How to change pixel values of background to zero

I have an gray image as uint16. the background of the image (air region)has got some pixel values. I want to make them all zero. Any idea how do do it? Your help is much appreciated. Thanks

Answer 1

[Matt J]

How about

 image(image<airthreshold)=0;

[Kavitha Srinivasan]

thanks for your reply. But how to get the airthreshold value?

[Matt J]

That depends on noise and what's going on in the rest of the image. In one of your tags, you seem to say that this is a CT image. In my experience, anything below 500 HU can usually be classified as air, but extreme noise-levels could change that.