Philips sell MRI scanners. I used to do angiograms with them. My workflow was to start a cheap and cheerful scan that showed the location of the injected contrast. As it got close to the vessel of interest I’d click and hold the button that started the high resolution angio run. When it was at the vessel of interest I’d lift my finger off the button and the scan would start.

This all happened over about 10-20 seconds after the injection. Blood moves quickly. Start the scan too early and you miss everything, too late and you get veins rather than arteries.

In a point release update with no mention in the release notes they changed the behaviour of the scanner - it then started the scan on clicking down in the button, not on lifting off.

A time critical scan on a patient due for theatre was messed up by me. Their renal function was poor and the scan couldn’t be repeated for a few days.