In recent year, X . ray photographs can be obtained by very low dose with high sensitivity photographic system . Such very 16w dose makes the image quality inferior because of the 且uctuation of . x −ray quanta so called ' ' quantum noise " . The prosess of th 巳 x − ray ilnage formation was evalttated from a point of view that the x − ray images are forlned by the accumulation of x − ray quanta distributed randemly . And the conditien that the i皿 age of a . simple . shaped small object can be

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... ognized into background qllanta was investigated , 、 Under the assumption that the quanta absorbed by an object are Ns and that the average number of background quanta per area which is in the same projeeted area of the object is Nb , the conditiDn that Ns