The cardiac MRI turned out to be, at least in my case, a three-hour operation. I had heart monitor probes stuck to my chest; on top of that, I was strapped to the table with a device that vaguely resembled a baseball umpire's chest protector. This, I was told, was a kind of RF transceiver that matched another of its kind on the underside of the table.
The MRI was done in two phases. In the first, much longer, phase, I was simply pushed into the tunnel, and then periodically, a very large number of times, instructed to hold my breath while an image was taken. I would say the longest interval I was required to hold was about 30 seconds. This went on for what seemed a very long time, and my legs and back got pretty stiff, before I was finally pulled out.
For the second phase, I was fitted with an IV, and some kind of contrast material was pumped in. Once this was done, I was pushed into the tunnel again for more breath-holding, but for a much shorter span of time.
And that's pretty much it. Things were slightly more complicated than usual due to my requirement for supplemental oxygen throughout, but the technical team handled this as being more or less routine for them, so it really didn't take any extra time.
So now all we can do is await the results, which should be available some time next week.