Add Shibuya (mainly around the station) and Shinjuku and you basically covered all foreigner hotspots! Those are the most non-representative areas!
But go to Shimokitezawa or Gakuendaigaku or anywhere off the beaten path and foreigners will be few and far between.
Shimokitazawa still have a lot of foreigners. It is surrounded by universities including Todai.
Both of those places have a TON of foreigners, and neither one of them are central Tokyo either :p
I've been to places with thousands of Japanese (and the occasional Korean, which can be difficult to notice) and then just me. But on the other hand I've never, not once, been made to feel like an outsider.
Even? Minato-ku has the highest density of foreigners of all the wards, something like 20% of the population.