Correlating PET-CT images using DICOM tags

This is actually a wonderful question, which i am yet to find a good answer.

Note that, PET's SOP UID is 1.2.840.10008.5.1.4.1.1.128 and CT's SOP UID is 1.2.840.10008.5.1.4.1.1.2.

But there isn't a SOP UID for both (PET/CT) because essentially these two are two physical copies only that they are scanned in one single gantry simultaneously or immediately sequentially.

Now, assuming that you have inserted a lot of DCMs into the pacs/db, and now you would like to retrieve a set of PET/CT.   How would you search or query for this pair of stacks?

Modality Tag and Study/Series UIDs
Initially, i would have assumed that the study/series UIDs would be the same; however, here is my finding:
Purely relying on either modality tag or study/series UIDs would fail to do the job.  

Frame of Reference UIDs
They would be different most of the time.

StudyTime/AcquisitionTime
Some suggest to look into study time and/or acquisition time; however, for simultaneous scanning, they may be advisable.  However, obviously, this won't work with sequential scanning.

Study Description
Study description is primarily input by the machines and if the machine is indeed a pet/ct scanner, the study description should somehow have the string (pet/ct, petct, ptct, ctpt etc...).  With that said, if that specific study is only for CT but using the same machine, this study description would also include the aforementioned list of possible strings.  In addition, some pet/ct machines have NM capabilities, so their study description would contain NM as well.

Referenced Study Sequence
This may refer to the same study; however, not all pet/ct image sets have this tag as this is an O(optional) key¹.

Conclusion

In short, there seems to be a missing link for a good, solid correlation between pet/ct image stacks.   Feel free to comment and, by all means, please correct me if this is wrong and let me know the right way.  Thanks!

Footnote
1.  According to DICOM standard PS3.4-2009, there are types of keys used in Q/R Information Models: U is unique key attribute, R is required key attribute, and O is optional key attribute.