1. A correction method for reducing error in cone beam computed tomography images, the method comprising: generating a plurality of two-dimensional projection images of a subject from a three-dimensional multi-detector computed tomography image of the subject, which is spatially registered with a three-dimensional cone beam computed tomography image of the subject; subtracting the plurality of two-dimensional projection images from a plurality of two-dimensional cone beam projection images of the subject, resulting in a plurality of two-dimensional estimated error projections, which comprise an estimated error in the plurality of two-dimensional cone beam projection images; subtracting the plurality of two-dimensional estimated error projection images from the plurality of two-dimensional cone beam projection images to generate a plurality of two-dimensional corrected cone beam projection images; constructing a three-dimensional corrected cone beam computed tomography image of the subject from the plurality of two-dimensional corrected cone beam projection images; converting the three-dimensional multi-detector computed tomography image of the subject from Hounsfield Units to linear attenuation coefficients; and spatially registering the converted three-dimensional multi-detector computed tomography image of the subject before generating the plurality of two-dimensional multi-detector projection images of the subject, wherein the spatially registering comprises: placing a data set from the converted three-dimensional multi-detector computed tomography image of the subject in a same coordinate system as a data set from the three-dimensional cone beam computed tomography image of the subject; and aligning a center of mass of the converted three-dimensional multi-detector computed tomography image of the subject with a center of mass of the three-dimensional cone beam computed tomography image of the subject.