An apparatus for alveoli based visualization in dark-field or phase-contrast X-ray imaging, comprising: - an input unit; - a processing unit; and - an output unit; wherein, the input unit is configured to provide the processing unit with a dark-field or phase-contrast X-ray image of a region of interest of an object, wherein image data are represented in units of a dark-field signal or a phase-contrast signal; characterised in that the input unit is configured to provide the processing unit with a calibration relating dark-field signal or phase-contrast signal to a number of alveoli in a lung; wherein the calibration is an experimentally generated calibration that relates to how much dark-field signal or phase-contrast signal is associated with an alveolus, wherein the calibration was generated on the basis of a comparison between a high resolution calibration CT image of a calibration lung and a calibration dark-field image or calibration phase-contrast image of the calibration lung, and wherein the calibration CT image had a resolution high enough to reveal a number of alveoli in the calibration lung; wherein, the processing unit is configured to utilize the calibration to convert the dark-field X-ray image or phase-contrast X-ray image of the region of interest of the object into an alveoli based image of the region of interest of the object, wherein the image data are represented in units of the number of alveoli; and wherein, the output unit is configured to output the alveoli based image. - an input unit; - a processing unit; and - an output unit; wherein, the input unit is configured to provide the processing unit with a dark-field or phase-contrast X-ray image of a region of interest of an object, wherein image data are represented in units of a dark-field signal or a phase-contrast signal; characterised in that the input unit is configured to provide the processing unit with a calibration relating dark-field signal or phase-contrast signal to a number of alveoli in a lung; wherein the calibration is an experimentally generated calibration that relates to how much dark-field signal or phase-contrast signal is associated with an alveolus, wherein the calibration was generated on the basis of a comparison between a high resolution calibration CT image of a calibration lung and a calibration dark-field image or calibration phase-contrast image of the calibration lung, and wherein the calibration CT image had a resolution high enough to reveal a number of alveoli in the calibration lung; wherein, the processing unit is configured to utilize the calibration to convert the dark-field X-ray image or phase-contrast X-ray image of the region of interest of the object into an alveoli based image of the region of interest of the object, wherein the image data are represented in units of the number of alveoli; and wherein, the output unit is configured to output the alveoli based image. wherein, the input unit is configured to provide the processing unit with a dark-field or phase-contrast X-ray image of a region of interest of an object, wherein image data are represented in units of a dark-field signal or a phase-contrast signal; characterised in that the input unit is configured to provide the processing unit with a calibration relating dark-field signal or phase-contrast signal to a number of alveoli in a lung; wherein the calibration is an experimentally generated calibration that relates to how much dark-field signal or phase-contrast signal is associated with an alveolus, wherein the calibration was generated on the basis of a comparison between a high resolution calibration CT image of a calibration lung and a calibration dark-field image or calibration phase-contrast image of the calibration lung, and wherein the calibration CT image had a resolution high enough to reveal a number of alveoli in the calibration lung; wherein, the processing unit is configured to utilize the calibration to convert the dark-field X-ray image or phase-contrast X-ray image of the region of interest of the object into an alveoli based image of the region of interest of the object, wherein the image data are represented in units of the number of alveoli; and wherein, the output unit is configured to output the alveoli based image.