A second-line problem-solver after an equivocal ultrasound — and it must be read against GESTATIONAL AGE, because the normal brain changes weekly.
Orient first
- Fetal MRI answers a QUESTION ultrasound has already raised. It is not a screening study, and the report should name the ultrasound finding it was asked to resolve.
- THE NORMAL APPEARANCE IS GESTATION-DEPENDENT. Sulcation, myelination and the germinal matrix all change week by week, so an appearance that is normal at 24 weeks is abnormal at 34. Every judgement is made against a gestation-specific expectation.
- Its main advantages over ultrasound are the posterior fossa, the corpus callosum, cortical malformations, and haemorrhage or ischaemia — all poorly seen through bone late in pregnancy.
- VENTRICULOMEGALY is a measurement, not a diagnosis: 10–12 mm mild, 12–15 mm moderate, above 15 mm severe. The important work is finding the CAUSE and any associated anomaly, because isolated mild ventriculomegaly has a very different outlook.
- Motion is the dominant technical problem and the main cause of a non-diagnostic study.
Acquire the study
- Single-shot fast T2 sequences in three orthogonal planes aligned TO THE FETAL BRAIN, not to the mother — and re-planned after every fetal movement.
- Add T1 for haemorrhage and for myelination, gradient-echo or susceptibility imaging for blood products, and diffusion for ischaemia.
- No gadolinium.
- Mother supine or left lateral; no sedation routinely — repeat the sequence instead.
- Confirm which sequence you are on before judging any signal — see the MRI sequence primer.
The manoeuvre
- State the gestational age and read every structure against that expectation.
- Measure the ATRIAL WIDTH of both lateral ventricles on a true axial plane, perpendicular to the ventricular axis.
- Assess the CORPUS CALLOSUM on a true midline sagittal image, and the cavum septi pellucidi.
- Assess the POSTERIOR FOSSA: cerebellar vermis, its rotation, the fourth ventricle and the cisterna magna.
- Assess SULCATION against the gestation-specific expectation — delayed or abnormally deep sulcation indicates a cortical malformation.
- Look for haemorrhage on susceptibility-weighted imaging and for ischaemia on diffusion.
- Assess the extracerebral spaces and for any mass or cyst.
- Look for associated anomalies OUTSIDE the brain in the same study, since these change counselling most.
- State whether the finding appears ISOLATED, because that is the strongest prognostic statement available.
What confirms it
- A structural diagnosis needs the abnormality to be visible in more than one plane and to be inconsistent with the gestation-specific normal appearance.
What licenses you to exclude it
- ⚠️ A motion-degraded study excludes nothing. Report it as non-diagnostic for the specific question rather than as normal.
- Some malformations evolve, and sulcation abnormalities in particular may not be assessable before about 26 weeks — say when a repeat would be informative.
- Fetal MRI does not exclude a genetic or metabolic diagnosis; imaging and karyotype answer different questions.
The classic misread
- Reading sulcation without reference to gestational age.
- Measuring the atrium on an oblique plane.
- Reporting ventriculomegaly without searching for a cause or associated anomalies.
- Assessing the vermis on a plane that is not a true midline sagittal.