1. A method for generating an optimized emission activity image, the method comprising: receiving raw emission projection data from an emission tomography scanner and raw radiography projection data from a radiography scanner for a subject; acquiring an initialization dataset of an initial emission activity image and at least one of an initial emission attenuation map, or an initial radiography image for the subject; applying a first model function to generate an estimated emission projection data using the initial emission activity image, the initial emission attenuation map, and one or more emission operators to map an emission activity image space into an emission projection data space; applying a second model function to generate an estimated radiography projection data using the initial radiography image and at least one radiography operator to map a radiography image space into a radiography projection data space; providing a scaling function to map a radiography image space into an emission attenuation map space; and using an optimization function based on the first model function, the second model function, the scaling function, the raw emission projection data, and the raw radiography data to determine iteratively an optimized emission activity image.