I had a PET-CT scan last fall. It was prescribed by a hematologist/oncologist as part of a workup for neutropenia and neuropathy to rule out an underlying malignancy. A neurologist involved in evaluating me said that PET-CT scans usually need to be prescribed by hematologists/oncologists in the US, as they know how to get insurers to authorize them here. The results were negative/normal.
Maybe 20 years ago, I also had an fMRI by a PhD radiology researcher at Columbia University named Dikamo Shungu, who was evaluating ME/CFS patients for abnormal levels of lactate in the brain. It was like a regular brain MRI.
@Pyrrhus is correct - fMRI is used in research settings and isn’t available in medical settings. The reason I know this is that I got in touch with Shungu a few years ago and he said I could only get an fMRI if I were a participant in one of his research studies, and even then he could not provide me with my results because of the need for blinding.