Image Storage Unit. The online storage can be referred to as the "image storage unit" in a PACS. Copies of all the images of the in-patients, hospitalised patients, or patients attending clinics are held in this form of storage. The images can be removed from the ISU when no longer required online. However they will always be available from the nearline or offline archive. See Working Storage Unit (WSU), Nearline archive and Offline archive.