I had a nuclear stress test a couple of years ago. I presume that's what you will have. It's not a worrying thing to go through and you don't go through any enclosed scanning equipment.
You have an injection of radioactive material into a cannula in a vein in your hand, then you're asked to lie down on a bed where a camera comes over your chest area and does its thing for, if I remember correctly, about 45 minutes or so. You have to stay still and keep your arms behind your head. I remember hearing my heartbeat in the background too. I believe there was then a break where I had a cup of tea.
I then had to go into another room and I was wired up to an ecg and blood pressure monitor. I then had to cycle on an exercise bike until I couldn't manage any more, all the while being monitored. You're given another injection into the cannula in your hand, after which you lie down on the bed again and the camera does its thing again.
After the procedure is finished the cannula is removed and you're free to go home. I was given a leaflet which advised me not to have contact with babies, small children and, if I remember correctly, pregnant women. Also, if you're flying within a week of the procedure you should have a letter stating you've had this procedure done.
In all honesty, it's absolutely nothing to worry about. The staff were marvellous and explained what was going to happen and put me at ease. The only difficult part of the procedure, I found, was trying to lie still for so long. It was uncomfortable due to my osteoarthritis.
I hope this helps!