Posterior fossa pathology: High-resolution US vs. Fetal MRI

Fetal MRI is claimed to be superior to US for diagnosing posterior fossa pathology. In contrast, our experience is that US allows better visualisation of the cerebellum, thus facilitating diagnosis. Thirteen fetuses with posterior fossa pathology were offered a fetal MRI (0.5 Tesla Philips Apollo MR system) following detailed high-resolution ultrasound (Acuson Sequoia TM 512). Three patients were diagnosed on US with classic Dandy-Walker (CDW), four with Dandy-Walker variant (DWV), three with enlarged cysterna magna (CM) and in three patients other cerebellar pathology was suspected. In 11 of 13 (85%) fetuses, the MRI did not provide additional information. In one fetus with CDW, the MRI confirmed the lesion and in addition showed agenesis of the corpus callosum, but did not change management. In one of the first MRIs on a fetus with DWV, the lesion in the vermis of the cerebellum could not be visualised and this changed the management. Our experience is that small vermal defects or subtle lesions in the posterior fossa are very difficult to visualise on MRI before 24 weeks, because of slice thickness and fetal movements. In these cases US seems superior