1. A method for operating a radiographic system comprising the steps of: emitting from X-rays which penetrate through an object to be detected being carried by a mechanical control device; receiving by a detector device the X rays and transmitting the received signal from the detector device to a data acquisition device; converting the signal into a digital signal and transferring the digital signal to a computer imaging device; the method further comprising the steps of, at the computer imaging device; normalizing an acquired image A(x,y) to form a normalized image B(x,y); filtering the normalized image B(x,y) by a low-pass filter to obtain an filtered image C(x,y); calculating a relative standard deviation for each pixel in the image A(x,y), three times the relative standard deviation being an edge threshold for each pixel, wherein the relative standard deviation is calculated as below: σ ⁡ ( x , y ) = { ∑ i , j = - N 2 N 2 ⁢ [ A ⁡ ( x i , y j ) - M ⁡ ( x , y ) ] 2 } / ( N 2 - 1 ) wherein M(x,y) is an average value of pixel neighborhood for pixel (x,y), and N denotes a width of pixel neighborhood; subtracting the filtered image C(x,y) from the normalized image B(x,y) to obtained a difference image; thresholding the difference image using the edge threshold for each pixel to form a threshold-processed image D(x,y); enhancing a contrast of the threshold-processed image D(x,y) by using a non-linear function to form a contrast-enhanced image E(x,y), wherein the non-linear function is arranged to implement a sine transformation; determining a enhancement coefficient a(x,y) for each pixel as below: α ⁡ ( x , y ) = c A ⁡ ( x , y ) A max , wherein c is a predetermined constant, Amax is the maximum value of pixel in the image A(x,y); obtaining a edge-enhanced image F(x,y) by multiplying the enhancement coefficient a(x,y), the contrast-enhanced image E(x,y) and the filtered image C(x,y), and generating a resulting image by multiplying a sum of the edge-enhanced image F(x,y) and the filtered image C(x,y) with the maximum value Amax.