In posterior pedicle screw instrumentation of thoracic idiopathic scoliosis, screw malposition might cause significant morbidity in terms of possible pleural, spinal cord, and aorta injury. Preoperative axial magnetic resonance images (MRI) in 12 consecutive patients with right thoracic adolescent scoliosis, all with King type 3 curves, were analyzed in order to evaluate the relationship between the inserted pedicle screw position to pleura, spinal cord, aorta. Axial vertebral images for each thoracic level were scanned and the simulation of pedicle screw insertion was performed using a digital measurement programme. The angular contact value for each parameter regarding the pleura and spinal cord was measured on both sides of the curve. The aorta-vertebral distance was also measured. Aorta-vertebral distance was found to be decreasing gradually from the cephalad to the caudad with the shortest distance being measured at T12 with a mean of 1.2 mm. Concave-sided screws on T5-T9 and convex-sided screws on T2-T3 had the greatest risk to spinal cord injury. Pleural injury is most likely on T4-T9 segments by the convex side screws. T4-T8 screws on the concave side and T11-T12 screws on the convex side may pose risk to the aorta. This MRI-based study demonstrated that in pedicle instrumentation of thoracic levels, every segment deserves special consideration, where computer scanning might be mandatory in immature spine and in patients with severe deformity.