1. A method of determining movement of biological tissue, using ultrasound data values, wherein the ultrasound data values represent an intensity of reflected ultrasound energy corresponding to a position of the biological tissue, the method comprising: obtaining a first set of ultrasound data values corresponding to positions of the biological tissue at a first time; obtaining a second set of ultrasound data values corresponding to positions of the biological tissue at a second time; locating a feature pattern in the first set of ultrasound data values so as to provide a reference feature pattern; determining a search region in the second set of ultrasound data values that corresponds to the located reference feature pattern in the first set of ultrasound data values; locating a feature pattern in the search region of the second set of ultrasound data values so as to provide a candidate feature pattern corresponding to the reference feature pattern; determining a correlation between the reference feature pattern in the first set of ultrasound data values and the candidate feature pattern in the search region in the second set of ultrasound data values; and selecting the candidate therefor feature pattern in the search region in the second set of ultrasound data values based on the correlation.