Supine Magnetic Resonance Imaging Cobb Measurements for Idiopathic Scoliosis Are Linearly Related to Measurements From Standing Plain Radiographs

Study Design. Retrospective. Objective. To demonstrate a relationship between adolescent idiopathic scoliosis Cobb angle measurements obtained with standing plain radiographs and standard supine magnetic resonance (MR) images. Summary of Background Data. Patients with adolescent idiopathic scoliosis are exposed to repeated doses of ionizing radiation during the course of their treatment with significant potential long-term health consequences. Supine MR images of the spine may allow measurements of coronal plane spinal deformity equivalent to plain radiographs while minimizing exposure to ionizing radiation. Methods. A retrospective chart and radiograph review was conducted for patients with adolescent idiopathic scoliosis. Cobb angle measures were derived from available plain radiographs and MR images obtained within 6 months of each other. Pearson correlation and linear regression analysis was used to test the relationship between plain radiographical and magnetic resonance imaging Cobb measures. Inter-and intraobserver reliability for the Cobb measures was tested with a random sample of 20 patients using intraclass correlation coefficients. Results. Supine MR images tended to underestimate plain radiographs by 10 on average. However, radiographical and MR measures showed a strong positive correlation (r = 0.90-0.94) for all curves, structural or nonstructural, and this correlation was not influenced by patient age or body mass index. The relationship allowed the development of a simple linear equation for converting MR image measures to radiograph measures with an acceptable absolute error of +/- 5 degrees. Conclusion. Cobb angle measures from supine magnetic resonance imaging of the spine can reliably be translated to the equivalent radiographical measures with an acceptable range of error. The data suggest that standard supine magnetic resonance imaging sequences may be a viable substitute for plain radiographs in the clinical diagnosis and serial evaluation of adolescent idiopathic scoliosis while obviating the associated dangers of ionizing radiation from plain radiographs.