A method and a data-processing system are disclosed for determining the proportion of calcium in coronary arteries using image data from CT angiography. In at least one embodiment of the method, anatomical landmarks are detected in the image data in the region of the heart and coronary arteries are segmented taking into account the detected landmarks. Regions with an increased HU value compared to a contrast agent surroundings are segmented in the segmented coronary arteries. A proportion of calcium respectively is calculated from the segmented regions for one or more of the segmented coronary arteries. At least the last two steps are carried out fully automatically by a data-processing system. Weighting factors for the individual regions are used when calculating the proportion of calcium, which weighting factors depend on both the threshold for segmenting the respective region and the volume of said region. The method and the data-processing system of at least one embodiment allow the dose exposure of the patient to be reduced and reduce the time expenditure of the user for determining the proportion of calcium.