Current techniques and future directions for fetal MRI

The widespread application of modern MRI techniques to the live fetus in utero is an evolving endeavor, contingent on ongoing improvement in hardware and software. At present, all major MRI techniques, including structural MRI imaging, diffusion weighted imaging (DWI), magnetic resonance spectroscopy (MRS), diffusion tensor imaging (DTI), and functional MRI, have been adapted for studies of the maternal–fetal unit with varying degrees of success. Issues affecting adaptation of MRI techniques to fetal imaging include safety, fetal motion, and the small but changing size, anatomy, and composition of the fetal organs. Consequently, clinical and research studies employing MRI techniques of the maternal–fetal unit are performed during the second and third trimester, eschewing the use of contrast (gadolinium) agents. Fetal movement is currently addressed by using multiple acquisitions with ultrafast sequences, to which T2-weighted imaging is most amenable. The changing landscape of fetal size and tissue compositions requires adaptation of the field of view and sequence parameters to the age of the fetus for best contrast and resolution in anatomical as well as physiological (DWI) and biochemical (MRS) imaging techniques. Techniques requiring longer acquisition times and thus more sensitive to fetal motion, such as MRS, DTI, and fMRI, are used mostly in the research setting. Future expansion of fetal MRI applications relies on the adaptation of 3D acquisition, automated motion correction, and volumetry as well as the establishment of multiple normative databases of fetal development parameters for each gestational week.