We read with much interest two recent articles published in Pediatric Radiology by Blondiaux et al.  and Chougar et al.  addressing the peculiarity of signal intensity of fat tissue on T1-weighted foetal MRI. Those studies focused respectively on subcutaneous fat tissue and pericallosal lipomas, while other authors have also described absent prenatal T1 hyperintensity in lipomas associated with spinal dysraphysms .
We have seen a case of a lipoma of the posterior fossa in a young child with proven congenital cytomegalovirus infection that was also not apparent on two foetal MRI studies performed during the third trimester. A 23-year-old primigravid woman was referred to our centre for prenatal consultation following cytomegalovirus immunoglobulin G seroconversion in the first trimester, which was confirmed by a positive polymerase chain reaction in amniocentesis aspirate. Foetal ultrasound revealed diffuse white matter hyperechogenicity and periventricular calcifications. The mother opted not to terminate the pregnancy and treatment with cytomegalovirus hyperimmune globulin was started. Foetal brain MRI was obtained at 29 weeks and at 33 weeks of gestation. Both studies revealed diffuse T2 hyperintensity in the supratentorial white matter and a left temporal cyst. No other brain abnormalities were demonstrated. The infant was born at 39 weeks’ gestation by Caesarian section. Treatment with intravenous ganciclovir was started at 2 days of age and was maintained for 6 weeks. Postnatal MRI was performed at 7 months of age, confirming the imaging findings previously detected at foetal MRI. Additionally, T1-hyperintense tissue was identified in the left ambiens cistern, disappearing on a post-gadolinium T1-weighted fat-suppressed sequence and thus compatible with a lipoma (Fig. 1). In light of this, both foetal MRI examinations were reviewed but no abnormal T1 hyperintensity could be detected in the corresponding location (Fig. 2).
As suggested [1, 2], in utero differences regarding fat signal intensity on T1-weighted MRI might be explained by progressive replacement of brown adipose tissue by white adipose tissue, a theory supported by histopathological analysis. Interestingly, as also observed by Chougar et al. , although the lipoma in our patient could not be detected on T1-weighted foetal MRI, subcutaneous fat displayed normal T1 hyperintensity as expected by gestational age . This suggests that intracranial lipomas might have a different composition or a delayed maturation when compared to foetal subcutaneous fat. Radiologists and obstetricians need to be aware of this phenomenon while interpreting T1-weighted sequences on foetal MRI and during prenatal counselling.
Blondiaux E, Chougar L, Gelot A et al (2018) Developmental patterns of fetal fat and corresponding signal on T1-weighted magnetic resonance imaging. Pediatr Radiol 48:317–324
Chougar L, Blondiaux E, Moutard ML et al (2018) Variability of T1-weighted signal intensity of pericallosal lipomas in the fetus. Pediatr Radiol 48:383–391
Nagaraj UD, Bierbrauer KS, Peiro JL, Kline-Fath BM (2016) Differentiating closed versus open spinal dysraphisms on fetal MRI. AJR Am J Roentgenol 207:1316–1323
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Geraldo, A.F., Valente, F., Almeida, N. et al. Posterior fossa lipoma without T1 hyperintensity in foetal magnetic resonance imaging performed in the third trimester. Pediatr Radiol 48, 1178–1179 (2018). https://doi.org/10.1007/s00247-018-4168-y