Some systems may perform an XA acquisition by automatically changing the Frame rate at predefined times during the same acquisition. An XA Acquisition Phase is defined as a collection of Frames in which the acquisition Frame rate remains constant. A new phase shall be defined whenever there is a change in the Frame rate, new phase can be defined at the acquisition device's discretion.

Each XA Acquisition Phase is encoded as an Item in this Sequence. An Acquisition Element of constant Frame rate for the duration of the acquisition is encoded with one single Item in this Sequence.

An Image Instance may contain one or more phases, but an Image Instance cannot span across multiple Protocol Elements. The X-Ray switch is kept ON along all the XA Acquisition Phases within an acquisition.