Variability of T1-weighted signal intensity of pericallosal lipomas in the fetus

International audience; Background: Pericallosal lipomas are often associated with corpus callosum dysgenesis. The diagnosis of lipoma, suggested on ultrasonography, relies on the classic T1 hyperintensity on magnetic resonance imaging (MRI). However, this feature may be absent prenatally.Objective: Our objective was to study the changes of T1 intensity in fetal lipomas with comparison to postnatal/postmortem data and to assess the factors influencing the signal variations of pericallosal lipomas on prenatal MRI.Materials and methods:Patients with callosum dysgenesis and interhemispheric hyperechogenicity suggestive of a pericallosal lipoma with available postnatal or postmortem data were included. Gestational age, lipoma size and pattern, corpus callosum size and changes in fetal fat T1 intensity were recorded. Comparison with postmortem neuropathology was available for one fetus.Results: Eleven patients with callosum dysgenesis and pericallosal lipomas (seven curvilinear and four tubulonodular) were included. All MRI scans were performed in the third trimester. Curvilinear lipomas were thinner and six cases were associated with prenatal T1 iso-intensity. Typical T1 hyperintensity appeared on postnatal MRI only. All tubulonodular lipomas were much larger and showed prenatal T1 hyperintensity. In two patients, the lipoma increased in size on postnatal MRI.Conclusion: The type and size of a lipoma influence T1 prenatal intensity. Absence of T1 intensity was observed in curvilinear lipomas only. Curvilinear lipomas are much thinner. Changes in T1 intensity may also be related to fat maturation within the lipoma and, subsequently, to gestational age. In the case of callosum dysgenesis, absence of prenatal T1 pericallosal hyperintensity should not exclude the diagnosis of pericallosal lipoma.