Researchers found that the number of patients who had inconclusive test results had doubled over 15 years.

Dr Raul Uppot and his team analysed all the radiology examinations performed at a Massachusetts hospital between 1989 and 2003, looking for records labelled "limited by body habitus" - meaning the patient's size hampered the quality of the test.

They found that while 0.10 per cent of inconclusive exams were due to patient size in 1989, the figure had climbed to 0.19 per cent by 2003, despite advances in technology.

It is estimated that 66 per cent of adults in the US are overweight, obese or morbidly obese, while there are 12.5 million overweight children and adolescents.

The team said CT and MRI scans could cause problems for obese patients because there were weight limitations on the imaging table and the size of the opening on the imager could be too small for them to get in.

Ultrasound beams and X-rays can also be hindered by fat and fail to produce clear pictures of the organs being examined.

Standard CT tables in the US can take patients weighing up to 450lbs, while MRI machines can usually obtain diagnostic-quality images in patients weighing up to 350lbs.

The team, writing in the journal Radiology, warned that incomplete examinations because of obesity could have serious consequences, such as misdiagnosis or the failure to find a diagnosis at all.

Dr Uppot, a staff radiologist at MGH, said: "Americans need to know that obesity can hinder their medical care when they enter a hospital."

He added: "In the short term, the medical community must accommodate these patients by investing in technology to help them.

"In the long term, this country must make cultural shifts that promote more exercise and a healthier diet."