The invention relates to a method of calculating a displacement of an object of interest comprising a step of calculating (101) a displacement model of said object of interest from adjacent images of a set of pre-acquired images of said object of interest, said displacement model reflects the position of said object of interest along the time. The method is characterized in that the method further comprises the following. A step of determining (102) a first sub-set of images (S1) from said set of pre acquired images within one periodical time cycle of said set of pre-acquired images on the basis of the displacement model. A first step of identifying (103) a second sub-set of images (S2) from newly-acquired images, wherein images in said second sub-set of images (S2) are consecutive and have the same most similar image in said first sub-set of images (S1), wherein a first 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 (S1), and wherein said most similar image has the largest similarity level in said first set of similarity levels. A first step of selecting (104) a given image in said second sub-set of images (S2) as a first reference image (I1). A second step of identifying (105) a third sub-set of images (S3) from said newly-acquired images, wherein images in said third sub-set of images (S3) are consecutive and have the same most similar image in said first sub-set of images (S1), 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 (S1), and wherein said most similar image has the largest similarity level in said set of similarity levels. A second step of selecting (106) a given image in said third sub-set of images (S3) as a second reference image (I2). A step of calculating (107) the displacement between said second reference image (I2) and said first reference image (I1). The invention also relates to a corresponding system of displacement calculation.