Multiple processes occur during embryological development of brain and spine

Neurulation : Ectodermal cells form midline neural plate in which folds develop then fuse to create a tube with openings at either end

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  Neurulation → neural tube + neural crest

  
  
  
  
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    Neural tube → brain, spinal cord

    
    
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    Neural crest → peripheral nerves, autonomic nervous system

  
  

Neuronal proliferation : Neurons are “born” in ventricular zone; migrate peripherally to form white/gray matter

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  Glioblast cells provide metabolic/structural support to neurons; ependymal cells produce cerebrospinal fluid

Histogenesis : Process of proliferation, migration, differentiation → development of mature cerebral cortex

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  Cerebral hemispheres formed by 11th week

  
  
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  Corpus callosum should be complete by 20 weeks

Neuronal migration : Peak activity occurs from 11-15 weeks; majority of neurons in correct location by 24 weeks; continues up to 35 weeks

Myelination : Occurs in orderly, predictable manner from caudal → cranial, deep → superficial, posterior → anterior

Operculization : Development of insular cortex and infolding of sylvian fissures during weeks 11-28

Gyral and sulcal development : Occurs in predictable fashion; continues through end of 35th week

Standard scan planes

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  Transventricular: Axial image superior to thalami

  
  
  
  
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    Size, shape, and orientation of lateral ventricles, choroid plexus, falx

  
  
  
  
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  Transthalamic: Axial image at level of thalami, includes cavum septi pellucidi (CSP)

  
  
  
  
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    3rd ventricle, CSP, cerebellar hemispheres

  
  
  
  
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  Transcerebellar: Oblique axial image at level of CSP, tipped to include posterior fossa structures

  
  
  
  
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    Cerebellar hemispheres, vermis, cisterna magna

  
  

Use highest resolution transducer possible

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  If fetus in cephalic presentation, use vaginal transducer for brain evaluation

  
  
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  If fetus in breech presentation, use vaginal ultrasound (US) for distal spine evaluation, particularly in late gestation, obese maternal habitus, or low fluid

  
  
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  9-MHz linear transducer provides exquisite detail of accessible structures

Image brain in more than axial planes; coronal and sagittal images can be obtained with transducer manipulation

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  3D volume acquisition allows reconstruction of dataset to “create” true orthogonal image planes

Use color Doppler to evaluate course of marker vessels

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  Normal anterior cerebral artery branches (callosomarginal and pericallosal) run along corpus callosum

Fetal MR is a problem-solving tool that can be used to clarify abnormal US findings

If distal spine looks abnormal, double check bladder, external genitalia, anal dimple

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  Common embryological precursor from caudal cell mass

Head shape and size

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  Familial head size variants are common

  
  
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  Microcephaly usually associated with severe brain abnormalities

  
  
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  Macrocephaly may relate to hydrocephalus, megalencephaly, intracranial tumors

  
  
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  Head shape may be a key to a diagnosis

  
  
  
  
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    Cloverleaf: Thanatophoric dysplasia, lemon: Chiari malformation, strawberry: Trisomy 18

  
  
  
  
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  Cephaloceles are most commonly occipital but may be at other locations

Midline

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  Falx cerebri creates midline linear echo bisecting cranium, separating cerebral hemispheres

  
  
  
  
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    Present in severe hydrocephalus, hydranencephaly

    
    
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    Absent in alobar holoprosencephaly

    
    
    
    
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      Variable posterior component in other forms of holoprosencephaly

    
    
    
    
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    On coronal images, midline echo continues from falx, lines up with CSP, 3rd ventricle

    
    
    
    
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      Subtle abnormality described as distortion of the interhemispheric fissure seen when anterior falx-deficient, medial surface gyri interdigitate across midline

    
    

  
  
  
  
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  CSP should be visible from 18-37 weeks

  
  
  
  
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    Box-like structure with bright linear echogenic walls surrounding an anechoic space, between frontal horns of lateral ventricles

    
    
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    Marker of normal midline development

    
    
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    By term, septi often fuse → septum pellucidum with obliteration of the cavum

  
  
  
  
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  Pitfall: Fornices, just caudal to CSP, create series of parallel black and white lines; do not form a box shape

Ventricles

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  Lateral ventricles should be symmetric in size with butterfly wing configuration; parallel orientation is abnormal

  
  
  
  
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    Frontal horns are narrow, almost slit-like at term

    
    
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    Widest portion is ventricular atrium; confluence of body with occipital, temporal horns

    
    
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    Ventricular diameter measured at atrium, perpendicular to long axis of ventricle, inner edge to inner edge

    
    
    
    
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      Should always be ≤ 10 mm

    
    

  
  

Cerebral hemispheres

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  Fissures and sulci develop as cortical mantle grows

  
  
  
  
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    Fissures are deeper infoldings than sulci with fixed position on cerebral surface

    
    
    
    
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      Sulci are shallower, more subject to individual variation

    
    
    
    
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    Interhemispheric fissure seats falx cerebri, traverses brain from anterior to posterior

    
    
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    Sylvian fissure initially appears as shallow indentation on lateral surface of brain (~ 18 weeks)

    
    
    
    
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      Indentation deepens, becomes “squared off,” shaped like an open box (~ 24 weeks)

      
      
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      Eventually becomes covered by process of opercularization, which is not complete until term

    
    

  
  
  
  
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  Gestational age when sulcus/fissure should be seen

  
  
  
  
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    Sylvian: US at 18 weeks, MR at 24 weeks

    
    
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    Parietooccipital: US at 18 weeks, MR at 22-23 weeks

    
    
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    Calcarine: US 18 weeks, MR at 22-23 weeks

  
  

Posterior fossa

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  Visually inspect occipital bone contour; cephaloceles may be quite small and subtle

  
  
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  Cisterna magna depth is measured in the midline, from posterior surface of vermis to inner table of calvarium

  
  
  
  
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    Should be < 10 mm throughout gestation

    
    
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    Linear echoes in the cisterna magna are thought to be vestigial remnants of the walls of the Blake pouch

  
  
  
  
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  Normal falx cerebelli bisects posterior fossa

  
  
  
  
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    If asymmetric position, look for space-occupying lesions (e.g., arachnoid cyst) or asymmetry of hemispheres (e.g., cerebellar hemihypoplasia)

  
  
  
  
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  Torcular Herophili marks confluence of transverse sinus with straight/superior sagittal sinuses

  
  
  
  
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    Enlargement of the cisterna magna (e.g., in Dandy-Walker malformation) causes torcular elevation

  
  
  
  
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  Normal cerebellum is composed of 2 rounded lobes joined in midline by vermis

  
  
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  Cerebellar vermis is more echogenic than hemispheres

  
  
  
  
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    On transcerebellar plane, transverse diameter of echogenic vermis is measured at level of 4th ventricle

    
    
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    On sagittal view, craniocaudal diameter can be measured at limits of a line drawn perpendicular to fastigial declive line

    
    
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    Tegmentovermian angle is angle between a line along dorsal brainstem surface parallel to tegmentum and a line along ventral surface of vermis

    
    
    
    
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      Normal angle is close to zero

      
      
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      Angle < 30 are likely due to Blake pouch cyst

      
      
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      Angle > 45° strongly associated with Dandy-Walker malformation

    
    

  
  
  
  
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  4th ventricle (V4) assessment is an integral part of vermian evaluation

  
  
  
  
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    On axial views, V4 is quadrangular with anteroposterior diameter < transverse diameter

    
    
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    Fastigial point is the posterior, superior recess of V4

    
    
    
    
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      Forms an acute angle at apex of triangular-shaped V4 on sagittal view

    
    
    
    
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    Declive is cerebellar lobule just inferior to primary fissure

    
    
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    Fastigial declive line used as landmark for vermian measurement

  
  
  
  
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  Brainstem and pons

  
  
  
  
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    Normal pons creates a prominent bulge anterior to 4th ventricle on sagittal view

    
    
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    Biometric data available

  
  

Check alignment in coronal and sagittal planes if possible

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  Coronal plane is best for scoliosis, sagittal plane best for kyphosis

  
  
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  If abnormal alignment look for hemivertebrae, block or butterfly vertebrae, spinal dysraphism

Count segments particularly in lumbar region

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  Mild caudal regression syndrome can be missed as spine may taper where it ends

  
  
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  Check that all lumbar and sacral segments are present

Assess relative size and ossification of vertebral bodies

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  Abnormal ossification and platyspondyly associated with skeletal dysplasia

Check skin line; should see amniotic fluid between spine and uterine wall to ensure intact skin

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  Myeloschisis has no sac (unlike myelomeningocele), look for defect in skin echo

  
  
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  Closed neural tube defects not associated with Chiari malformation; need to look for subcutaneous mass

Check position of conus

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  By 18 weeks, should be superior to L3/4, above L2/3 by term

Characterize abnormalities

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  Is an intracranial finding within the substance of the brain (intraaxial) or not (extraaxial)?

  
  
  
  
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    Differential diagnosis is different for intraaxial vs. extraaxial lesions

  
  
  
  
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  Is a mass cystic or solid?

  
  
  
  
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    If cystic, is it vascular? Use color Doppler!

    
    
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    If cystic, is it a developmental abnormality or a destructive process?

    
    
    
    
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      A porencephalic cyst replaces a focal area of brain destruction, arachnoid cyst is a space-occupying lesion displacing adjacent brain

      
      
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      A schizencephalic cleft is due to abnormal neuronal migration → local abnormality of brain architecture

    
    

  
  
  
  
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  Is a spine alignment fixed or variable? More likely to be structural if fixed

Is the finding isolated?

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  Aneuploidy or syndrome more likely with multiple abnormalities

Normal structures mistaken for pathology

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  Yolk sac confused with cephalocele in 1st trimester

  
  
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  Rhombencephalon confused with posterior fossa cyst in 1st trimester

  
  
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  Fluid in ventricular atrium mistaken for choroid plexus cyst

  
  
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  Fornices mistaken for CSP

  
  
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  Rotation of vermis may be mistaken for vermian dysgenesis

Unossified coccyx confused with dermal sinus

Failure to recognize interrupted skin line in myeloschisis

Failure to recognize vascular lesions; misdiagnosed as cystic mass unless color Doppler used