I had gel breast implants (under muscle) in 2009, 1 is now noticeably smaller. could this be a rupture? is an mri scan the only way to be sure?

No: The MRI is the gold standard for evaluating gel breast implants for a possible rupture but are expensive and may not be covered by insurance. A less expensive way which might give the information would be an ultrasound. In any case you should see a board certified plastic surgeon to have this evaluated. Also, are you up to date on your mammograms?