You can continue to follow up with your yearly mammograms and if the mammograms pick up a suspicious lesion a MRI can be done. Usually you should do regular monthly self breast exams and a clinical evaluation by a physician every 6 months and a follow up mammogram evrery year. In case of dense breasts the lesions are not easily picked up on mammogram and then a USG or MRI is advised. Discuss with your doctor why he thinks you need an MRI at 6 months and what is he suspecting. Goodluck.
It is not possible to reason out 6monthly follow up with MRI breast.Probably discuss with your doctor regarding this as he may be thinking of some other condition.
Annual mammography with daily self breast examination is all that required at present.