MRI Fetal — brain
Whole fetal brain in three orthogonal planes referenced to the fetal head, not to the mother; a survey of the maternal pelvis and the placenta is usually included.
Typical, not policy
Timings, volumes and delays here are representative values drawn from published guidance. Scanner generation, injector, cardiac output and local preference all move them. Confirm against your department's own protocol before you rely on a number.
When to use it
- Ventriculomegaly on ultrasound, to identify an underlying cause.
- Suspected corpus callosal or posterior fossa malformation.
- Suspected cortical malformation or migrational abnormality.
- Suspected acquired brain injury, including after twin-twin transfusion, haemorrhage or congenital infection.
- Family history of a specific brain malformation with a non-diagnostic ultrasound.
Technique
- Single-shot fast spin-echo T2 in axial, coronal and sagittal planes relative to the fetal head, each plane re-planned from the previous acquisition because the fetus moves.
- Typical in-plane resolution around 0.5 mm with slice thickness of roughly 3-5 mm at 1.5 T.
- T1, diffusion-weighted and gradient-echo or susceptibility-sensitive sequences are added for haemorrhage, ischaemia and calcification.
- Field strength of 1.5 T is generally preferred; 3 T imaging is used in some centres, with higher specific absorption rate and greater artefact as the trade-off.
Where it goes wrong
- Planes prescribed relative to the mother, so no image is truly in a standard fetal anatomical plane.
- Not re-planning each stack after fetal movement, producing three obliquely angled and non-comparable series.
- Slice thickness too great for a small fetus, volume-averaging the cortex and posterior fossa structures.
- Scanning too early in gestation, when small size and rapid movement limit resolution.
- Maternal supine positioning causing discomfort and aortocaval compression in the third trimester, which shortens the achievable study.
Clinical questions that reach this study
Contrast
None
- No gadolinium is given in fetal MRI. Gadolinium crosses the placenta and is not part of any routine fetal protocol.
Acquisition
- Breathing
- Maternal free breathing. Every sequence is ultrafast because the fetus moves between and during acquisitions.
- Preparation
- Supply the ultrasound report and the exact question; a fetal MRI without a stated ultrasound abnormality is rarely justifiable. Gestational age is essential — the study is generally most informative in the second half of pregnancy and small fetuses move more. Gadolinium is not given. Maternal sedation is not routine and should be a documented decision if considered. Position the mother for comfort; left lateral decubitus is often better tolerated in the third trimester than supine.
Safety checks this protocol carries
Derived from the contrast agent and phases above, not authored here — which is why they cannot drift apart from what the protocol actually does.
- MR safety screening for implants and foreign bodies· radiographer at scan
- Gadolinium in known or possible pregnancy· radiographer at scan