St Elsewhere's Snow-Globe Ending

The final and controversial ending to the popular 80s drama, St. Elsewhere.
From Wikipedia:
Tommy shakes the snow globe, he is told by his father to come and wash his hands for dinner. Donald places the snow globe on the family's television set and walks into the kitchen with Tommy and Auschlander; as they leave the room, the camera closes in on the snow globe-which holds a replica of St. Eligius.
The most common interpretation of this scene is that the entire series of events in the series St. Elsewhere had been a product of Tommy Westphall's imagination, with elements of the above scene used as its own evidence.