The method is performed in a computer and includes acts of importing medical image data sets of an object, selecting regions of interest of the object in each medical image data set, processing the medical image data and constructing an optimal symmetry plane. The act of selecting the regions of interest of the object in each medical image data set defines the regions of interest of the object in each medical image data set and locates the regions of interest of the object in each medical image data set. The act of processing the medical image data executes an error equation and obtains optimal parameters of the optimal symmetry plane with a symmetry value. The act of constructing an optimal symmetry plane constructs an optimal symmetry plane with the obtained optimal parameters. Based on the discovered Optimal Symmetry Plane, slanted images from medical imaging can be rectified.