DICOM PS3.3 2019b - Information Object Definitions
C.7.6.21.2 Equipment Coordinate System Identification

The Equipment Coordinate System Identification (0028,9537) identifies the Reference Coordinate System to which the Image to Equipment Mapping Matrix (0028,9520) is related.

The Defined Term ISOCENTER refers to a coordinate reference system where the origin corresponds with the center of rotation of the projections.

DICOM PS3.3 2019b - Information Object Definitions