Fetal MRI

Fetal MRI allows for detailed imaging of the developing fetus in utero. Fast sequences are required due to fetal movement ¹. Fetal MRI is most commonly utilized when ultrasound findings are equivocal. Fetal anatomy can be evaluated in detail including the brain, upper aerodigestive tract, thorax, pelvis and abdomen. Imaging can be performed from the second trimester ². Biosafety and small fetal size in the first trimester makes fetal MRI of equivocal/controversial use prior to the second trimester ².

General indications

• abnormality suspected on ultrasound but poorly seen due to fetal position, maternal habitus, oligohydramnios, overlying ossification or limited field of view ⁴

• abnormality on ultrasound is poorly defined or equivocal and further clarification is required for ongoing management or prognostication

• the fetus is considered to be at high risk for a pathology that cannot be evaluated on ultrasound

Specific indications

• suspected brain and spine anomalies on ultrasound requiring further evaluation:

  • familial disorders (e.g. tuberous sclerosis, lissencephaly, corpus callosal dysgenesis)

  • cephalocele

  • solid/cystic masses

  • cerebral cortical malformations / migrational anomalies

  • posterior fossa abnormalities

  • holoprosencephaly

  • abnormalities of the corpus callosum or cavum septum pellucidum

  • ventriculomegaly

  • suspected cerebral vascular anomalies

    • vascular malformations

    • hydranencephaly

    • infarct

    • hemorrhage

    • monochorionic twin pregnancy complications

  • suspected congenital anomalies of the spine

    • neural tube defects

    • sacrococcygeal teratoma

    • caudal regression / sacral agenesis

    • vertebral anomalies

  • head and neck

    • suspected mass of the face/neck

      • vascular malformation

      • lymphatic malformation

      • teratoma

      • clefts

      • congenital cysts / cystic masses

    • airway concerns

  • thorax

    • congenital lung and airway malformations

    • congenital diaphragmatic hernias

    • effusions

    • mediastinal masses

    • esophageal atresia

    • volumetric assessment of lungs in fetus at risk for pulmonary hypoplasia

  • abdominopelvic

    • suspected mass/cyst

    • suspected genitourinary anomaly, inadequately assessed by ultrasound (e.g. secondary to severe oligohydramnios)

    • suspected anorectal malformation / complex bowel obstruction / abdominal wall defect

  • musculoskeletal

    • assessment of suspected masses, malformations or dysplasias

  • multiple gestation pregnancies

    • monochorionic twins

      • investigation of vascular anatomy to facilitate laser therapy

    • conjoined twins

      • optimize parental counseling, delivery planning and postnatal management

  • pre-interventional planning

    • outline anatomy prior to fetal intervention

  • placenta

    • placental anomalies not adequately assessed by ultrasound

      • gestational trophoblastic disease

      • disorders of placentation

Limitations

• reduced signal-to-noise ratio and partial volume artifact (especially prior to 18 weeks gestation)

• maternal weight/size exceeds table recommendations or MRI caliber

• claustrophobia

• implanted ferromagnetic devices

• assessment of cardiac structures is limited by rapid fetal heart rate and fetal movement

Standard sequences

• single-shot fast spin-echo (SSFSE) T2-weighted imaging ¹

• T1-weighted imaging (fat, calcification, hemorrhage) ¹

• steady-state free-precession (SSFP) can be used for imaging of the heart and blood vessels ¹

Biosafety

Fetal MRI is a non-invasive examination and is not associated with ionizing radiation. However, the effects on the developing fetus of static magnetic fields, noise, radiofrequency pulses and rapidly changing gradients are yet to be fully elucidated.

Imaging in the first trimester is not recommended. The ACR recommends imaging after 18 weeks, as the effects of static magnetic fields, radiofrequency pulses (inherent potential for tissue heating) and rapidly changing gradients are yet to be defined prior to 18 weeks. While high levels of acoustic exposure have been linked to low birth weights, shorter gestations and hearing loss, the use of 1.5 T MRI in the 2^nd and 3^rd trimesters has not been associated with hearing abnormality in multiple studies.

Practical approach

Review pregnancy structures

• placenta

  • chronicity/amnionicity

  • location (anterior/posterior, previa, low lying)

  • accreta/increta/percreta

  • morphology (bilobed, single lobed, succenturiate, circumvallate)

• cord

  • insertion (central, marginal, velamentous)

  • three or two vessel cord

• amniotic cavity

  • presence of amniotic bands

• cervix

  • long and closed

  • shortened

  • effaced

• presentation

Review fetal signs of life

• normal cardiac signal void present with normal cardiac activity

• "swirling" of amniotic fluid infers normal fetal movements

Review central nervous system

• check cranial vault, BPD, OFD and HC

  • the correct plane is at the level of the cavum and tentorial hiatus

  • BPD measured from inner table to outer table of skull

  • OFD measured outer to outer

  • HC = (OFD + BPD)/2*π

    • anencephaly

    • exencephaly

    • macrocephaly

    • microcephaly

• ventricular caliber and choroid plexus

  • correct plane slightly above the BPD/HC plane

  • perpendicular to the lateral wall

  • measurement of ventricular caliber is just posterior to the choroid plexus

    • hydrocephalus

• Cerebellum and cisterna magna

  • the correct plane is just below the BPD/HC plane, tilted to the posterior fossa

    • Dandy-Walker spectrum

• cerebellar vermis

  • the correct plane is the midline sagittal plane, with the primary fissure and decline seen posteriorly

    • vermian hypoplasia

• orbits

  • binocular distance (BOD)

    • BOD between the two malar margins of each vitreous

  • interocular distance (IOD)

    • IOD is measured between the two ethmoidal margins of each vitreous

  • ocular diameter (OD)

    • OD is measured between the malar and ethmoidal margins of the vitreous

  • microphthalmia, hypertelorism, persistent hyperplastic primary vitreous in T2

• midline structures

  • septo-optic dysplasia (absent cavum, fused fornices)

  • holoprosencephaly (monoventricle)

  • callosal agenesis

  • frontonasal dysplasia

• sulcation and gyration ⁶

  • the formation of sulci in the healthy fetus is so precise, that sulcation and gyration can be used as a reliable estimate of gestational age and marker of brain maturation ⁵

  • 22-23 weeks: the parieto-occipital fissure should be seen on the medial aspect of the posterior cortex on axial view

  • 24-25 weeks: the calcarine fissure should be seen on the medial aspect of the posterior cortex

  • 26-27 weeks: the Rolandic sulcus is seen on the superolateral aspect of the cortex on axial view

  • 29 weeks: the superior temporal sulcus is seen on the lateral aspect of the cortex on the coronal view

• lamination ^7,8

  • 15-26 postovulatory weeks:

    • typical fatal lamination pattern present in five distinct layers on T1 weighted imaging ^7,8

      1. ventricular zone / germinal matrix (high signal intensity)

      2. periventricular fiber-rich zone (low signal intensity)

      3. intermediate zone (moderate signal intensity, includes the subventricular cellular zone and fetal white matter)

      4. subplate zone (low signal intensity)

      5. cortical plate (high signal intensity)

  • 27-30 postovulatory weeks

    • gradual blueing of the laminar structure, parallel to cerebral gyri formation

    • increased signal intensity of the subplot zone compared to adjacent white matter, reducing contrast resolution ⁸

  • 31-36 postovulatory weeks

    • further reduction in subplate signal intensity, the "subplate dissolution stage" ⁸

    • subplate zone and white matter/ intermediate zone become almost isointense ⁸

Review face and calvarium

• facial profile and lips

  • lips and alveolar margins should be in alignment on the sagittal view

  • Pierre Robin, cleft lip and palate

• ears

  • external auditory meati

  • pinnae

  • cochlea

    • cup ear deformity

    • branchial arch syndromes

    • microtia in trisomy 22

Review spine

• should be assessed with the amniotic fluid between the fetus and uterine wall

  • spina bifida

  • neurenteric cyst

  • teratoma

Review body

• biometry: abdominal circumference measured axially at the level of the junction of the portal veins

• situs

• the aortic arch will appear as a shepherd's crook-shaped flow void on sagittal imaging

• diaphragms present bilaterally, dividing thorax from the abdomen

• stomach on left, seen to change in size during the examination

• no stomach seen may indicate esophageal atresia 

• the liver is low signal intensity on SSFSE

• the gallbladder is ovoid in appearance

• renal pelvis 5 mm AP or less

• renal length is approximately equal to the gestational age in weeks

• the bladder should appear ovoid or bean-shaped and should be seen to empty and fill during the scan 

Review limbs

• confirm four limbs, with both proximal and distal components present

• talipes can only be assessed if feet are away from the uterine wall 

Review imaged maternal structures

• maternal physiological hydronephrosis

• maternal ovaries