How different is an introverts brain different from an extroverts?
I am not sure if this answers your question, but according to the article "The Difference Between Introverts and Extroverts" (Gallagher, 1999), the main differences between introverts and extraverts are where the main brain activity occurs (as seen by positron emission tomography (PET) measurements).  The full PET study is in the article "Cerebral Blood Flow and Personality: A Positron Emission Tomography Study " (Johnson et al. 1999).

The conclusion of the Johnson et al. paper states:


  This study revealed a pattern of increased blood flow in the frontal lobes associated with introversion. ... in the present study, there were more cortical regions associated with introversion than with extraversion. ...in the present study, extraverts did show lower blood flow in several regions in the behavioral inhibition system, namely, the frontal lobes and the hippocampus.


Some images of brain activity is included in a more recent article "Differences in regional brain volume related to the extraversion–introversion dimension—A voxel based morphometry study" (Forsman et al. 2012).  Forsman's article looks at activity in  the white and grey matter of the brain.

I hope this helps.