A derivative of the RIM Observation class that represents a region of interest on an image, using an overlay shape. RegionOfInterest is used to make reference to specific regions in images, e.g., to specify the site of a physical finding by "circling" a region in a schematic picture of a human body. The units of the coordinate values in RegionOfInterest.value are in pixels, expressed as a list of integers. The origin is in the upper left hand corner, with positive X values going to the right and positive Y values going down. The relationship between a RegionOfInterest and its referenced ObservationMedia or ExternalObservation is specified by traversing the entryRelationship or reference class, respectively, where typeCode equals "SUBJ". A RegionOfInterest must reference exactly one ObservationMedia or one ExternalObservation. If the RegionOfInterest is the target of a <renderMultimedia> reference, then it shall only reference an ObservationMedia and not an ExternalObservation. An XML attribute "ID" of type XML ID, is added to RegionOfInterest within the CDA Schema. This attribute serves as the target of a renderMultiMedia reference (see renderMultiMedia). All values of attributes of type XML ID must be unique within the document (per the W3C XML specification).