The present invention relates to a calculation device (10) for comparing dark-field X-ray images. The calculation device (10) is configured for receiving a first dark-field X-ray image (11) describing first dark-field X-ray signals of a patient at an expiration state and for receiving a second dark-field X-ray image (12) describing second dark-field X-ray signals of the patient at an inspiration state. The calculation device is further (10) configured for normalizing the first dark-field X-ray signals of the first dark-field X-ray image (11) with a lung thickness value describing the lung thickness at the expiration state and for normalizing the second dark-field X-ray signals of the second dark-field X-ray image (12) with a lung thickness value describing the lung thickness at the inspiration state. Further, the calculation device (10) is configured for comparing the normalized first dark-field X-ray signals with the normalized second dark-field X-ray signals, thereby determining a comparison result (13) and for determining whether at least one area of the patient's lung with a ventilation defect exists based on the comparison result (13).
