Hospitals track MRIs and other images ordered per doctor, and doctors are under pressure to hit their quota.
"Doctors Under Pressure to Meet Quotas and Fill Hospital Beds" [1].
"Once hospitals have made such a huge investment, experts like Dr. Zietman say, doctors will be under pressure to guide patients toward proton therapy when a less costly alternative might suffice." [2]
"While payments vary widely, doctors often collect $500 to $700 for a typical M.R.I. done on an outpatient basis in this area." [3]
[1] https://www.thehealthlawfirm.com/blog/posts/doctors-under-pr...
[2] https://www.nytimes.com/2007/12/26/business/26proton.html
[3] https://www.nytimes.com/2004/03/13/business/an-mri-machine-f...
Article 2 is addressing a cancer treatment, not diagnostic tests. And has one quote by a doctor concerned about oncologists being pressured to use proton therapy. No mention of quotas.
Article 3 is about the proliferation of advanced diagnostic imaging (MRI, CT, etc) in private clinics and the concerns about over utilization of these diagnostic tests as well as potential conflicts of interest with doctors having a financial stake in the tests they're ordering. These are both real problems in the US medical system. However, no mention of US hospitals placing quotas on diagnostic tests ordered by doctors on their staff.
Is it possible these types of quotas have occurred? Sure. But is this typical practice in the US? No.
Your experience is different from mine. Ordering unnecessary tests is typical practice in my experience. And doctors are quick to reject tests done at another hospital, as if tests done at another hospital are untrustworthy.
Any time a US hospital has expensive equipment, doctors working at the hospital are under pressure to keep it running. You can find any number of articles on this topic if you're willing to spend a few minutes googling.
Anyway, I will concede that my personal experience practicing medicine is not necessarily representative of the practice of medicine in all areas of the country.
edit: grammar