Obese patients are getting too big to fit into some MRI scanning machines, NHS chiefs have said.
Cardiff and Vale University Health Board said it has had to abandon more than 200 scans since 2014 and has added a mobile scanner to tackle the problem.
Patients have been warned they could be putting their own lives at risk if they can't get scans to spot illnesses.
The British Dietetics Association says the size of scanning equipment is not growing in line with obesity rates.
And chief executive of the Society of Radiographers Richard Evans said growing waistlines was piling on pressure on the NHS.
"It causes inconvenience to waiting lists - particularly if it's not clear the patient will not fit in the machine until the day of the appointment itself," said Mr Evans.
"Then there is the additional problem that comes with rearranged appointments such as the added costs to the NHS when services are already under such pressure."
Some health boards have been forced to buy extra-large scanners or send patients to out-of-area hospitals for examinations.
The hole size of standard scanner measures 68cm (26in) and some hospitals put a 25-stone (158kg) limit on patients using them.
"The statistics are quite clearly showing increasing rates of obesity but services and equipment doesn't seem to be growing in line with that," said Sioned Quirke of the British Dietetics Association.
MRI scanners are used to detect a number of internal illnesses, injuries or conditions and are a vital tool for doctors wanting to diagnose ailments.
A spokeswoman for the Cardiff and Vale health board said: "We do our very best to ensure that all radiology patients have equity of service.
"Patients who require a conventional MRI scan should fall within certain parameters, the scanner table holds a maximum weight of 25 stone and the bore of a conventional MRI scanner is 68 inches."