1. A method for determining a boundary of an object for positron emission tomography (PET) scatter estimation, the method comprising: obtaining positron emission tomography (PET) data and computed tomography (CT) data for an object, the PET data and CT data acquired from an imaging system; determining a PET data boundary of the object based on the PET data; determining a CT data boundary of the object based on the CT data; and determining a combined boundary for PET scatter correction, the combined boundary encompassing the PET data boundary and the CT data boundary.