STUDY DESIGN: Use of magnetic resonance imaging (MRI) with Constructive Interference in Steady State (CISS) sequence and isometric voxels to demonstrate the anatomic variations of vertebral artery in C2 vertebra. OBJECTIVES: To determine the transarticular screw trajectory on CISS MRI and to identify patients with anatomic variations of vertebral artery in C2 vertebra. SUMMARY OF BACKGROUND DATA: Atlantoaxial transarticular screw fixation has been reported to be biomechanically superior to other posterior techniques for atlantoaxial arthrodesis. Vertebral artery injury can be associated with catastrophic sequelae. Anatomic variation of vertebral artery is well recognized and computed tomography scan is the traditional preoperative assessment. However, no report has evaluated the use of MRI in preoperative assessment for the screw trajectories and the anatomic variation of vertebral artery. METHODS: The 3-dimensional (3D) CISS MRI with isometric voxels was performed in 30 local Chinese patients. The 3D reconstruction images were created to determine the proposed screw trajectories and their relationship with the vertebral arteries. RESULTS: In 12 patients (40%), the vertebral arteries were lying within the screw trajectories prohibiting transarticular screw fixation on at least one side. Bilateral variations with high risk of vertebral artery injuries were found in 6 patients. The remaining 6 patients had unilateral variations prohibiting the insertion of transarticular screws on one side. CONCLUSION: The 3D CISS MRI with isometric voxels is a safe and simple imaging technique to outline the vertebral arteries in C2. Reconstruction images are easily created and undistorted. It is one of the useful imaging in preoperative planning of transarticular screw fixation and determination of anatomy of vertebral artery.