(a) Only X1 (central group); (b) X1, X2 (central, peripheral); (c) only X2 (peripheral); (d) stepwise. (a) Only the central group was the model, the area under the receiver operating characteristic (ROC) curve for differentiation of the benign and malignant nodules is 0.9766. (b) The central group and the peripheral group were the model, the area under the ROC curve for differentiation of the benign and malignant nodules is 0.983. (c) Only the peripheral group was the model, the area under the ROC curve for differentiation of the benign and malignant nodules is 0.9842. (d) For stepwise regression, the optimal model is the X2 model.