Hi, 
thanks for your reply.

Instead of downloading images directly, I use Catalog XML option. It opens a new webpage that has got xml format. I read this file and parse it to get "Cat:entry" elements using python. I read "uri" attribute which has this format:  " http://www.nitrc.org/ir/data/experiments/NITRC_IR_E03372/scans/MPRAGE/resources/DICOM/files?format=zip&projectIncludedInPath=true&subjectIncludedInPath=true" 

Then I use wget/curl to download this link in my terminal. Since downloading needs username and password, I use --http-user and --http-password ( and other flags). But I can not download these images. I get these messages in the terminal : 

Connecting to http://www.nitrc.org|132.239.16.23|:80..... connected.
HTTP request sent, awaiting response... 401 Unauthorized
Reusing existing connection to http://www.nitrc.org:80.
HTTP request sent, awaiting response... 404 Not Found
2015-08-01 05:21:51 ERROR 404: Not Found.

Thanks for your help.