Foetal MRI

Foetal MRI is a problem solving examination of the foetus, placenta and maternal pelvis, done after ultrasound has shown or suggested an abnormality.

It adds anatomical detail, tissue contrast and a wide field of view that ultrasound lacks, and it is not affected by maternal obesity, oligohydramnios or foetal position to the same degree.

The study uses a 1.5 tesla scanner (3 tesla in some centres) and a surface coil placed over the abdomen. The mother lies supine or left lateral, and a wedge relieves caval compression.

Fast single shot T2-weighted sequences (about 0.5 seconds for each slice) freeze foetal motion. Slices are 3 to 4 mm thick in three planes aligned to the foetal part being studied.

T1-weighted sequences show fat, meconium, blood products and the liver. Diffusion weighted imaging, balanced steady state free precession images, cine sequences and volume reconstruction are added when needed. Gadolinium is not used in pregnancy. The examination takes 30 to 45 minutes.

Foetal MRI is done from 18 weeks, and is most useful from 20 to 22 weeks (for the brain after 20 to 24 weeks) onward, with best results in the third trimester.

Congenital anomalies affect 2 to 4% of births, and ultrasound is the first test. MRI changes the diagnosis or management in 20 to 50% of cases referred for brain anomalies, and it adds information in about 15 to 30% of body anomalies.

The foetal brain, spine, chest masses, congenital diaphragmatic hernia, abdominal and pelvic anomalies and placental disease are the main uses. There is no evidence of harm to the foetus at 1.5 tesla, and the technique is used in all trimesters when clinically indicated.

Indications.

• Central nervous system: ventriculomegaly, agenesis of the corpus callosum, posterior fossa anomalies, cortical malformations, haemorrhage, ischaemia, infection, tumour and spina bifida.
• Congenital diaphragmatic hernia (lung volume, liver position), lung masses and thoracic anomalies.
• Neck and face masses, airway obstruction and cleft assessment (for planning delivery).
• Abdominal wall defects, bowel obstruction, renal and urinary tract anomalies, and pelvic and sacrococcygeal masses.
• Skeletal dysplasia and complex multi-system anomalies.
• Twin pregnancies: complications of monochorionic twins, and the fate of a co-twin after demise.
• Placenta: suspected accreta spectrum and other placental abnormalities.
• Maternal indications when ultrasound is limited: tumours and pelvic masses, appendicitis, and anomalies of the uterus.
• Foetal growth restriction and brain injury after maternal events.
• Pre-surgical planning for in utero or neonatal surgery.

Not for routine screening of normal pregnancy, and not before an adequate ultrasound.

Contraindications and precautions.

• The usual MRI safety screen for the mother. Foetal exposure is not a contraindication.
• Gadolinium crosses the placenta and is avoided in pregnancy unless essential for the mother.
• Claustrophobia: sedation of the mother is avoided where possible. Lorazepam in a small dose or other measures are used only when needed.
• Maternal body size exceeding the bore, and maternal inability to lie flat or hold still.
• Foetal motion limits studies in early gestation, and maternal sedation is not used routinely to reduce it.
• Counselling before the scan covers the aims, the limits and the possibility of new findings.

Preparation and technique.

No fasting is needed, though a light meal before the scan is reasonable. The mother lies supine with the wedge on the right, or in the left lateral position, with the coil over the abdomen. She is told to breathe quietly.

The foetal head and body are scanned in three planes. For the brain, axial, coronal and sagittal single shot T2 images are taken, with repeat acquisition if motion occurs. T1 and diffusion sequences follow. For the chest and abdomen, coronal and sagittal planes show the diaphragm, lungs and organs.

The foetus is measured and volumes are calculated: total lung volume and the observed to expected lung volume ratio in congenital diaphragmatic hernia. Slice-to-volume reconstruction produces 3D datasets of the brain. The placenta, cervix and the maternal pelvis are included when relevant.

Normal appearances.

The foetal brain at 20 to 24 weeks has a smooth surface with few sulci, and sulcation increases with gestational age (the main sulci appear in a known sequence). The germinal matrix is thick and dark on T2 early on and regresses.

The lateral ventricles are under 10 mm at the atrium. The corpus callosum is complete by 20 weeks.

The lungs are bright on T2 and grow with gestation. The liver is bright on T1, and meconium in the distal colon is bright on T1 from about 20 weeks. The stomach and bladder contain fluid, bright on T2. The kidneys are moderately bright on T2.

The placenta is of intermediate signal with uniform lobules, and the myometrium shows a dark layer behind it. The umbilical cord and amniotic fluid are bright on T2.

Interpretation.

• Ventriculomegaly: atrial width 10 to 12 mm is mild, 12 to 15 mm moderate, and over 15 mm severe. MRI looks for associated anomalies (corpus callosum, cortical development, haemorrhage, posterior fossa), which are found in about 10 to 40% of apparently isolated cases and affect prognosis.
• Agenesis or dysgenesis of the corpus callosum: absent or short corpus callosum with parallel lateral ventricles, high riding third ventricle and radial gyral pattern on the medial surface. MRI separates isolated cases from those with other brain anomalies.
• Posterior fossa anomalies: separates cystic malformations (Dandy-Walker malformation, vermian hypoplasia, mega cisterna magna, arachnoid cyst), based on vermis size and rotation and the position of the tentorium and the fourth ventricle.
• Chiari II malformation and spina bifida: open neural tube defect with a small posterior fossa, beaked tectum, ventriculomegaly and the lesion level, important for counselling and in utero repair assessment.
• Cortical malformations: lissencephaly, polymicrogyria, heterotopia and schizencephaly are best seen after 24 to 28 weeks. Delayed sulcation is judged against gestational age.
• Haemorrhage and ischaemia: blood is dark on T2 and bright on T1, with periventricular changes, porencephaly and white matter injury. Diffusion restriction shows acute ischaemia.
• Infection: cytomegalovirus causes ventriculomegaly, periventricular calcification, cortical malformation and white matter changes, and toxoplasmosis and Zika virus produce similar patterns.
• Tumours: teratoma, choroid plexus tumour, and neck and sacrococcygeal teratomas.
• Congenital diaphragmatic hernia: bowel, stomach or liver in the chest, with lung compression. Observed to expected lung volume under 25 to 30% and liver herniation indicate severe disease and the need for specialist therapy.
• Lung masses: congenital pulmonary airway malformation (cystic or solid, bright on T2) and bronchopulmonary sequestration (systemic arterial supply), with mediastinal shift and hydrops risk. Lung hypoplasia shows low T2 signal and volume.
• Neck masses and airway obstruction: lymphatic malformation, cervical teratoma and goitre, with assessment of the airway for planned delivery with specialist airway support.
• Abdominal anomalies: bowel atresia (dilated loops with low T1 signal in the colon, microcolon), gastroschisis and omphalocele, anorectal malformation, and cloacal anomalies.
• Renal and urinary tract: renal agenesis, multicystic dysplastic kidney, obstructive uropathy (posterior urethral valves with a dilated bladder), and renal size and signal.
• Skeletal and body: thoracic dystrophy, and spinal anomalies such as vertebral and sacral defects.
• Twin complications: brain injury after demise of one monochorionic twin, and structural assessment.
• Placenta: placenta accreta spectrum (heterogeneous placenta, T2 dark bands, bulge, loss of the myometrial line, bladder invasion), previa, and abruption.
• Maternal findings: fibroids, ovarian masses, appendicitis and uterine anomalies.

Critical values and findings needing immediate action.

• A major brain anomaly with poor prognosis, or lethal anomaly: multidisciplinary counselling by foetal medicine, neonatology, neurology and surgery, with discussion of options and delivery planning.
• Open neural tube defect: referral for in utero repair assessment (before about 26 weeks) and delivery planning.
• Severe congenital diaphragmatic hernia: referral to a centre for foetal therapy and neonatal intensive care.
• Airway obstruction (neck mass): planned delivery with specialist airway support at a tertiary centre.
• Placenta accreta spectrum: planned delivery with an experienced team and blood availability.
• Evidence of hydrops, or acute foetal brain injury: urgent foetal medicine review.
• Maternal emergency found (appendicitis, torsion, mass): obstetric and surgical review.

Limitations and artefacts.

• Foetal movement blurs the images. Repeated sequences, short acquisition times and post-processing help, and early gestation is hardest.
• Image quality depends on gestational age, maternal size, position and field strength. Before about 20 to 22 weeks, brain detail is limited, and cortical malformations may only appear later.
• MRI cannot replace ultrasound for foetal heart anatomy, growth, Doppler flow, and real time assessment. Cardiac MRI is limited.
• A normal MRI does not exclude all abnormalities, particularly migrational and microscopic changes, and genetic, metabolic and functional disorders.
• Normal development is time dependent, so gestational age must be exact and interpretation uses reference standards for age.
• Susceptibility artefact from gas in maternal bowel and from the foetal skull base may distort images, and the 3 tesla scanner has more.
• Findings may be uncertain, and the prognosis for many findings is unclear (mild ventriculomegaly, vermis hypoplasia, isolated callosal agenesis). Careful counselling and follow up are needed.
• Access to the technique and to experienced readers is limited.
• Gadolinium cannot be used, which limits tumour and vascular characterisation.

Special groups.

• Multiple pregnancy: each foetus is assessed separately, and the fate of a co-twin after demise needs imaging 2 to 4 weeks after the event.
• Obese mothers: MRI is preferred when ultrasound is poor.
• Mothers with claustrophobia: preparation and a wide bore scanner. Sedation is avoided.
• Late gestation (third trimester): best for brain malformations, cortical development, and the lungs.
• Women with a previous caesarean section or placenta praevia: placental assessment for accreta.
• Regions: late booking, limited ultrasound and few foetal medicine services. Foetal MRI is available in a few centres, and most anomalies are managed with ultrasound, parental counselling and planned delivery at a referral hospital. Neural tube defects, hydrocephalus and congenital infections are relatively common, so folic acid, vaccination and infection control are important.

Follow up tests.

• Detailed foetal anomaly ultrasound and foetal echocardiography, with serial growth and Doppler studies.
• Amniocentesis or chorionic villus sampling for chromosomal microarray, karyotype and genetic panels or exome sequencing.
• Maternal serology and amniotic fluid polymerase chain reaction for cytomegalovirus, toxoplasma, rubella, syphilis, Zika and parvovirus.
• Repeat foetal MRI later in pregnancy for evolving brain and body findings.
• Postnatal MRI, ultrasound or CT to confirm the diagnosis.
• Foetal medicine, genetics, neonatology, paediatric surgery, neurosurgery or neurology consultation, and counselling.