Chiari malformation: Present Diagnostic criteria and possible refinements in flow studies

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Chiari malformation Present Diagnostic
criteria and possible refinements in flow
studies

• Victor Haughton
• University of Wisconsin

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Chiari I malformation
"Ever since the initial postmortem description by
Chiari in 1891 of the group of malformations that
bears his name, it seems there have always been
more questions on this subject than answers."
--- Ball and Crone 1995
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Radiologic diagnosis of Chiari I

• Tonsilar ectopia 5 mm or more below the foramen
  magnum Barkovitch
• 0.6 to 0.8 of MR studies of the head and
  cervical spine
• One half to a third have no symptoms of a Chiari
  malformation
• One third of symptomatic Chiari I malformation
  have less than 5 mm tonsilar herniation Milhorat
  TH et al., 1999
• Syringomyelia without tonsilar ectopia (Chiari 0)
  may respond to surgical treatment
• Tonsilar ectopia has limited utility in the
  diagnosis of symptomatic Chiari I malformation.

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Milhorat, et al, 1999
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Role of CSF flow studies in Chiari I
malformation

• Distinguish obstructed CSF flow
• Identify patients who will benefit from
  cranio-occipital decompression
• Limited effectivity of current imaging
• Comparisons of symptomatic and asymptomatic
  Chiari I malformation lacking
• Pressures not measured non-invasively
• Velocities measurements typically at only one
  level

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Technique for PC MR

• Cardiac gated PC MR images
• flip angle 20
• TR/TE 20/5 ms
• slice thickness 5 mm
• field of view 180 mm
• matrix 256 x 256
• encoding velocity (venc) 10 cm/s
• location and projection according to reference
• flow velocity for each time point and each voxel
  from the phase shift

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Characterization of CSF flow in the foramen magnum

• Normal subjects
• Relative preponderance of flow anteriorly
• Inhomogeneity of flow, presence of small jets
• Unidirectional flow
• Patients with Chiari I
• Diminished flow posteriorly
• Greater preponderance of flow anteriorly
• Large jets in anterior subarachnoid space
• Synchronous bidirectional flow.

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Chiari I patient with a syrinx
2D FIESTA cine
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Chiari I patients with syrinx
Sagittal PC MR, flow in superior to inferior
direction
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Chiari I patient with syrinx
Axial PC MR cine loop
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Interpretation of CSF flow studies

• PC MR images in axial or sagittal projection
• Examine for evidence of flow obstruction
• Compare flow in anterior and posterior
  subarachnoid space
• Evaluate homogeneity of flow
• Evaluate for synchronous bidirectional flow

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CSF flow study in a patient with tonsilar
ectopia, no symptoms of Chiari malformation
Readers called flow pattern non-obstructed
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CSF flow in a symptomatic Chiari I patient
Readers called flow obstructed
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Symptomatic Chiari I patient with Valsalva
induced headaches
Readers disagreed
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Asymptomatic Chiari I, with incidental tonsilar
ectopia and no symptoms suggestive of a Chiari I
malformation
Readers disagreed
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Accuracy of interpretation of flow images

• Reader sensitivity for the finding of obstructed
  flow in symptomatic Chiari I cases averaged 76
  for both sagittal and axial images
• Reader specificity for finding unobstructed flow
  in the subjects with incidental tonsilar ectopia
  averaged 62

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Computational flow analysis in the evaluation of
CSF flow

• Techniques borrowed from blood flow analysis
• Boundary conditions assumed
• Geometry of the space
• Flow through the space
• Velocity or pressure calculated throughout the
  volume

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multi-echo 3D radial acquisition (VIPR)

• VIPR imaging achieves isotropic spatial
  resolution and fat/water separation, a 20x20x20
  cm3 image, 256x256x256 voxels, in 5 minutes of
  scan time. The high contrast between CSF and the
  surrounding tissues and the high spatial
  isotropic submillimeter resolution are ideally
  suited for the creation of an accurate 3D model

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3D model of the subarachnoid space

• Created from the VIPR acquisition with
  commercial software (Mimics 9.0, Materialize Ann
  Arbor, MI, USA) using semi-automatic
  segmentation. The surfaces of this 3D model are
  meshed (discretized) with another commercial
  software ANSYS using isoparametric elements of 8
  nodes (Figure 4).

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Boundary element method

• To analyze flow phenomena using this method, a
  series of points are set within the flow domain.
• Advantages (BEM) over finite difference method
  (FDM) and finite element method (FEM) include
  satisfaction of partial differential equations
  that relate pressure and velocity

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Velocity distribution at a cross-section near the
inferior end of the spinal canal

• Higher velocities are identified lateral to the
  spinal cord

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Velocity distribution at a cross-section near the
superior end of the spinal canal

• Higher velocities seen anterior and lateral to
  the spinal cord

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Pressure distribution from BEM

• Pressures against the spinal cord and boundary of
  the subarachnoid space, calculated for a normal
  subject

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PC MR measurements of CSF velocity
mmmmmmmmmmmmmmmmmmmm
Chiari I Patient 14 Consecutive images in
cardiac cycle
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Effect of Surgery
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Summary

• CSF flow visibly more complex in Chiari I than in
  normal subject
• Chiari I characterized by flow jets,
  bi-directional flow, diminished posterior flow
• Further research needed to characterize CSF flow
  in the posterior fossa and spinal canal
• Goal of accurate diagnostic test based on flow
  imaging.

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Future studies

• Comparisons of flow patterns in symptomatic and
  asymptomatic Chiari I malformation
• Evaluation of CSF flow in hydromyelia without
  tonsilar ectopia (Chiari 0)
• Measure velocities in a volume rather than a
  slice of tissue
• Calculate pressures fields throughout the
  subarachnoid space from velocity measurements and
  test hypothetical relationship of pressures and
  syrinx formation

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Chiari I patients with syrinx
Sagittal PC MR, flow in all directions, magnitude
images, flow left to right and flow superior to
inferior