Prenatal diagnosis of infratentorial anomalies: ultrasound, fetal MRI, and implications for counseling.

Assessment of the fetal brain is a routine part of standard and detailed prenatal ultrasound. Further evaluation of central nervous system abnormalities identified by prenatal ultrasound is a common indication for fetal MRI. Infratentorial malformations identified by diagnostic imaging may be isolated or may occur with supratentorial anomalies and with multisystemic anomalies or syndromes. The interpretation of posterior fossa abnormalities and their implications for diagnosis and prenatal counseling can benefit from recognizing the dominant morphological feature. This review discusses the ultrasound and MRI characteristics of developmental anomalies affecting the cerebellum and brainstem, focusing on their primary imaging characteristic: cystic anomalies, cerebellar dysgenesis, and brainstem malformations. Key findings on imaging are provided to guide the radiologist in arriving at a reasonable diagnosis. Genetic implications are also provided.