Most often these tests take approval from your insurance company, sometimes denied and needing to be justified by your doctor - which can take weeks just to get the ok.  Also as PJ mentioned the MRI is not as accurate as other test.  A PET CT Scan is more reliable, and while it will not show just a cancer cell, it does detect a group of cells or a tumor.  But since an MRI can show false-positive results many doctors like to do the whole range of tests then compare them to one another before making a diagnosis or recommendation.  Even though your cancer is labeled as "aggressive", it did not spread over night.  It still takes many months for a new tumor to grow.  Your doctors are most likely recommending a mastectomy because they did not get clear margins and other tumors show present - and rather than biopsy each tumor or try to remove them, they feel it better to just remove the whole breast tissue - which is pretty standard.  If you did not have other areas, then going back into the orirginal site and removing more tissue would most likely be done - based on the findings of other tests.  Don't think this as being what is easiest for the doctor to do - they obviously think that the benefit of such outweighs any risks, etc. 

my suggestion to you would be to find out what kind of procedure they are recommending (a modified radical, a radical, or just a mastectomy) and read up on the different procedures; consult with a plastic surgeon about possible immediate reconstruction - and don't forget to find out if he intends on taking any or all of your nodes, as resulting lymphedema is also a possibility.  This way you can be best prepared for what is to come.

Best of luck to you!

Angi