Fetal MRI

Fetal MRI

Fetal MRI

General Indications:

• Abnormalityabnormality suspected on ultrasound but poorly seen due to fetal position, maternal habitus, oligohydramnios, overlying ossification or limited field of view ⁴.
• Abnormalityabnormality on USS is poorly defined or equivocal and further clarification is required for ongoing management or prognostication.
• Thethe fetus is considered to be at high risk for a pathology that cannot be evaluated on ultrasound.

Specific indications ⁴:

• Suspected Brain and spine anomalies on USS requiring further evaluation:
  • Familialfamilial disorders (ie TS, lissencephaly, corpus callosal dysgenesis)
  • Cephalocelecephalocele
  • Solidsolid/cystic masses
  • Cerebralcerebral cortical malformations/migrational anomalies
  • Posteriorposterior fossa abnormalities
  • Holoprosencephalyholoprosencephaly
  • Abnormalitiesabnormalities of the corpus callosum or cavum septum pellucidum
  • Ventriculomegalyventriculomegaly
  • Suspectedsuspected cerebral vascular anomalies
    • Vascularvascular malformations
    • Hydranencephalyhydranencephaly
    • Infarctinfarct
    • Haemorrhagehaemorrhage
    • Monochorionicmonochorionic twin pregnancy complications
  • Suspectedsuspected congenital anomalies of the spine
    • Neuralneural tube defects
    • Sacrococcygealsacrococcygeal teratomas
    • Caudalcaudal regression/sacral/sacral agenesis
    • Vertebralvertebral anomalies
  • Headhead and Neckneck
    • Suspectedsuspected mass of the face/neck
      • Vascularvascular malformation
      • Lymphaticlymphatic malformation
      • Teratomateratoma
      • Cleftsclefts
      • Congenitalcongenital cysts/cystic masses
    • Airwayairway concerns
  • Thoraxthorax
    • Congenitalcongenital lung and airway malformations
    • Congenitalcongenital diaphragmatic hernias
    • Effusionseffusions
    • Mediastinalmediastinal masses
    • Oesophagealoesophageal atresia
    • Volumetricvolumetric assessment of lungs in fetus at risk for pulmonary hypoplasia
  • Abdominopelvicabdominopelvic
    • Suspectedsuspected mass/cyst
    • Suspectedsuspected genitourinary anomaly, inadequately assessed by ultrasound (ie secondary to severe oligohydramnios)
    • Suspectedsuspected anorectal malformation/complex bowel obstruction/abdominal wall defect
  • Musculoskeletalmusculoskeletal
    • Assessmentassessment of suspected masses, malformations or dysplasias
  • Multiple Gestation Pregnanciesmultiple gestation pregnancies
    • Monochorionicmonochorionic twins
      • Investigationinvestigation of vascular anatomy to facilitate laser therapy
    • Conjoinedconjoined twins
      • Optimiseoptimise parental counseling, delivery planning and postnatal management.
  • Pre-Interventional Planningpre-interventional planning
    • Outlineoutline anatomy prior to fetal intervention
  • Placentaplacenta
    • Placentalplacental anomalies not adequately assessed by ultrasound
      • Gestationalgestational trophoblastic disease
      • Disordersdisorders of placentation.

Limitations of Fetal MRI:

• Reducedreduced signal to noise ratio and partial volumingvolume artifact (especially prior to 18weeks gestation).

• Maternalmaternal weight/size exceeds table recommendations/MRI caliber
• Claustrophobiaclaustrophobia
• Implantedimplanted ferromagnetic devices
• Assessmentassessment of cardiac structures is limited by rapid fetal heart rate and fetal movement.

Standard sequences:

• Singlesingle-shot fast spin-echo (SSFSE) T2-weighted imaging ¹
• T1-weighted imaging (fat, calcification, haemorrhage) ¹
• Steadysteady-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 ionising radiation.

• However, the effects upon 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. 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, use of 1.5T 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)

  • Acreta/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

Reviewfetal 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*Pi
    • 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
• Cerebella vermis
  • The correct plane is the midline sagittal plane, with the primary fissure and declive 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 T21
• Midline structures
  • Septo-optic dysplasia (absent cavum, fused fornices)
  • Holoprosencephaly (monoventricle)
  • Callosal agenesis
  • Frontonasal dysplasia
• Sulcation and gyration⁶
  • 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 fetus and the uterine wall.

  • Spina bifida

  • Neuroenteric 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 oesophageal atresia 
• The liver is low signal intensity on SSFSE
• Gallbladder ovoid in appearance. 
• Renal pelvis 5mm 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