An apparatus for determining decomposed spectral image data of a subject, wherein the apparatus comprises: a spectral image data providing unit configured to provide spectral image data of the subject that has been injected with a contrast agent, wherein the spectral image data of the subject has been acquired using a spectral x-ray imaging apparatus, a spectral image data decomposition unit configured to calculate a basis decomposition for the spectral image data, wherein the basis decomposition results in decomposed spectral image data, wherein the decomposed spectral image data are indicative of weights provided to at least two basis functions of the spectral image data, a virtual non-contrast image generation unit configured to generate a virtual non-contrast image based on a predefined combination of the decomposed spectral image data, and a contrast agent residual identification unit configured to identify a residual region comprising a contrast agent residual in the virtual non-contrast image based on an expected structural characteristic of the contrast agent residual, wherein the contrast agent residual is a contribution of the contrast agent to an attenuation of a part of the subject represented by image values of the virtual non-contrast image, wherein the spectral image data decomposition unit is configured to: determine a residual measure for the residual region, wherein the residual measure is indicative of an intensity of the contrast agent residual of the residual region, and calculate a new basis decomposition in the residual region by correcting the weights provided to the basis functions of the spectral image data in the residual region by adding or subtracting a correction value to the weights of at least one of the basis functions, wherein the correction value is determined based on the residual measure, such that the contrast agent residual is reduced, wherein the spectral image data decomposition unit is configured to define a residual area containing the residual region in the virtual non-contrast image, and comprises a margin of one or two voxels surrounding the residual region, wherein determining the residual measure is further based on the defined residual area. a spectral image data providing unit configured to provide spectral image data of the subject that has been injected with a contrast agent, wherein the spectral image data of the subject has been acquired using a spectral x-ray imaging apparatus, a spectral image data decomposition unit configured to calculate a basis decomposition for the spectral image data, wherein the basis decomposition results in decomposed spectral image data, wherein the decomposed spectral image data are indicative of weights provided to at least two basis functions of the spectral image data, a virtual non-contrast image generation unit configured to generate a virtual non-contrast image based on a predefined combination of the decomposed spectral image data, and a contrast agent residual identification unit configured to identify a residual region comprising a contrast agent residual in the virtual non-contrast image based on an expected structural characteristic of the contrast agent residual, wherein the contrast agent residual is a contribution of the contrast agent to an attenuation of a part of the subject represented by image values of the virtual non-contrast image, wherein the spectral image data decomposition unit is configured to: determine a residual measure for the residual region, wherein the residual measure is indicative of an intensity of the contrast agent residual of the residual region, and calculate a new basis decomposition in the residual region by correcting the weights provided to the basis functions of the spectral image data in the residual region by adding or subtracting a correction value to the weights of at least one of the basis functions, wherein the correction value is determined based on the residual measure, such that the contrast agent residual is reduced, wherein the spectral image data decomposition unit is configured to define a residual area containing the residual region in the virtual non-contrast image, and comprises a margin of one or two voxels surrounding the residual region, wherein determining the residual measure is further based on the defined residual area. determine a residual measure for the residual region, wherein the residual measure is indicative of an intensity of the contrast agent residual of the residual region, and calculate a new basis decomposition in the residual region by correcting the weights provided to the basis functions of the spectral image data in the residual region by adding or subtracting a correction value to the weights of at least one of the basis functions, wherein the correction value is determined based on the residual measure, such that the contrast agent residual is reduced, wherein the spectral image data decomposition unit is configured to define a residual area containing the residual region in the virtual non-contrast image, and comprises a margin of one or two voxels surrounding the residual region, wherein determining the residual measure is further based on the defined residual area.