The present invention relates to an apparatus (10) for alveoli based visualization in dark-field or phase-contrast X-ray imaging. It is described to provide (210) 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 or a phase-contrast signal. A calibration is provided (220) relating dark-field or phase-contrast signal to an alveoli index in a lung. The calibration is utilized (230) to convert the dark-field 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 alveoli index. The alveoli based image is outputted (240).