1. A method for performing 2-dimensional discrete Fourier transform of a subject image data to be performed in one or more digital processors, the method comprising: step (a) of extracting boundary image data from the subject image data; step (b) of performing 2-dimensional fast Fourier transform of the subject image data by performing 1-dimensional fast Fourier transform on each row of the subject image data and performing 1-dimensional fast Fourier transform on each column of the subject image data; step (c) of performing a simplified 2-dimensional fast Fourier transform processing on the extracted boundary image data without performing column-by-column 1-dimensional fast Fourier transform so as to derive fast Fourier transform of a smooth component, the step (c) including: performing 1-dimensional fast Fourier transform only on a first column vector in the extracted boundary image data, using scaled column vectors to derive fast Fourier transform of remaining columns of the extracted boundary image data, and performing 1-dimensional fast Fourier transform on each row of the extracted boundary image data; and step (d) of deriving fast Fourier transform of a periodic component of the subject image data with edge-artifacts removed by subtracting the fast Fourier transform of the smooth component derived in step (c) from the 2-dimensional fast Fourier transform of the subject image data performed by step (b).