Chiari I malformations: assessment with phase-contrast velocity MR.

PURPOSE: To assess movement of the medulla, tonsils, and upper cervical cord as well as that of the posterior fossa cerebrospinal fluid pathways in both normal subjects and those with Chiari I malformations.
METHODS: Nine healthy volunteers and eight patients with Chiari I malformations were examined with phase-contrast cine MR. With a region-of-interest cursor, the directions and intensities of the brain and cerebrospinal fluid were assessed and intensity-versus-time graphs generated.
RESULTS: Cerebrospinal fluid flow patterns of the patients with Chiari I malformations were normal except for absence of valleculla flow. In addition, increased velocities (10 times normal) of the tonsils of all patients with Chiari I malformations together with posterior movement of the medulla (rather than the expected anterior movement seen in volunteers) occurred.
CONCLUSIONS: Increased velocities of the tonsils may be the result of the carotid systolic pulse being delivered to a structure (the tonsil) without the normal surrounding cerebrospinal fluid, resulting in impact of the tonsils in the confined foramen magnum and a consequent caudocranial recoil. An alternative explanation would include the Bernoulli effect caused by the confined location of the tonsils. There may be a decrease in the peak tonsillar velocities after surgery.