superimposing bw mask on a grayscale image

I have a bw image which i obtained by modifying a grayscale image. Now i want to use the bw image as a mask on the grayscale image so that all the other parts except for the white part of the mask in the greayscale image vanishes. i have tried multiplying the images but all i get is bw image with some outer noise around the border.

Answer 1

[Image Analyst]

Try this:

grayImage(~mask) = 0;