The invention relates to an image processing device (10) for processing diagnostic image data, comprising: a data input (11) for receiving vector-valued diagnostic image data and a quantification unit (12) for, for each pixel, determining a subset of identifiers, selected from a predetermined set of template identifiers, and determining for each identifier of the subset a quantification value indicative of a presence, proportion or significance in the pixel of a material or condition corresponding to the identifier. The device comprises a dictionary definition unit (14) for providing a dictionary that assigns an index to each unique identifier subset, and an image data encoder (16) for encoding the image data. The encoder is adapted for, for each pixel, calculating a bit sequence comprising a first set of bits encoding the index and further sets of bits encoding quantification values. The device also comprises a data packager (18) for packing the encoded image in a diagnostic image format and for inserting the dictionary into metadata of that format. Further aspects relate to a decoding device, a system, corresponding methods for encoding and decoding and derived computer related products.