If you are willing to pay for the MRI (very expensive, about 900$), you can do it immediately at the radiology clinic at 1 westmont square 1st floor. Ask your doctor for a reference note for the MRI and go to the radiology clinic for an appoitment (you would obtain it within 1-2 days, I don't have the phone # of the clinic anymore, but you can go personally by metro and get off at atwater metro station. there is a tunnel which allow you to go to Westmount square without going out).
My mom did the MRI for her liver tumor and she obtained the results 2 days after.