It is also useful to save references in converted SOP instances to their source. Accordingly, converted instances are required to contain such references, both for image conversions as well as for ancillary instances that may be updated, such as Presentation States and Structured Reports.

Obviously, the references to the source instances for the conversion are excluded from conversion themselves. If the instances have been converted on different systems, however, there is a possibility that the source references will be "replaced" and a record of the "chain" of multiple conversions will not be persisted.

There is no mechanism to define forward references in the source to the converted instances, since that would imply changing the source instances from their original form, and while this is acceptable within the scope of the normal "coercion" that a Storage SCP is permitted to perform, it is probably not sufficiently useful to justify the effort. This does imply some asymmetry however, depending on the direction of conversion (classic to enhanced or vice versa); only one set will contain the references.

In performing round trip conversion, without access to the source instances, the referenced source UIDs can be used as the UIDs for the newly created converted instances.