This is because you don't have any medical training and thus are making assumptions based on the technology used. The main risk of scanning everyone is you will find all sorts of abnormalities that probably have nothing to do with anything. These are called "incidentalomas" (i.e. incidental abnormalities completely unrelated to the patient's symptoms). When these lead to harm in the patient, the patient is called VOMIT (Victim of Medical Imaging Technology).
If someone's problems are behavioral, psychological, or due to drugs, some incidental finding then becomes the "cause" of all their problems, and the patient no longer has any incentive to address their problems.
It is very common for headache patients who get (inappropriately) scanned, to get worse when some incidental tumor or spots that are totally unrelated to the headache are unearthed. In some cases the location of the headache shifts hemisphere or other region to "match" the incidental imaging findings
Lots of people have tumors - including brain tumors since we're talkng about neuroimaging - that will never cause a problem for them. But you unearth some tiny meningioma that was never going to cause problems, and tell someone they have a brain tumor. It may not go over well. They may become excessively anxious or distressed. They may undergo repeated imaging because now we've found this tumor we can't just ignore it - or the patient can't seem to forget about it.
You might see white matter hyperintensities etc that trigger additional, potentially invasive workup that is totally unnecessary. In the process of this workup (LP, biopsy etc), the patient may experience physical injury, infection, bleeding, pain etc etc.
As for contrast, we are realizing that gadolinium seems to pool in the brains of some people who have had repeated MRIs... the significance of this is not known but is a potential issue for MS patients who undergo lots of serial MRI.
More pts than you might expect require conscious sedation for MRI which has some inherent risks.
You're right that I don't have medical training, but I was actually aware of the risks of fishing expeditions when it comes to any type of test that can result in an obligation to do further testing/treatment that can be damaging. So, you're right that saying that the risk is low is not accurate when you include that facet. But there have been a huge increase in the number of CT scans performed that have immediately damaging effects, and some of those are fishing expeditions or are done in the name of defensive medicine. So, OK, in the face of what you said, I change my opinion that there's no such thing as a not useful MRI. I do still think there's a . . . not epidemic, that's too strong . . . a cavalier attitude toward liberal CT scans.
The point you made about incidentolomas made me think about troponin testing, and how any time you have a new protocol where you have more data, you need new algorithms.
Europe has used high-sensitivity troponin tests for well over a decade now in assessing chest pain. These tests are such that every person tested has detectable troponin. It requires a different algorithm than what is used in the US. The tests can detect anything on the spectrum from stable angina to myocardial infarction, to non-cardiac issues, along with normal levels of troponin that every person has. It presents a more complicated clinical picture than what we currently have in the US, which is one of the reasons the FDA hasn't approved these tests.
I don't know how Europe deals with these test results. They must have different cut-off levels than we do, and possibly some more complicated algorithms. I know they have a much higher NPV, but not sure how they deal with equivocal results.
But if it is possible to deal with new technology that gives you more information when it comes to a blood test, I wonder if it would be with neuro-imaging, as well. I mean I know within my own family someone who had an MRI and had a brain plaque that was deemed part of normal aging even given a family history of MS and that was given no further workup, so I guess that speaks to the fact that doctors already have discretion with imaging. I had a number of incidentalomas after my CT scan for appendicitis: trace plaque in abdominal aorta, some tiny thing in a lung (didn't worry me enough to even remember what it was), some sort of bone island in my pelvis.
I guess I should re-state my desire and observation that led to my idea about no-bad MRI: Many of the things that are done with CT scans today are possible to do with MRIs, but the health of patients has not been a large enough impetus to move to MRI en masse quickly.
I don't believe in handing out MRIs like candy (except to the end that I optimistically espoused: that it might hasten a move away from CT scans) and I accept what you're saying about fishing expeditions.
As for the pooling in the brain of gadinolium, I was not aware of that. I knew about NSF and the risk for that in renally impaired patients. I am not medically trained, but I do try to educate myself. When I had my most recent CT scan and contrast for the first time, I told the doctor I should probably delay my beta blocker dose as it increases the risk of an allergic reaction to the iodinated contrast material. The doctor was medically trained but was not aware of that increase in risk. I know that I can't see medicine from as broad of a scope as a physician, but my interest can sometimes result in specific knowledge. And there are medically trained physicians who are very concerned about the number of CT scans being performed, which hasn't seemed to result in any more judiciousness. So my spitballing was order more MRIs to change the balance. That is in no way based on specific knowledge, just spitballing on what might hasten a change from CT scans to MRIs (increase the demand, lower the costs, increase the ubiquity and normalcy of MRI as default over CT in situations where it's equal or better from an imaging standpoint since whether it's equal or better from an adverse event standpoint hasn't tipped the scales).
EDIT: I would also add that is precisely medically trained professionals who have offered whole body CT scans as a preventative medical fad for the very wealthy. I remember Oprah back when she had a TV show had one and had a doctor on promoting it. It takes a judicious mind, beyond just a medically trained one, to know radiating your entire body when there are no presenting symptoms is a bad idea.
EDIT 2: Here is a medically trained professional who understands things much better than I do.
He suggests 20 mSv for everyone! You get 20 mSv, and you get 20 mSv, and you get 20 mSv!
And I'm not talking about Oprah; I'm talking about the profiteer doctor who came on the show to hawk this: