PRENATAL IMAGING ADVANCES: PHYSIOLOGY AND FUNCTION TO MOTION CORRECTION AND AI: PICTORIAL REVIEW Algorithmic approach to neuroradiological diagnosis with pre- natal MRI: non- visualization of the fetal cavum septi pellucidi on mid- trimester screening ultrasound

This pictorial review provides an algorithmic approach and framework for systematic diagnostic evaluation of the fetus with intrauterine/fetal MRI (iuMRI) failed visualiza-tion of the cavum septum pellucidum (CSP) flanked by the septal leaflets (SLs) at the mid-trimester ultrasound. Pitfalls in sonographic diagnosis of normal or abnormal CSP/SLs, iuMR evaluation and differentiation of failed formation from perforation of the SLs, the latter secondary to obstructive hydrocephalus (OH), usually at the cerebral aqueduct, are discussed. Diagnosis with iuMR of malformations associ-ated with obstructive hydrocephalus that are commonly underdiagnosed with ultrasound are highlighted. Failed formation of SLs can be an isolated abnormality or associ-ated with abnormal formation of the corpus callosum, the holoprosencephaly spectrum, malformations of cortical development and Apert Syndrome. Septo-optic dysplasia is absent post-natally in most fetuses with isolated absence of the SLs. We describe the "five spots sign" to evaluate the pituitary and optic nerves on iuMR to help risk stratify the fetus with isolated failed SL formation for the presence of septo-optic dysplasia. This review will help clinicians to provide more accurate, evidence-informed counselling of pregnant females with a fetus who has absence of the SLs.Advances in knowledge: A systematic approach by the radiologist to analysis of imaging and other clinical data in the fetus with absent septal leaflets suspected on ultrasound will improve diagnostic efficiency, accuracy, and pre -natal counselling.