First check on the type of your data. You can cast the type of the data by example double() or uint16() etc. (Check the help for typecasting). Here is an example how you rescale your values to the intensity-range of uint16, unsigned integers with ~65k possible different values. The casting of course leads to less precision of the intensity values.
new_img(:,:) = uint16((new_img(:,:)./max(max(new_img(:,:),[],1)))*65536);
Afterwards you should be able to write the data to your file.