One of the factors in the deterioration of the reconstructed image of the single‐photon emission CT (SPECT) is the respective characteristics of the collimator used in the detection of the γ‐ray. The characteristics depend on the shape and material of the collimator, and produce shiftvariant blurs on the reconstructed image. This kind of error cannot be eliminated by the deconvolution, which regards the blur as shift‐invariant. This paper discusses the effect in the spatial frequency domain, and it is shown that the effect of the blur by collimator can be approximated analytically by a shift‐invariant blur on the annular ring determined for each frequency. A method of improving the quality of the reconstructed image is proposed utilizing this property. By the numerical simulation using a computer, it is verified that the proposed method is effective independently of the attenuation and quantization noise.
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