Contemporary fetal diagnosis should be a multidisciplinary endeavour. The relative contributions of MRI and ultrasound (US) for diagnosis of fetal cranial abnormalities in clinical practice depend upon:
1. The nature of the fetal abnormality and gestational age at which it is suspected
2. Availability of expertise in neurosonography and MRI, technological and medical
3. Accessibility and quality of equipment and laws in some jurisdictions that limit field strength of MRI when it used to evaluate the fetus
4. Jurisdictional funding / reimbursement mechanisms for both modalities
5. The medicolegal environment - whether patients, practitioners and / or the legal system perceive that failure to refer a pregnant patient for fetal MRI when a cranial abnormality is suspected could be construed as medical negligence
6. Clinical practice guideline recommendations that may be designed for a specific healthcare delivery system but inappropriate or unimplementable in another
7. The nature and timing of counselling provided to the pregnant woman, and the craft group of the provider of this counselling, regarding the relative value of US and MR for determining diagnosis, prognosis and likelihood of recurrence
8. Availability of legal medical termination of pregnancy and fetal therapies, such as laser photocoagulation of placental anastomoses in twin ñ twin transfusion syndrome.
In the era of personalised medicine, it is no longer acceptable to provide prognostic counselling for a fetus with severe ventriculomegaly as if all fetuses with this finding had the same medical condition. Evaluation of the ganglionic eminences, brainstem, cerebral cortex and cerebellum with MRI, when integrated with ultrasound evaluation of the fetus as a whole, can result in accurate prenatal diagnosis of tubulinopathy, mTOR / PI3K pathway mutations, lissencephalies, severe peroxisomal disorders, PHACES, Joubert syndrome, pontocerebellar atrophy and hypoplasia (PCAH), prognostically important cortical injuries in fetuses affected by CMV, twin ñ twin transfusion or the death of a monochorionic co ñ twin when these diagnoses unfeasible with ultrasound, even in the hands of the best neurosonologist.
Although proton MR spectroscopy is feasible in the fetus, and diffusion tensor imaging can demonstrate aberrant white matter tract trajectories in agenesis of the corpus callosum, PCAH, L1 CAM (X ñ linked) related aqueductal stenosis and Joubert syndrome, in practice these advanced techniques often do not work especially when the fetus is very young and the head not immobilised in the maternal pelvis. Education internationally continues to lag well behind what is daily being achieved with fetal MR in specialist centres.