurements of the transverse diameter of the cerebellum have shown that the value of thisparameter expressed in millimeters corresponded approximately to the gestational age expressed inweeks during the second trimester (Goldstein, Reece et al. 1987; Hata, Hata et al. 1989). The depthof the cisterna magna is measured as the midline diameter from the inner table of the occiput to theposterior aspect of the cerebellum in the standard horizontal plane of the cerebellum. The meandepth of the cisterna magna in normal 2. and 3. trimester fetuses is 5 ± 3 mm, with a maximum of10 mm (Mahony, Callen et al. 1984). Typically one can find septa-like linear echoes extendingapproximately from the edges of the vermis towards the posterior wall of the posterior fossa. Thesecisterna magna septa represent the walls of Blake’s pouch, the remnants of which are probablypresent in the neonatal period (Robinson and Goldstein 2007). A persistent Blake’s pouch becauseof a failure of fenestration of Blake’s pouch and the foramen of Luschka might lead to dilatation ofthe fourth ventricle and elevation of a normal vermis.THIRD TRIMESTERAPPROXIMATELY 28 WEEKS TO TERM, BPD ≥ 75 MMIn principle, it is possible to depict all hemispherical sulci by ultrasound. Still, the complexity of thecerebral surface makes the correlation of a sonographically described sulcus with the correctdefinition on the anatomical map difficult. Therefore, usually only the main sulci may be identified,especially those on the median surfaces of the hemispheres, which are rather easily identifiable bysagittal and coronal sections (Timor-Tritsch and Monteagudo 1996). The increased ossification ofthe skull at the end of the pregnancy impedes the accessibility for the ultrasound imaging of thebrain, such that the transfontanelle approach becomes essential.VASCULATURE OF THE BRAINThe blood supply of the brain originates from an anterior circulation through the internal carotidarteries, and a posterior circulation through the vertebral arteries via the common single basilarartery. These arterial circulations communicate with each other through the circulus arteriosus ofWillis. The middle cerebral artery (MCA) is the largest branch of the circle and runs laterally in theSylvian fissure as a direct continuation of the internal carotid artery. The two MCAs supply most ofthe cerebral cortex on the convexity of the hemispheres, and the deep parts of the cerebrum, such asthe basal ganglia and the internal capsule. The brain stem and cerebellum are supported from thevertebral arteries and their branches. The venous circulation drains the blood into dural sinuses,