A medical system (100, 300, 500) comprising: - a processor configured for controlling the medical system; - a memory for storing machine executable instructions, wherein execution of the instructions causes the processor to: - receive four-dimensional Dixon magnetic resonance image data, wherein the four-dimensional Dixon magnetic resonance imaging data is T1 weighted, wherein the four-dimensional Dixon magnetic resonance image data is synchronized to a respiratory signal; and - reconstruct synthetic four-dimensional computed tomographic image data from the four-dimensional Dixon magnetic resonance imaging data, wherein the four-dimensional Dixon magnetic resonance imaging data comprises a first Dixon image and a second Dixon image for each of a set of discrete breathing phases, wherein the reconstruction of the synthetic four-dimensional computed tomographic image data from the four-dimensional Dixon magnetic resonance imaging data comprises: - constructing a water Dixon image using the first Dixon image and the second Dixon image for each of a set of discrete breathing phases; - constructing a fat Dixon image using the first Dixon image and the second Dixon image for each of the set of discrete breathing phases; - constructing an in-phase Dixon image for each of the set of discrete breathing phases using the first Dixon image and the second Dixon image; - segmenting a body mask for each of the set of discrete breathing phases using the in-phase Dixon image; - segmenting a bone mask for each of the set of discrete breathing phases using the in-phase Dixon image; - segmenting regions between the body mask and the bone mask into soft tissue regions for each of the set of discrete breathing phases using the fat Dixon image and the water Dixon image; - assigning the soft tissue regions a soft tissue Hounsfield Unit value for each of the set of discrete breathing phases using a non-rigid registration or using a soft tissue classification model; and - assigning within the bone mask a bone tissue Hounsfield Unit value for each of the set of discrete breathing phases using the non-rigid registration or using a bone tissue classification model. - a processor configured for controlling the medical system; - a memory for storing machine executable instructions, wherein execution of the instructions causes the processor to: - receive four-dimensional Dixon magnetic resonance image data, wherein the four-dimensional Dixon magnetic resonance imaging data is T1 weighted, wherein the four-dimensional Dixon magnetic resonance image data is synchronized to a respiratory signal; and - reconstruct synthetic four-dimensional computed tomographic image data from the four-dimensional Dixon magnetic resonance imaging data, wherein the four-dimensional Dixon magnetic resonance imaging data comprises a first Dixon image and a second Dixon image for each of a set of discrete breathing phases, wherein the reconstruction of the synthetic four-dimensional computed tomographic image data from the four-dimensional Dixon magnetic resonance imaging data comprises: - constructing a water Dixon image using the first Dixon image and the second Dixon image for each of a set of discrete breathing phases; - constructing a fat Dixon image using the first Dixon image and the second Dixon image for each of the set of discrete breathing phases; - constructing an in-phase Dixon image for each of the set of discrete breathing phases using the first Dixon image and the second Dixon image; - segmenting a body mask for each of the set of discrete breathing phases using the in-phase Dixon image; - segmenting a bone mask for each of the set of discrete breathing phases using the in-phase Dixon image; - segmenting regions between the body mask and the bone mask into soft tissue regions for each of the set of discrete breathing phases using the fat Dixon image and the water Dixon image; - assigning the soft tissue regions a soft tissue Hounsfield Unit value for each of the set of discrete breathing phases using a non-rigid registration or using a soft tissue classification model; and - assigning within the bone mask a bone tissue Hounsfield Unit value for each of the set of discrete breathing phases using the non-rigid registration or using a bone tissue classification model.