“Real” in the sense it exists within your mind, which exists (somehow) within your brain, which is itself likely real. So your imagination must have some physical basis, but it would exist entirely within your brain (or body, to the extent the rest of your body influences the brain itself).

Your brain imagining a specter in a doorway does not mean there is any anomaly within the physical space of the doorway at all that anyone else could perceive or measure - they would need to measure your brain to (theoretically) detect the physical basis of it.

What you see, feel, hear, taste is an interpretation of your physical environment, and may not accurately reflect it at all times.

Imagination is real; imagined things are not necessarily.

Imaginary numbers are also not real.

The whole situation is quite complex.