It is very unconvenient to doctor if there are blood vessel

flab skeletion etc. interweaved on one encephalo-angiogram negative. Therefier it is need to use subtractive instrumect to subtract some unnecessary and disorderly background image. As a result only the blood vessle image is left for doctor to diagnose.This paper describes instrument which is different form the digital subtraction and the film subtract. Using the nonlinear foot part of exposure characteristic of the film and the fog part close to it

only one time exposure can eliminate the unnecessary and disorderly back-grund image and makes a good blood vessel image for doctor to diagnose.