Objective To evaluate normal maturation of the fetal brain with half-Fourier rapid acquisition with relaxation enhancement(RARE) MRI.Methods The normal brains of 25 fetuses of 12-38 weeks gestational age were examined in utero with half-Fourier RARE imaging.Gyrus maturation,gray and white matter differentiation,ventricle-to-brain diameter ratio,and subarachnoid space size were evaluated with respect to gestational age.Results At 12-23 weeks,the brain had a smooth surface,and two or three layers were differentiated in the cerebral cortex.At 24-26 weeks,only a few shallow grooves were seen in the central sulcus,and three layers,including the immature cortex,intermediate zone,and germinal matrix,were differentiated in all fetuses.At 27-29 weeks,sulcus formation was observed in various regions of the brain parenchyma,and the germinal matrix became invisible.Sulcation was seen in the whole cerebral cortex from 30 weeks on.However,the cortex did not undergo infolding,and opercular formation was not seen before 33 weeks.At 23 weeks and earlier,the cerebral ventricles were large;thereafter,they gradually became smaller.The subarachnoid space overlying the cortical convexities was slightly dilated at all gestational ages,most markedly at 21-26 weeks.Conclusion Changes in brain maturation proceed through stages in an orderly and predictable fashion and can be evaluated reliably with half-Fourier RARE MRI.