1. A method for generating a projection image projected on a two-dimensional frame from image data of three or more dimensions that includes a plurality of first pixels of three or more dimensions, wherein each first pixel has a value and a coordinate, and the projection image includes a plurality of second pixels, the method comprising: setting a plurality of rays that pass through some of the first pixels, with each ray corresponding to one of the second pixels; selecting one of the first pixels existing on each of the rays; storing a value of the selected first pixel as a first value for each selected first pixel; storing a coordinate of the selected first pixel as a first coordinate for each selected first pixel; and determining a noise position in a plurality of selected first pixels based on the first coordinate for each selected first pixel, wherein said determining a noise position includes determining a stored coordinate of a selected first pixel associated with a certain ray as a noise position when the distance between the stored coordinate of the selected first pixel associated with the certain ray and a stored coordinate of a selected first pixel associated with a ray near the certain ray is greater than a predetermined threshold value.