A UK expert has reassured parents after American research suggested children undergoing CT scans may be receiving more radiation than is necessary

The report suggests that many radiologists could reduce the dose and still produce a useable image.

The x-ray scans, which are often used to diagnose cancer, are more frequently used in the US than in the UK, where there are fewer scanners.

There is also European legislation which limits the amount of radiation to which children undergoing scans can receive.

However, Professor Adrian Dixon, a consultant radiologist at Addenbrookes Hospital near Cambridge, and the University of Cambridge, said the older equipment still used in many parts of the NHS meant that many patients were receiving higher doses.

He said that a "league table" of doses compiled for the NHS showed wide variations in the dosage given by different hospitals.

The research was completed by doctors at the Children's Hospital Medical Center of Cincinnati, and published in the American Journal of Roentgenology.

Cross section

CT scanning accounted for about one in 25 medical x-ray examinations - but contributes to an estimated 40% of the total radiation dose to the population.

CT, which stands for computed tomography, builds up a more revealing picture than a simple x-ray by taking a series of cross-section pictures.

The team found that trained radiologists could actually reduce the dose given to a child by five times and still display an image good enough to diagnose tumours and other medical problems.

Dr Kathleen Emery, a radiologist who participated in the study, said: "It began to bother us that we used parameters based on adults studies.

"Unlike general radiography, there is no penalty for using a higher dose of radiation, whereas in regular x-rays, too high a dose would result in a black picture.

"Efforts must be made to override these automatic parameters when imaging children."

Professor Dixon told BBC News Online: "The dose received does vary.

"But increasingly, radiologists are trying to use MRI scans on children in preference to CT scans wherever possible.

"We wouldn't recommend giving a CT scan unless it's absolutely necessary."