A system for single-photon emission computed tomography (SPECT) is provided. The system includes a computer device comprises at least one processor in communication with at least one memory device. The at least one processor is programmed to: a) store a model trained to generate an attenuation map of a subject being examined; b) receive a scatter-energy window projection of a first subject to be examined; c) execute the model with the scatter-energy window projection of the first subject as an input, wherein the model generates an attenuation map; d) receive a photopeak-energy window projection of the first subject to be examined; and e) perform attenuation compensation on the photopeak-energy window projection using the generated attenuation map.