The Effect of Image Magnification on Measuring Radiographic Parameters for Scoliosis

Study design Reliability and agreement study Background Clinical evaluation of adolescent idiopathic scoliosis requires serial radiographic examinations. The purpose of this study was to determine whether image magnification can improve interobserver and intraobserver reliability and accuracy in the measurement of radiographic parameters of adolescent idiopathic scoliosis. Methods The radiographs of randomly selected scoliosis patients were reviewed. Three observers measured coronal Cobb angle, thoracic kyphosis, apical vertebral translation and sagittal vertical axis. The curves were measured as it first appeared on the screen (screen fit) without magnification (Technique 1x), 200% of screen fit (Technique 2x) and 400% of screen fit (Technique 4x). Interobserver and intraobserver reliability was assessed by intraclass correlation coefficient. We assessed the percentage of variation > 5° between the observers for each technique Results For coronal Cobb angle, the percentage of measurement variation > 5° was 12.5%, 1.6% for and 0% for Technique 1x, Technique 2x and Technique 4x respectively. Technique 2x was significantly better than Technique 1x in interobserver and intraobserver reliability. For thoracic hyphosis, the percentage of variation > 5° was 18.3%, 6.7% and 1.7% for Technique 1x, Technique 2x and Technique 4x respectively. Technique 4x significantly increased the interobserver and intraobserver reliability compared to Technique 1x. Measurements of distance parameters such as apical vertebral translation and sagittal vertebral axis demonstrate better reliability and were not influenced by image magnification. Conclusions Image magnification improves the reliability of coronal Cobb angle and thoracic kyphosis measurements. Reliability of apical vertebral translation and sagittal vertebral axis measurements are not influenced by image magnification. The authors recommend that at least 200% magnification of screen fit when measuring coronal Cobb angle and 400% magnification when measuring thoracic kyphosis.