value a normalized value of minimum index an average brightness value a normalization function obtained by normalizing an accumulated function of brightness values first index information on brightness values over a predetermined critical value a dispersion of brightness values a standard deviation of brightness values a skewness of brightness values a kurtosis of brightness values a minimum brightness value a brightness value of an n th ultrasound image wherein n is a natural number a orientation change function of a brightness value gradient and a phase change function of the brightness value gradient for simplicity of the below description it is considered that the feature values may include the maximum brightness value the normalized value of minimum index and the average brightness value however the feature values are not limited thereto furthermore the processor 120 may form a change trend image showing the extracted feature values with colors over time fig3 is a flowchart showing a process of forming a change trend image over time based on the extracted feature values the processor 120 may form the ultrasound images including index information based on the ultrasound data provided from the ultrasound data acquisition section 110 at step s 102 the index information may include information.