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Fetal MRI

Fetal MRI is a problem-solving adjunct to obstetric ultrasound, most often used to clarify central nervous system anomalies and to characterise thoracic, abdominal and placental abnormalities. Ultrafast T2 sequences freeze fetal motion without sedation, and the examination is performed without gadolinium to avoid fetal exposure, usually after the first trimester.

Indications

  • Confirmation and characterisation of CNS anomalies (ventriculomegaly, agenesis of the corpus callosum, posterior fossa malformations, cortical malformations)
  • Thoracic lesions: congenital pulmonary airway malformation, congenital diaphragmatic hernia and lung volume assessment
  • Abdominal and pelvic masses, bowel and genitourinary anomalies
  • Complex or equivocal ultrasound findings limited by maternal habitus, oligohydramnios or fetal position
  • Assessment of placenta accreta spectrum and complicated monochorionic twin pregnancies
  • Surgical/fetal intervention planning and prognostication

Contraindications & Cautions

  • Non-conditional maternal implants (pacemakers, ICDs, neurostimulators) and ferromagnetic devices
  • First trimester is generally avoided; imaging is preferred after 18 weeks when justified
  • Maternal severe claustrophobia or inability to lie still/supine (use left lateral decubitus)
  • Gadolinium is contraindicated (crosses the placenta and recirculates in amniotic fluid)
  • Sedation is avoided; rely on ultrafast motion-tolerant sequences

Patient Preparation

  • Maternal MRI safety screening and confirmation of gestational age
  • No fasting required; empty bladder for comfort
  • Position supine or left lateral decubitus to avoid aortocaval compression in later pregnancy
  • Explain repeated short breath-holds and reassure regarding safety without contrast or radiation
  • Plan sequences relative to the fetal (not maternal) anatomical planes and re-localise as the fetus moves

Technique & Parameters

  • Ultrafast single-shot T2 (SSFSE/HASTE) in three orthogonal fetal planes as the primary sequence
  • Balanced steady-state free precession (true FISP/FIESTA) for vessels, heart and fluid interfaces
  • Echo-planar T2* to detect haemorrhage, calcification and bone
  • T1-weighted gradient-echo to demonstrate meconium, fat, haemorrhage and thyroid/liver signal
  • Diffusion-weighted imaging for the brain and kidneys where indicated
  • 1.5T is preferred over 3T for a favourable specific absorption rate and fewer artefacts; small field-of-view and multiple short acquisitions

Systematic Review

  • Confirm number of fetuses, lie, placental position and amniotic fluid
  • Assess fetal brain: biometry, ventricular width (atrium >10 mm abnormal), midline structures, posterior fossa and cortical sulcation for gestational age
  • Evaluate the face, neck and airway
  • Assess the thorax: lung volumes and signal, cardiac position, diaphragm integrity
  • Review the abdomen and pelvis: bowel signal (meconium T1-bright), kidneys, bladder and abdominal wall
  • Examine the spine, limbs and the placenta/myometrial interface

Key Findings & Significance

  • Ventriculomegaly with atrial width >10 mm and associated anomalies affecting prognosis
  • Agenesis of the corpus callosum with colpocephaly, parallel ventricles and absent cavum septi pellucidi
  • Posterior fossa anomalies: Dandy-Walker malformation, vermian hypoplasia, Blake pouch cyst
  • Congenital diaphragmatic hernia with herniated viscera and reduced lung volume; measured lung-to-head ratio for prognosis
  • Congenital pulmonary airway malformation or sequestration as a T2-hyperintense lung lesion
  • Placenta accreta spectrum: dark intraplacental T2 bands, myometrial thinning and bladder-wall interruption

Differential Considerations

  • Cystic posterior fossa: Dandy-Walker malformation, mega cisterna magna, Blake pouch cyst, arachnoid cyst
  • Fetal chest mass: CPAM, sequestration, congenital diaphragmatic hernia, bronchogenic cyst
  • Abnormal bowel signal: meconium peritonitis, atresia, cystic fibrosis-related changes
  • Intracranial cyst/midline abnormality: interhemispheric cyst, holoprosencephaly, arachnoid cyst
  • Abdominal cystic lesion: ovarian cyst, mesenteric cyst, duplication cyst, hydronephrosis

Pearls & Pitfalls

  • Fetal MRI complements, and does not replace, a detailed anatomy ultrasound; interpret them together
  • Gadolinium is avoided because it crosses the placenta and accumulates in amniotic fluid with unknown fetal effect
  • 1.5T is generally preferred to keep the specific absorption rate low and reduce dielectric artefact
  • Meconium is characteristically T1-bright and T2-dark, helping to localise normal distal bowel
  • Sulcation and myelination milestones must be judged against gestational age to avoid overcalling delay

Structured Report

  • State gestational age, field strength, sequences and that no contrast or sedation was used
  • Report the targeted anomaly with measurements (e.g. ventricular atrial width, lung volumes/ratios)
  • Describe associated findings that alter prognosis and correlate with ultrasound
  • Comment on the placenta and amniotic fluid where relevant
  • Provide an impression with prognostic implications and multidisciplinary/counselling recommendation

References

  • Prayer D, et al. ISUOG Practice Guidelines: performance of fetal MRI. Ultrasound Obstet Gynecol
  • Griffiths PD, et al. MERIDIAN study: use of MRI in fetal brain abnormalities. Lancet
  • ACR-SPR Practice Parameter for the Safe and Optimal Performance of Fetal MRI
  • Saleem SN. Fetal MRI: an approach to practice. Radiographics

Educational clinical decision support only. Protocols vary by institution and equipment; always confirm with a qualified radiologist and local guidelines before clinical use.