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.