To me N and S draw in data (well defined or not) and it is T and F that processes the data (defines it, gives it meaning, decides what to do with it etc.).
Data is passive (it just is) and evaluation is active (makes a statement about what is).
Perception (N and S) is passive (it just is) and Judgement (T and F) is active (makes a statement about something).
Therefore to me imagination is simply the internal combination of a judging and a percieving process/es. Perception providing the matierials and judgement constructing the internal world or concept.
I wonder if that makes any sense.