1. A computer-implemented method for initializing a global model of a set of hierarchical models, comprising: accessing a localizer image acquired as part of a patient examination being currently performed using an imaging system, wherein the localizer image comprise a two-dimensional projection of three-dimensional data acquired by the imaging system; generating one or more two-dimensional profiles of radiation attenuation based on the localizer image; determining extents of one or more anatomical structures within the patient based on the one or more two-dimensional profiles by applying an intensity-based threshold to the one or more two-dimensional profiles; and initializing the global model of the set of hierarchical models based on the determined extents.