It's very non-intuitive but scans and tests have the potential to cause more harm than good.

From TFA:

>Overtesting has also created a new, unanticipated problem: overdiagnosis. This isn’t misdiagnosis—the erroneous diagnosis of a disease. This is the correct diagnosis of a disease that is never going to bother you in your lifetime. We’ve long assumed that if we screen a healthy population for diseases like cancer or coronary-artery disease, and catch those diseases early, we’ll be able to treat them before they get dangerously advanced, and save lives in large numbers. But it hasn’t turned out that way. For instance, cancer screening with mammography, ultrasound, and blood testing has dramatically increased the detection of breast, thyroid, and prostate cancer during the past quarter century. We’re treating hundreds of thousands more people each year for these diseases than we ever have. Yet only a tiny reduction in death, if any, has resulted.

>H. Gilbert Welch, a Dartmouth Medical School professor, is an expert on overdiagnosis, and in his excellent new book, “Less Medicine, More Health,” he explains the phenomenon this way: we’ve assumed, he says, that cancers are all like rabbits that you want to catch before they escape the barnyard pen. But some are more like birds—the most aggressive cancers have already taken flight before you can discover them, which is why some people still die from cancer, despite early detection. And lots are more like turtles. They aren’t going anywhere. Removing them won’t make any difference.

>Over the past two decades, we’ve tripled the number of thyroid cancers we detect and remove in the United States, but we haven’t reduced the death rate at all. In South Korea, widespread ultrasound screening has led to a fifteen-fold increase in detection of small thyroid cancers. Thyroid cancer is now the No. 1 cancer diagnosed and treated in that country. But, as Welch points out, the death rate hasn’t dropped one iota there, either. (Meanwhile, the number of people with permanent complications from thyroid surgery has skyrocketed.) It’s all over-diagnosis. We’re just catching turtles.

We get overdiagnosis and overtreatment of the population precisely because it is done at an individual level.

Checking occasionally for something specific that's reasonably suspected due to symptoms is good. If you're always checking for everything, you have a high risk of finding something you don't actually have. It turns out that that risk overrides the beneficial chance of finding something real before it has any symptoms.

But that scan would be limited to the head and not cover the entire body.

I'm also not convinced that MRIs are a great, general purpose diagnostic tool, if it's not used to diagnose concrete issues. The experience of your friend, who I hope recovered from the tumor not withstanding.

In the US, studies have been shown that physicians who own shares in CT (not so much any more, as it becomes almost 'commodity') and MRI (which is a lot, manufacturers approach physicians to form imaging cooperatives) are ordering imaging up at more than 2 standard deviations from expectations.

As noted, MRIs in themselves aren't harmful. But with prices in the four digits in the US... you can easily pay off your $1.5M MRI machine in 18 months and be printing money with your over-imaging.