A method for determining a surrogate respiratory signal for four-dimensional computed tomography using ultrasound data includes acquiring computed tomography data with a computed tomography imaging system (402), acquiring ultrasound data with an ultrasound probe of an ultrasound imaging system (404) concurrently with acquiring the computed tomography data during one or more respiratory cycles, wherein the ultrasound probe is aligned to acquire an image of a diaphragm of a subject, synchronizing the acquired computed tomography data and the acquired ultrasound data, and determining a surrogate respiratory signal from the acquired ultrasound data.
