The effect of added fat on the accuracy of Cobb angle measurements in CT SPR images: A phantom study

Introduction: Adolescent idiopathic scoliosis (AIS) is a spinal deformity that mostly affects females aged between 10 and 17 years old. Cobb's method is the gold standard for assessing AIS. Being overweight is a common characteristic in AIS patients; therefore, the aim of this study is to investigate the effect fat mass has on the accuracy of Cobb angle measurements in 10-year-old female AIS patients. Methods: A purpose-built phantom representing an AIS patient was scanned after adding several thicknesses of lard fat (0,2,4 and 8 cm). The phantom was scanned in an antero-posterior position using the scout mode of the CT scanner. 18 observers performed Cobb angle measurements on the images. Results: The average Cobb angle at 0 cm of fat was 10.83° (SD = 3.06), at 2 cm it was 10.90° (SD = 3.16), at 4 cm it was 10.64° (SD = 3.06) and at 8 cm it was 10.88° (SD = 3.02). No significant difference was observed between the measurements at these thicknesses. Conclusion: Cobb angle measurements are not affected by the presence of fat. Implications for practice: When assessing overweight AIS patients, it not necessary to manipulate the acquisition parameters, which could lead to increased patient dose, in order to get more accurate Cobb angle measurement.