For improvement of nodule detection in CT, we have evaluated image preprocessing employed before our Com-puter-aided diagnosis (CAD) system for lung cancers. The preprocessing method consists of the median and tophat fil-ters. The median filter can reduce high frequency noises, and the tophat filter can remove low frequency changing in density. This paper describes an evaluation method by use of artificial nodules. Experimental results show the relation-ship between the parameters for the filters and the sizes and attenuation of detectable nodules.

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