New ultra-sound technology is being developed
Patients above a certain weight have had appointments for MRI or CT scans cancelled because of fears they are too big to fit in the scanner.
MRI scanners have a weight limit of 24 stone (152kg), and doctors at the Royal Cornwall Hospital in Truro are worried people may get stuck inside.
Instead, overweight patients are sent as far away as Cheltenham for scans.
New technology is being developed but a health adviser says "just building everything bigger" is not the answer.
Zanita Jones had a scan cancelled several years ago because she weighed 34 stone (216kg).
She said: "They found there wasn't a machine in the country they know of that takes over 25 stone in weight."
She now weighs 18 stone (114kg) but has still not had her scan.
"I am just at the weight, but because of where I hold my weight I am not quite sure, because it also goes on the circumference of you.
"They would have to measure the size of the hole [in the scanner] and the size of my stomach, and I haven't been brave enough to ask that question."
Dr Amanda Liddicoat, the hospital's director of clinical imaging, said: "We simply can't put patients on above a certain weight limit.
"The scanner will malfunction and not move.
"If we go close to the limit the risk is the patient might go into the scanner, but the table will then jam and perhaps we can't get them out."
She admitted that the hospital would have to invest in equipment that will be "able to cope" with larger patients.
Currently the size of a patient can also affect the clarity of the scan, but new systems are being designed to rectify that.
Peter Sutton works for Philips Healthcare, which makes scanners. He said: "We've developed new technology in ultra-sound which will actually allow doctors to see through the extra fat to get a better image and therefore a better diagnosis."
Lindley Owen, a public health consultant for the Cornwall and Isles of Scilly Primary Care Trust, thinks there needs to be a combination of approaches to deal with the issue of overweight patients and scanners.
"Simply [encouraging people to lose weight] is not the answer," she said.
"On the other hand, neither is building everything bigger and stronger."