If you want to squeeze the B image into the range of the A image, you can use imhistmatch(), mat2gray(), or imadjust(), or even simple mathematical scaling manually. For example
minA = min(A(:));
maxA = max(A(:));
newB = (maxA - minA) * mat2gray(double(B)) + minA;
imshow(newB, []); % The [] are needed if you want to see it.
newB is a floating point image. Cast back to uint8 if you want but if you do that, you'll have quantization error.
