Background Evaluation of fetal central nervous system (CNS) agenesis by ultrasonography (US) is frequently limited, but magnetic resonance imaging (MRI) has its own advantages and is gaining popularity in displaying suspected fetal anomalies. The purpose of this study was to explore the value of MRI in detecting fetal CNS agenesis. Methods Thirty-four women (aged from 22 to 35 years, average 27 years) with complicated pregnancies (16- 39 weeks of gestation, average 30 weeks) were examined with a 1.5 T superconductive MR unit within 24 hours after ultrasonography. Half-Fourier acquisition single-shot turbo spin-echo (HASTE) T2-weighted imaging (T2WI) sequence were performed in all patients, and fast low angle shot (FLASH) T1-weighted imaging (T1WI) sequence were applied sequentially in seven of them. Comparison of the results was made between the MRI and US findings as well as autopsy or postnatal follow-up MRI findings. Results The gyrus, sulcus, corpus callosum, thalamus, cerebellum, brainstem, and spinal cord of fetus were shown more clearly on T2-weighted MR images than on T1weighted MR images. MRI corrected the diagnosis of US in 10 cases (10/34, 29.41%) and the diagnosis was missed only in 1 case (1/34, 2.94%). Conclusion MRI has advantages to US in detecting fetal CNS anomalies and is a supplement to US in complicated pregnancies. Chin Med J 2006; 119(15): 1272-1277 Background Evaluation of fetal central nervous system (CNS) agenesis by ultrasonography (US) is frequently limited, but magnetic resonance imaging (MRI) has its own advantages and is gaining popularity in displaying suspected fetal anomalies. The purpose of this study was to explore the value of MRI in detecting fetal CNS agenesis. Methods Thirty-four women (aged from 22 to 35 years, average 27 years) with complicated pregnancies (16-39 weeks of gestation, average 30 weeks) were examined with a 1.5 T superconductive MR unit Half-Fourier acquisition of single-shot turbo spin-echo (HASTE) T2-weighted imaging (T2WI) sequence were performed in all patients, and fast low angle shot (FLASH) T1- weighted imaging were applied sequentially in seven of them. Comparison of the results was made between the MRI and US findings as well as autopsy or postnatal follow-up MRI findings. Results The gyrus, sulcus, corpus callosum, thalamus, cerebellum, brainstem, and spinal cord of fetus were shown more clearly on T2-weighted MR images than on T1weighted MR images. MRI corrected the diagnosis of US in 10 cases (10/34, 29.41%) and the diagnosis was missed only in 1 case ( 1/34, 2.94%). Conclusion MRI has advantages to US in detecting fetal CNS anomalies and is a supplement to US in complicated pregnancies. Chin Med J 2006; 119 (15): 1272-1277