Abstract: BACKGROUND: The development and evolution of atlas are complex, and there are a few research reports. CT imaging can clearly show the atlas. It is of great clinical significance to clarify the normal development process, anatomical structure, variation and malformation of atlas.
OBJECTIVE: To show the normal development and variation of atlas using CT imaging.
METHODS:  The atlas CT data of patients aged 0-14 years in the picture archiving and communication system of Xiamen Children’s Hospital and Zhongshan Hospital of Xiamen University were collected, and the CT data of 732 patients were finally included. The observation items included: The ossification centers of anterior arch and vertebral arch, the epiphyseal plates of vertebral arch, anterior and posterior median, the tubercle of transverse ligament and the variation and deformity of atlas. 
RESULTS AND CONCLUSION: (1) Among 732 cases of atlas, CT images exhibited that 679 (92.76%) had normal development, 53 (7.24%) variation or deformity. Among them, there were 27 cases of incomplete bone ring, 23 cases of accessory ossification center and accessory epiphyseal plate, and 3 cases of atlanto-occipital joint deformity. (2) There were three ossification centers, including two lateral vertebral arches and one anterior arch. The bilateral ossification centers of two vertebral arches appeared in the fetal period; the anterior arch ossification center of one vertebral arch appeared from 2 days to 2 years after birth, the median age with 6 months and the interquartile-range with 4-11 months. (3) There were four epiphyseal plates, including two vertebral arch epiphyseal plates, one anterior median epiphyseal plate, and one posterior median epiphyseal plate. The two epiphyseal closures of vertebral arch were at the median age of 6.2 years, interquartile-range of 4.5-7.0. The occurrence rate of anterior median epiphyseal plate was 37.43%. That of anterior median was at the median age of 4.8 years, interquartile-range of 2.8-6.0 years and the posterior median at 2.8 years, interquartile-range of 2-3.8 years. (4) The appearance of transverse ligament nodules was at the median age of 9 years, interquartile-range of 5.5-10. (5) These findings have concluded that there is a specific age range between the appearance of ossification center and the closing of epiphyseal plate. The variations or deformities of atlas are mainly the incomplete bone ring, accessory ossification center or epiphyseal line. CT image showing the development or deformity of atlas is clear and accurate, which has many advantages. 

Key words: atlas, ossification center, epiphyseal plate, variation or deformity, CT image