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 poster ior fossa anatomy in fetuses and, if so, to document a template on MR imagi ng for normal posterior fossa development. MATERIALS AND METHODS. A retrospective review found 66 MR imaging studies o f 63 fetuses, 16-39 weeks' gestation age (mean, 25 weeks gestation), who we re referred between June 1996 and May 1999 for evaluation of non-central ne rvous system anomalies revealed on prenatal sonography. All fetuses had nor mal brains and spines on prenatal sonography. The standard MR imaging proto col included axial, sagittal, and coronal half-Fourier acquisition single-s hot turbo spin echo (HASTE): sagittal and coronal two-dimensional fast low- angle shot (FLASH); and axial turbo T1-weighted FLASH images through the fe tal brain. Structures that we analyzed were the fourth ventricle, the ciste rna magna, the vermis, the cerebellar hemispheres, and the brainstem. Using the HASTE sequences, we documented gestational age-specific signal intensi ty changes in the cerebellar hemispheres and the brainstem. RESULTS. The posterior fossa anatomy was sufficiently well defined to exclu de 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 ultra fast MR images, which can be helpful when prenatal sonography is equivocal.