A method for automatically detecting the presence of a contrast agent in an x-ray image includes acquiring a preliminary x-ray image. A background image is estimated. The contrast agent is administered. A plurality of image frames is acquired. The background image is subtracted from each image frame. An image having a highest image intensity is selected. A predefined shape model is fitted to the selected image using a semi-global optimization strategy. The fitting of the shape model is used to fit the shape model to each of the subtracted images. A feature value is calculated for each image frame based on pixel intensities of each pixel fitted to the shape model for the corresponding subtracted image. An image frame of peak contrast is determined by selecting the image frame with the greatest feature value.