Hi Martin, when I checked the Allow lossy image conversion option. It worked and I got the nhdr format image in the output directory. It worked both in Windows and Mac OS. 
However, it further raised two problems because I want to do this in the loop. 

First, it seemed that I couldn't install the Slicer well in my own macbook. Each time I opened Slicer by clicking the icon, it came out the warning as follows: "Slicer" can't be opened because the identity of the developer cannot be confirmed. I don't know why this happens everytime.
 
Secondly, i tried to use the software in Linux OS. I did as the tutorial suggested, I opened the tar.gz archive and copy directory to the location of my current folder. But when i typed the "DWIConvert", I got an error message "command not found". So do the "DWIConverter" command. (I saw your suggestion to others' problems.)Just the same error. Is there any software I need to install? So far I just got only Slicer 4.8.1 for Linux.

Thank you,
Xiao