NewfieGal,
I totally agree with CastingPearls on this one - call ahead, in fact make a pest out of yourself doing it if necessary. I have been in the same situation as you with hospital test equipment and have learned a few things that help.
One is to get a list going of the equipment that will take fat bodies. This is a sliding scale too. Some devices have a limit of 500 lbs. Some devices have a limit of 750 lbs, etc. Your best bet is to find a hospital that does WLS (weight loss surgery) 'cause they often have machines that can handle larger bodies. I usually call the hospital's imaging office directly myself and verify the info with the supervisor of the dept that runs the test I am to have. The only draw back is some of the more open machines do not get as good of resolution (picture quality) as the smaller machines. However, the newer MRIs have overcome most of these issues.
Second, if you're getting a MRI, ask for an OPEN MRI machine. It is like a table instead of being like a tunnel. You can also ask for a sedative before you go inside, but your GP or doc that orders the test should give you this before you go. This makes the test a little more fun - more like a ride at Disneyland than being buried alive. You just have to bring a ride home. Most of the MRI folks will let you bring a recorded CD or two of music and play it for you, to help you stay calm too. Don't drink too much before you go inside the machine tho, you may have to be in there for an hour or two before you can pee.
Third, keep a cool head about you if things go south. I have a brain disease that requires MRIs every couple of years. So, I have had the distinct pleasure of having at least a half dozen or more MRIs. I have also had three major surgeries and a couple of accidents that required other scanning equipment. If the machine doesn't look right (too small, too high, the technician or attendant has a crappy attitude) I will and have turned and walked out. I am not getting my fat ass in something that looks like I will need the fire dept to pull me out of. I try and be courteous to the staff, and treat them with respect. However, I am likely to morph into Hogzilla if I get pushed too far - something that happens if my temper gets rilled up.
Don't forget you are the customer in this deal and you deserve a high quality of customer care. One MRI test may cost thousands of dollars or more depending on the fancy frills. Sometimes they shot dyes inside of you to watch how things move. Your insurance or your health care agent is paying for these tests - so you need to get your money's worth and value from the information that the scans are supposed to give. You also deserve and should demand a high quality of care and service from the hospital staff.