I walked into an urgent care because I had severe back pain. They put me through a bunch of chest x-rays where they told me to inhale and hold my breath, then they bring me over to the CAT scanner and tell me to hold my breath when the light goes green. So I inhale and hold my breath. Then the tech comes out and tells me "did you inhale? don't do that, it ruins the results".
They decided not to re-run them because the odds I had something wrong even with those symptoms were apparently less than the odds of the additional CAT scan giving me cancer down the line. Certainly didn't help my peace of mind.
But the muscle relaxants and a proper desk chair seems to clear it up within a week so I guess they were right...
"Fortunately" they only have to scan my head, so no issues arise from (wrong) breathing.
I also have to do MRI every year and it's much more frustrating for me because they always complain I'm breathing wrong, and I have asthma so it's not easy to "breath right". It takes ages.