The diffractive imaging is to reconstruct an object image from its diffraction pattern. It was found that the phase-retrieved images from the incomplete diffraction pattern （contaminated by noise） construct a spherical shell structure centered on the average. In this paper, we show that spherical shell structures are generated even if the diffraction pattern is not contaminated by noise or the noise is weak. Moreover, we found that applying PCA （Principal Component Analysis） can greatly reduce the complexity of the data without destroying the spherical shell structure.