Ultrafast MR imaging of the normal posterior fossa in fetuses

OBJECTIVE: The purpose of our study was to determine if a standard imaging protocol using ultrafast MR sequences could adequately reveal normal posterior fossa anatomy in fetuses and, if so, to document a template on MR imaging for normal posterior fossa development. MATERIALS AND METHODS: A retrospective review found 66 MR imaging studies of 63 fetuses, 16-39 weeks' gestation age (mean, 25 weeks' gestation), who were referred between June 1996 and May 1999 for evaluation of non-central nervous system anomalies revealed on prenatal sonography. All fetuses had normal brains and spines on prenatal sonography. The standard MR imaging protocol included axial, sagittal, and coronal half-Fourier acquisition single-shot turbo spin echo (HASTE); sagittal and coronal two-dimensional fast low-angle shot (FLASH); and axial turbo T1-weighted FLASH images through the fetal brain. Structures that we analyzed were the fourth ventricle, the cisterna magna, the vermis, the cerebellar hemispheres, and the brainstem. Using the HASTE sequences, we documented gestational age-specific signal intensity changes in the cerebellar hemispheres and the brainstem. RESULTS: The posterior fossa anatomy was sufficiently well defined to exclude abnormalities of the fourth ventricle and cerebellar vermis in all cases. Because of high T2-weighting, good contrast enhancement, and good signal-to-noise ratios, HASTE images provided the best anatomic definition of the posterior fossa. CONCLUSION: Normal posterior fossa anatomy can be adequately shown on ultrafast MR images, which can be helpful when prenatal sonography is equivocal.