An image data processing device for determining a calcium content by analyzing cardiac spectral CT data, the image data processing device comprising: - a data input adapted for receiving spectral CT projection data obtained by scanning a cardiac region of a subject using a spectral CT scanning unit; - a modelling unit adapted for applying a material decomposition algorithm to said spectral CT projection data such as to provide a calcium-specific component of said spectral CT projection data; - a tomographic reconstruction unit adapted for reconstructing said spectral CT projection data such as to provide a first 3D image of said cardiac region and for reconstructing said calcium-specific component of the spectral CT projection data such as to provide a second 3D image representative of calcium content in said cardiac region; the image data processing device characterized by: - a segmentation unit adapted for segmenting said first 3D image such as to provide an image mask corresponding to a cardiovascular structure of interest in said cardiac region; - a selection unit adapted for selecting a part of said second 3D image based on said image mask; and - a computation unit adapted for calculating a calcium content in said cardiovascular structure of interest based on said selected part of the second 3D image, wherein said selection unit is adapted for selecting a second set of voxels of said second 3D image that is the set complement of the voxels that form said cardiovascular structure of interest, wherein said image data processing device further comprises a forward projection unit adapted for projecting the second set of voxels to generate simulated projection data corresponding to the same projections as said spectral CT projection data, and wherein said computation unit is adapted for subtracting said simulated projection data from said calcium-specific component of the spectral CT projection data to obtain subtracted projection data representative of calcium specific attenuation in specifically the cardiovascular structure of interest. - a data input adapted for receiving spectral CT projection data obtained by scanning a cardiac region of a subject using a spectral CT scanning unit; - a modelling unit adapted for applying a material decomposition algorithm to said spectral CT projection data such as to provide a calcium-specific component of said spectral CT projection data; - a tomographic reconstruction unit adapted for reconstructing said spectral CT projection data such as to provide a first 3D image of said cardiac region and for reconstructing said calcium-specific component of the spectral CT projection data such as to provide a second 3D image representative of calcium content in said cardiac region; - a segmentation unit adapted for segmenting said first 3D image such as to provide an image mask corresponding to a cardiovascular structure of interest in said cardiac region; - a selection unit adapted for selecting a part of said second 3D image based on said image mask; and - a computation unit adapted for calculating a calcium content in said cardiovascular structure of interest based on said selected part of the second 3D image, wherein said selection unit is adapted for selecting a second set of voxels of said second 3D image that is the set complement of the voxels that form said cardiovascular structure of interest, wherein said image data processing device further comprises a forward projection unit adapted for projecting the second set of voxels to generate simulated projection data corresponding to the same projections as said spectral CT projection data, and wherein said computation unit is adapted for subtracting said simulated projection data from said calcium-specific component of the spectral CT projection data to obtain subtracted projection data representative of calcium specific attenuation in specifically the cardiovascular structure of interest.