There are a couple of ways to accomplish this.
1) You can use a calculated container field, where the results of the field are equal to one of two (or more) containers stored in a separate table, based on the value in a third field. For example:
Case ( doctor = "Doc1" ; commonTable::doc1sig ; doctor = "Doc2" ; commonTable::doc2sig )
This method is OK if you have a limited number of signatures, but falls down very quickly if your number of signatures expands. It's also not very flexible.
2) A better, more flexible, more relational method involves the use of a "doctors" table that includes the key for the doctor in question, along with a container field that stores each doctor's signature. You then set up a relationship between the letter table and the doctor table on the doctor key field. You then display the appropriate container field as a related field on the letter. That way, if you add a new doctor, you just create a new record (along with his scanned signature) in the doctors table, and everything works just fine.
I tried the better way you suggested and it worked like a charm.
Thank you so much for your help,