Preoperative imaging of the craniovertebral junction. (A) Lateral radiography shows malalignment of the craniovertebral junction. (B) Midsagittal reconstruction from computed tomography shows an anteriorly tilted odontoid process of C2 and migration of a hypoplastic C1 posterior arch into the foramen magnum. (C) Posterolateral view of three-dimensional computed tomography shows migration of the C1 posterior arch into the foramen magnum and rachischisis (spina bifida) of the C1 posterior arch. (D) Midsagittal T2-weighted magnetic resonance imaging shows compression of the spinal cord by the basilar invagination and an anteriorly shifted C1 posterior arch. (E) Posteroinferior view of three-dimensional computed tomography angiography shows anomalous courses of bilateral vertebral arteries (persistent first intersegmental artery), which were located between the C1 posterior arch and the pars interarticularis of C2. Asterisks, odontoid process; arrowheads, C1 anterior arch.