Fetal MRI

Last revised by Joshua Yap on 28 Aug 2023

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
  • 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


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.

Currently, 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 2nd and 3rd 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

  • amniotic cavity

  • 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

  • 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

  • Cerebellum and cisterna magna

  • 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

  • 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
Review spine
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

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Cases and figures

  • Case 1: normal brain fetal MRI - 22 weeks
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  • Case 2: congenital diaphragmatic hernia
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  • Case 3: normal brain fetal MRI - 24 weeks
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  • Case 4: normal fetal MRI (30 weeks)
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  • Case 5: normal brain fetal MRI - 26 weeks
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  • Case 6: microcephaly
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  • Case 7: dysplasia of CSP and anterior corpus callosum
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  • Case 8: agenesis of the corpus callosum
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  • Case 9: congenital diaphragmatic hernia
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  • Case 10: aqueductal stenosis
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  • Case 11: Dandy Walker malformation
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  • Case 12: vein of Galen malformation
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