New structure of a low intensity X-ray image system is mainly made of plane plate mode X-ray intensifier of single proximity focus and CCD data acquisition and processing system. By the system composition

the paper analyses the image noise source of low X-ray imaging system

and points out that the random noise is white noise which is obeyed by Poisson distribution in the whole body

yet the positive-negative interfering impulse is excited in the some locality. Then the compound methods of the "multi-frame mean + extremum median filter" is submitted which deals with the imaging noise. Firstly

some frame images is superimposed

then mean image is calculated from those images

which is under the principle of noise non-correlation. By the method

the information is enhanced and the noise is compressed. Secondly

based on the standard median filtering algorithm

the extremum median filter is ordered as much as possible to preserving the detail of the image when the noise are removed. That is to say

all the pixels are separated into signal and noise pixels according to the decision criterion given in the following; then

noise pixels are replaced with the median value of their neighborhood in the input image. The decision criterion: if a pixels value is the extremum (max or min) of its neighborhood

it is a noise pixel; else it is a signal pixel. This decision criterion is under such an assumption: inherent relationships exist among neighbor pixels. If a pixels value is far higher or lower than the others' value of its neighborhood

we may consider that it had been contaminated with noise. Else

if it is similar to the others

we consider that it represents an effective signal. By the calculation of PSNP

the methods are supper to any single method greatly. And the effect of image filter is satisfied.