Imaging diagnosis--Vertebral canal porcupine quill with presumptive secondary arachnoid diverticulum.

A 3-year-old Gordon Setter developed cervical hyperesthesia and a stiff gait. Upon magnetic resonance (MR) imaging, an arachnoid diverticulum was detected at the C1 level. Upon surgical resection, a porcupine quill was identified within the vertebral canal in the area of the cyst. At a retrospective review of the MR images, the quill appeared as a circular well-demarcated T2-hypointense lesion. Porcupine quill migrations are common in the dog but migration into the central nervous system is rare.