Vertebral artery (VA): 4 segments

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  V1 segment (extraosseous segment)

  
  
  
  
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    Arises from 1st part of subclavian artery

    
    
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    Courses posterosuperiorly to enter C6 transverse foramen

    
    
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    Branches: Segmental cervical muscular, spinal branches

  
  
  
  
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  V2 segment (foraminal segment)

  
  
  
  
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    Ascends through C6-C3 transverse foramina

    
    
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    Turns superolaterally through inverted L-shaped transverse foramen of axis (C2)

    
    
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    Courses short distance superiorly through C1 transverse foramen

    
    
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    Branches: Anterior meningeal artery, unnamed muscular/spinal branches

  
  
  
  
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  V3 segment (extraspinal segment)

  
  
  
  
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    Exits top of atlas (C1) transverse foramen

    
    
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    Lies on top of C1 ring, curving posteromedially around atlantooccipital joint

    
    
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    As it passes around back of atlantooccipital joint, turns sharply anterosuperiorly to pierce dura at foramen magnum

    
    
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    Branches: Posterior meningeal artery

  
  
  
  
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  V4 segment (intradural/intracranial segment)

  
  
  
  
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    After VA enters skull through foramen magnum, courses superomedially behind clivus

    
    
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    Unites with contralateral VA at or near pontomedullary junction to form basilar artery (BA)

    
    
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    Branches: Anterior, posterior spinal arteries (ASA, PSA), perforating branches to medulla, posterior inferior cerebellar artery (PICA)

    
    
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    Arises from distal VA, curves around/over tonsil, gives off perforating medullary, choroid, tonsillar, cerebellar branches

  
  

BA

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  Courses superiorly in prepontine cistern (in front of pons, behind clivus)

  
  
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  Bifurcates into its terminal branches, posterior cerebral arteries (PCAs), in interpeduncular or suprasellar cistern at or slightly above dorsum sellae

  
  
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  Branches: Pontine, midbrain perforating branches (numerous), anterior ICA (AICA), superior cerebellar arteries (SCAs), PCAs (terminal branches)

VA

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  ASA: Upper cervical spinal cord, inferior medulla

  
  
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  PSA: Dorsal spinal cord to conus medullaris

  
  
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  Penetrating branches: Olives, inferior cerebellar peduncle, part of medulla

  
  
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  PICA: Lateral medulla, choroid plexus of 4th ventricle, tonsil, inferior vermis/cerebellum

BA

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  Pontine perforating branches: Central medulla, pons, midbrain

  
  
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  AICA: Internal auditory canal, CNVII, and CNVIII, anterolateral cerebellum

  
  
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  SCA: Superior vermis, superior cerebellar peduncle, dentate nucleus, brachium pontis, superomedial surface of cerebellum, upper vermis

Normal variants

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  VA: Variation in size from right to left, dominance common; origin from aortic arch in 5%

Anomalies

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  VA/BA may be fenestrated or duplicated (may have increased prevalence of aneurysms)

  
  
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  Embryonic carotid-basilar anastomoses (e.g., persistent trigeminal artery)

V1 and V2 segments are amenable to US examination

Examination usually starts in V2 segment and proceeds downward to V1 segment, then to its origin

Examination of V2 segment

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  Transducer oriented longitudinally in midcervical region between trachea and sternocleidomastoid muscle

  
  
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  Angle transducer laterally from common carotid artery (CCA) and locate V2 segment posterior to acoustic shadowing of transverse processes

Examination of V1 segment

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  Trace caudally from V2 to its origin

  
  
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  Left VA more difficult to visualize than right VA

  
  
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  Do not confuse with vertebral vein lying adjacent to VA, which can appear pulsatile

  
  
  
  
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    Color flow imaging helps to differentiate

  
  

Normal waveform of VA on spectral Doppler analysis

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  Low-resistance flow

  
  
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  Similar to that of CCA but with lower amplitude

  
  
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  PSV: 59 ± 17 cm/s; EVD: 19 ± 8 cm/s

  
  
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  Flow velocity asymmetry is common and related to caliber of VA

VA distal to V2 cannot be properly assessed by US

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  Abnormalities in spectral Doppler waveform of VA at V1/V2 segment provide clue for disease beyond V2

Plexiform longitudinal anastomoses between cervical intersegmental arteries → VA precursors

Paired plexiform dorsal longitudinal neural arteries (LNAs) develop, form precursors of BA

Transient anastomoses between dorsal LNAs develop, and ICAs appear (primitive trigeminal/hypoglossal arteries, etc.)

Definitive VAs arise from 7th cervical intersegmental arteries, anastomose with LNAs

LNAs fuse as temporary connections with ICAs regress → definitive BA, vertebrobasilar circulation formed