I think mammograms do a better job of finding calcifications and MRI do a better job of finding and identifying lesions. If an early BC is mostly calcifications then a mammogram would see it, maybe a little more clearly than an MRI.
My DCIS had very few calcifications and with the mammogram they thought it might just be an unusual lymph node, partly because of location. An ultrasound indicated a biopsy. MRI identified the DCIS.