A method for reducing the effect of physiological motion in image data acquired from a sequence of medical images of a subject comprising: acquiring a sequence of medical images containing ultrasound image data of a target region of a subject during the occurrence of cyclical physiological motion affecting the target region; using image data of the images to identify cycles of the physiological motion; identifying images acquired during a certain range of phases of the identified motion cycles; and processing image data (56, 68) of the identified images to assess a property of the target region; characterized in that using image data comprises: extracting a motion information matrix, MIM, from the image data by forming an average intensity image which is an average intensity of all the image frames in the sequence, then subtracting each image frame of the sequence from the average image and binarizing the subtracted image relative to a threshold pixel intensity value to produce a binary image of bright structures in each image frame, then adding those binary images together to form the MIM; selecting from the image sequence a trigger frame to be included among the images to be processed, by virtue of its inclusion of a clear definition image of a structure as by a high intensity or large size in the image or of an image which appears virtually identical to previously and/or successively acquired images in the sequence; comparing structures in the image of the trigger frame with structures in the MIM to select, for motion curve identification, one which varies over a wide intensity range or positional range by physiological motion; and applying the selected structure as a mask to the sequence of images to extract a series of mean intensity values which vary in intensity with the cyclic physiological motion, to identify the motion cycles as a motion curve. acquiring a sequence of medical images containing ultrasound image data of a target region of a subject during the occurrence of cyclical physiological motion affecting the target region; using image data of the images to identify cycles of the physiological motion; identifying images acquired during a certain range of phases of the identified motion cycles; and processing image data (56, 68) of the identified images to assess a property of the target region; extracting a motion information matrix, MIM, from the image data by forming an average intensity image which is an average intensity of all the image frames in the sequence, then subtracting each image frame of the sequence from the average image and binarizing the subtracted image relative to a threshold pixel intensity value to produce a binary image of bright structures in each image frame, then adding those binary images together to form the MIM; selecting from the image sequence a trigger frame to be included among the images to be processed, by virtue of its inclusion of a clear definition image of a structure as by a high intensity or large size in the image or of an image which appears virtually identical to previously and/or successively acquired images in the sequence; comparing structures in the image of the trigger frame with structures in the MIM to select, for motion curve identification, one which varies over a wide intensity range or positional range by physiological motion; and applying the selected structure as a mask to the sequence of images to extract a series of mean intensity values which vary in intensity with the cyclic physiological motion, to identify the motion cycles as a motion curve.