The effect of the braces or instrumentation was studied using stereoradiography or axial tomography. The CTM brace was only studied clinically using frontal radiography. This study addresses a 3D analysis of the immediate effect of initial CTM brace wearing on the scoliotic spine using an MRI method developed previously. The MRI examination had been performed on eight scoliotic children wearing a first CTM brace. The MRI protocol was performed first with the brace and then without the brace. Using an in house processing software SIP and the pre and post processor Patran, two geometrical models of the vertebral bodies and intervertebral discs, with and without brace, were obtained. Different geometrical parameters for CTM brace effect analysis were computed. Seven braces decrease the scoliosis, transforming a C shape in the top view into a S or I shape and correcting the maximum deformity plane. One brace do not correct the scoliosis and aggravated this plane. Three braces aggravated and five corrected the maximum deviation plane. The intervertebral disc shift forward is corrected by the brace. Three braces rotated the apical vertebra and translated the end vertebrae. Two braces translated the apical vertebra and rotated the end vertebrae. Two braces translated the three vertebrae. One brace translated and rotated the three vertebrae. The method developed using MRI gave quantitative and qualitative data for understanding the CTM brace effect on the scoliotic spine. Our results show that the CTM brace has a local and global three dimensional action. This method is limited to small curvatures and to braces only worn at night. In the future, double curvatures will be analysed and the brace effect simulated using mechanical modelling.