Serviço de Neurorradiologia/Radiologia, Hospital Dona Estefânia, Centro Hospitalar Lisboa Central, E.P.E.
- Conferência Imagem Fetal – VIII Congresso Português de Neurorradiologia, 25 e 26 de Maio 2012, Porto (Palestrante 20').
Introduction: Knowledge about the normal anatomy and development of fetal brain is essential for the correct interpretation of fetal MRI. Development and maturation of the CNS is characterized by changes in brain morphology and composition, which follows a predictable and precisely programmed pattern, with predetermined chronology, sequence and time points.
Objective: We will review the three most apparent processes related with brain development with fetal MRI, which are neuronal migration, myelination and gyration.
Methods: Several exams of fetal brain MRI are presented, illustrating these patterns of normal development in different gestational ages and important landmarks.
Results: The progression of MYELINATION is reflected by the appearance of bright signal on T1-weighted image and low signal on T2-weighted image, related to changes in water content, cellular density and magnetic resonance properties of membrane lipids. It is seen by week 20 in dorsal brainstem, by week 33 in posterior limb of internal capsule and by week 35 in optical tracts and perirolandic white matter.
NEURONAL MIGRATION as also a schedule program; the migrating cells are encountered within the white matter, between the germinal matrix and the cortical ribbon in the so called intermediate layer, resulting in a multilayered pattern. This multilayered pattern is seen in fetal MRI between 20 and 28/30 weeks and his regularity and signal is an important issue to evaluate.
GYRATION and appearance of fetal sulci and fissures also follows a predictable sequence, with time points landmarks, in such way that sulcal development is one of the markers of cortical development and gestational age. It is reflected by changes in brain surface and brain morphology, with sequential appearance of sulci: until week 24 the brain surface is agyric with open primitive Sylvian fissures, central sulcus begins folding by week 24-25 and it is achieved by week 34-35, pre and postcentral sulcus begins folding by week 26 and are achieved by week 34-35, superior temporal sulcus begins posterior folding by week 26 and it reaches anteriorly by week 28-29 and calcarine fissure appears by week 24 until week 29-30.
The evolution described is associated with subsequent modification of the volume of the ventricles and subarachnoid spaces: the subarachnoid spaces are prominent in young fetuses and a decrease in volume after 30 weeks and the ventricular size is quite constant from 14 to 40 weeks, with normal size at the atria inferior to 10 mm.
Conclusions: We can evaluate brain development and maturation with fetal MRI, mostly by morphological changes which follow a predictable and schedule program. The knowledge of the MRI appearance of fetal brain maturation is mandatory to be able to recognize impairment of fetal brain development or other fetal brain pathology.
Keywords: Fetal MRI; brain anatomy; brain development; pattern; landmarks.