Misunderstanding here. The question is at what age is the base rate in the population sufficiently high that a test with a certain sensitivity and specificity useful net risks. Multiple studies have shown that these screening programmes don’t have a huge impact on mortality - excluding lead time bias etc. and noting the c. 10 in 100k extra cases of cancer caused by the screening. Here’s a review for prostate screening that’s even more damning about its usefulness - https://med-fom-urlgsci.sites.olt.ubc.ca/files/2007/06/P-ca-...
MRI scans take longer, are often more uncomfortable for the patient (being in an enclosed, noisy machine), require much larger capital investment than an X-ray (and therefore supply is more limited), and have a higher cost per exam than an X-ray.
It almost certainly would be possible to use MRI for screening, but the impact would be a reduction in availability and a higher cost.
