In the conventional successive approximation image reconstruction method (SPS method), in order to provide an arithmetic unit, an X-ray CT apparatus, and an image reconstruction method capable of shortening the processing time while maintaining the noise reduction effect, For each image update performed, forward projection is performed on the updated image, the forward projection data is subtracted from the original subject projection data, and the difference image is obtained by backprojecting the forward projection data and the forward projection data, and the forward projection process and the back projection process are repeated In the present invention, however, the forward projection processing and the back projection processing, which require computation time, are replaced with processing with a relatively small amount of calculation, which is the difference between the updated image and the reference image, and as a result, the sequential approximate image reconstruction processing It is possible to greatly reduce the calculation amount and shorten the processing time.ノイズ低減効果を維持しつつ処理時間を短縮することが可能な演算装置、X線CT装置、及び画像再構成方法を提供するために、従来の逐次近似画像再構成方法(SPS法)では、繰り返し行われる画像更新の都度、更新画像を順投影し、順投影データを元の被写体投影データと差分しこれを逆投影することで差分画像を得て、順投影処理と逆投影処理とを繰り返し行っていたが、本発明では、演算時間を要する順投影処理と逆投影処理を、更新画像と基準画像との差分という比較的演算量の少ない処理に置き換え、その結果、逐次近似画像再構成処理の演算量が大幅に減少し、処理時間を短縮する事が可能となる。
