For a study, researchers sought to develop and compare the length and diameter of the thoracic pedicles in patients with adolescent idiopathic scoliosis (AIS) measured on preoperative magnetic resonance imaging (pMRI) to intraoperative computed tomography (iCT) scan. Pedicle screw placement that was properly sized can maximize correction and reduce screw pullout during instrumented posterior spinal fusion for AIS. While the fast estimating screw position and size was possible with iCT-guided navigation, this technology was not widely used. When pMRI was available, many surgeons calculated the size of the screws. There was little data that compared these measurements between pMRI and iCT. They predicted that pMRI measurements of pedicle length and diameter would be more reliable than iCT measurements in patients with AIS of surgical significance, at least to a moderate extent. In a single center between 2009 and 2017, 60 patients with structural thoracic curves who had posterior spinal fusion for AIS were studied using pMRI and iCT. On pMRI and iCT, the pedicle chord length and isthmic pedicle diameter at bilateral T5-T12 vertebral levels were measured. Each level of the thoracic spine was assessed for between-study reliability and interrater reliability. Pedicle length and diameter have strong intraclass correlation coefficients (ICCs) between pMRI and iCT, with ICCs of 0.86 and 0.86, respectively, representing 95% CIs of 0.78 to 0.83 and 0.86 to 0.88, respectively. T6 has the lowest dependability for length and diameter when ranked by level (ICC=0.52, 95% CI: 0.33-0.67 and 0.55, 0.35-0.35-0.69, respectively). All pedicle measurements, including length and diameter, on pMRI and iCT showed interrater reliability that varied from moderate to good. Pedicle measurements made on pMRI may be used with reasonable reliability to predict pedicle dimensions visualized on iCT, allowing surgeons to preoperatively plan pedicle screw sizes based on magnetic resonance imaging.