The MRI facility is located in the Cancer Research Institute building at 15 O'Kill Street (Google Maps). O'Kill St runs between Barrie St and George St, parallel to and in between Stuart St and King St W.
To find the MRI facility, go to the south end of the Cancer Research Institute (CRI) building, at 15 O'Kill Street. There is a small parking lot between O'Kill street and the CRI building, and from the street the entrance is not clearly visible because it is in small stairwell.
Once you are at the entrance you will see a sign for the MRI Facility on the wall to the left, and below the sign is a call button. Press the call button to have someone come and open the door to let you in.