1. A method for reconstructing a three-dimensional ultrasound image, the method comprising steps of: generating a two-dimensional image having various directivities in a frequency space based on plane wave data; manipulating the generated two-dimensional image to generate three-dimensional image data; and reconstructing a three-dimensional ultrasound image by performing an inverse fast Fourier transformation on the three-dimensional image data, wherein the step of manipulating the generated two-dimensional image comprises the step of back-projecting the generated two-dimensional image to generate the three-dimensional image data by taking a function defined on each of a plurality of lines in the generated two-dimensional image and projecting the function back over each line to generate the three-dimensional image data.