1. A computer-implemented method for generating patient models for use in radiation therapy, comprising: receiving, by a computing device having a processor, multiple sets of image data representing a volume of a patient, each set of image data is spatially aligned with each other and is acquired in a different manner so as to create contrast amongst tissue types of the patient; classifying, by the computing device, tissue in each voxel in the volume into different classes using a classification algorithm, where the multiple sets of image data serve as input to the classification algorithm and each class represents a type of tissue; assigning, by the computing device, properties to each voxel in the volume bases on the classification of tissue by the classification algorithm; and generating a patient model for the volume from the properties assigned to each voxel.