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

1. Chiari malformation: Present Diagnostic criteria and possible refinements in flow studies Victor Haughton University of Wisconsin

2. 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

3. 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.

4. Symptomatic Asymptomatic Symptomatic Milhorat, et al, 1999

5. 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

6. 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

7. 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.

8. Chiari I patient with a syrinx 2D FIESTA cine

9. Chiari I patients with syrinx Sagittal PC MR, flow in superior to inferior direction

10. Chiari I patient with syrinx Axial PC MR cine loop

11. 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

12. CSF flow study in a patient with tonsilar ectopia, no symptoms of Chiari malformation Readers called flow pattern “non-obstructed”

13. CSF flow in a symptomatic Chiari I patient Readers called flow “obstructed”

14. Symptomatic Chiari I patient with Valsalva induced headaches Readers disagreed

15. Asymptomatic Chiari I, with incidental tonsilar ectopia and no symptoms suggestive of a Chiari I malformation Readers disagreed

16. 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%

17. 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

18. 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

19. 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).

20. 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

21. Velocity distribution at a cross-section near the inferior end of the spinal canal Higher velocities are identified lateral to the spinal cord

22. Velocity distribution at a cross-section near the superior end of the spinal canal Higher velocities seen anterior and lateral to the spinal cord

23. Pressure distribution from BEM • Pressures against the spinal cord and boundary of the subarachnoid space, calculated for a normal subject

24. PC MR measurements of CSF velocity mmmmmmmmmmmmmmmmmmmm Chiari I Patient: 14 Consecutive images in cardiac cycle

27. Effect of Surgery

28. 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.

29. 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

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