Downloading large amounts of data can take a while and it is important to find the most efficient way. If you find yourself clicking from session report to session report, then there is probably a better way.
XNAT includes a download applet for downloading files from multiple imaging sessions. The Download Applet is available from the Project Report -> Download Images or from the Options box on any listing of MR, PET, CT, etc. Either of these links will take you to the Image Data Download page.
On the Image Data Download page, you will need to select which scan, reconstruction and assessment types you would like to download. These requests will be passed into the Download Applet and download one collection at a time. Behind the scenes, the applet uses the REST api to download the files.
A common mechanism for downloading data is to use the REST API. Using this tool usually requires some mild scripting, but it gives you the most flexibility for choosing what to download. More details: Download Files with REST API.
Download Script: ArcGet
The most common script for downloading data is called ArcGet. ArcGet uses a host, username/password, and imaging session id to retrieve data from an application. ArcGet supports a number of parameters which help to streamline exactly which files are downloaded.