Just to point out, cancer isn't the only reason to get these. Aneurisms, hemachromatosis, etc can all be serious. I know someone who got scanned for $500 and they caught hemachromatosis via iron deposits in the liver. Much better than eventual chirrosis and liver failure.
A blood test doesn't scan for other issues. A genetic test and iron study is going to run a few hundred dollars. Add in the cumulative cost for the other conditions checked for, like aneurism, and a $500 MRI isn't that bad.
If you get the MRI you are performing simultaneous blind tests for a thousand rare, unsuspected conditions. The blood test only tests for one of them, and eats 10ml of blood. There isn't enough blood in your body for all those tests.
We should arguably be, in an ideal healthcare system, getting annual MRIs with the other typical tests in a physical, and feeding that data into an AI, and have the AI issue recommendations for informed hazard & secondary testing/biopsy priorities. Tomographic analysis and differential diagnosis is exactly the sort of multidimensional pattern recognition task an AI is great at compared to a human doctor, if you can provide the training data.
Normalizing an annual MRI is the biomedical equivalent to survey astronomy and opening up those gargantuan datasets - you expect lots of simultaneous serendipitous discoveries in unrelated areas without securing an organizational mandate to fund a dedicated research effort into each one of them. The worst the process is ever going to be diagnostically is right now before we're collecting any data in a concerted fashion - every bit of training improves it.
MRI does not diagnose hemachromatosis. It detects iron deposits (could be due to other harmful issues). To my knowledge it would not produce a false positive. Hemachromatosis is the most common genetic issue in white people, so pretty common (I'm too lazy to look up stats).
