My impression (might be wrong) is that one can get this for some subset of blood tests but not say an MRI or x ray, let alone more complex tests. Are these just insurance company rules? If I found a way to make it profitable can I open a diagnostic lab independent of insurance companies?

Diagnostic labs are just for-profit companies. They’re happy to do tests as long as they get paid by somebody. The only practical restrictions are if you want to convince insurance to pay for it. If you’re paying cash, you can get anything from a simple blood test up to a full-body MRI scan [0].

I got a CT angiogram from a cardiologist here who started her own business specifically to do them. (It’s just her, a nurse, and a CT machine, in a trailer in a parking lot!) Insurance doesn’t pay for imaging like this until things have already gone very wrong, and she feels it should be used much earlier for people who have risk factors.

[0] https://prenuvo.com/

The preventative heart health program i linked above at Stanford does an echocardiogram calcium score rather than a CT scan for the first phase. Saves you from radiation unless they see something that warrants it.

It is no issue at all to ask for and receive an echocardiogram referral since it has no risk, just ask for a referral.

The unwillingness to do anything involving radiation unless warranted seems reasonable. Just do the echocardiogram and go from there.

You can have those tests done, it's just timing and cost.

For example: https://radiologyassist.com/MRI.html