how do i filter a part of image

Question

i user convolution to find circle in one image,like imfilter(image,circlefilter,'same'), i find o method reckon the area of centre of a circle in image,example,size of originality image is 400*400,my reckon the area of centre'size is 40*40,in imfilter values outside the bounds of image is define set to 0, but i want set the bounds of the reckon area is bounds of reckon area in originality image.