A computer-implemented method of calculating a displacement of an object of interest, comprising a step of calculating a displacement model of said object of interest from adjacent images of a set of pre-acquired images of said object of interest, wherein said displacement model reflects the position of said object of interest along the time, characterized in that said method further comprises: - a step of determining a first sub-set of images from said set of pre-acquired images within a periodical time cycle of said set of pre-acquired images on the basis of the displacement model; - a first step of identifying a second sub-set of images from newly-acquired images, wherein images in said second sub-set of images are consecutive and have the same most similar image in said first sub-set of images, wherein a set of similarity levels is determined by comparing a given image in said newly acquired images with each image of said first sub-set of images, and wherein said most similar image has the largest similarity level in said set of similarity levels; - a first step of selecting a given image in said second sub-set of images as a first reference image; - a second step of identifying a third sub-set of images from said newly-acquired images, wherein images in said third sub-set of images are consecutive and have the same most similar image in said first sub-set of images, wherein a set of similarity levels is determined by comparing a given image in said newly acquired images with each image of said first sub-set of images, and wherein said most similar image has the largest similarity level in said set of similarity levels, and wherein the same most similar image of the third sub-set of images is different to the same most similar image of the second sub-set of images; - a second step of selecting a given image in said third sub-set of images as a second reference image; - a step of calculating the displacement between said second reference image and said first reference image. - a step of determining a first sub-set of images from said set of pre-acquired images within a periodical time cycle of said set of pre-acquired images on the basis of the displacement model; - a first step of identifying a second sub-set of images from newly-acquired images, wherein images in said second sub-set of images are consecutive and have the same most similar image in said first sub-set of images, wherein a set of similarity levels is determined by comparing a given image in said newly acquired images with each image of said first sub-set of images, and wherein said most similar image has the largest similarity level in said set of similarity levels; - a first step of selecting a given image in said second sub-set of images as a first reference image; - a second step of identifying a third sub-set of images from said newly-acquired images, wherein images in said third sub-set of images are consecutive and have the same most similar image in said first sub-set of images, wherein a set of similarity levels is determined by comparing a given image in said newly acquired images with each image of said first sub-set of images, and wherein said most similar image has the largest similarity level in said set of similarity levels, and wherein the same most similar image of the third sub-set of images is different to the same most similar image of the second sub-set of images; - a second step of selecting a given image in said third sub-set of images as a second reference image; - a step of calculating the displacement between said second reference image and said first reference image.