Big-bellied Britons are causing problems for doctors who have found that patients' excess fat makes it impossible for them to be probed properly with medical scanners.

As a result, researchers have had to exploit powerful new sonar techniques - used to pinpoint underwater mines - to create machines that can peer deeper inside the nation's fatties and spot damaged organs.

'Current ultrasound scanners which take images of kidneys, livers and other organs can work effectively up to a depth of six inches,' said Ross Gooding of QinetiQ, formerly the government's Defence Evaluation and Research Agency.

'However, for really fat people - of whom there are increasing numbers nowadays - this is not much use. They have too much fat surrounding them. That is why we have developed our system. It will double the depth to which we can peer inside someone - to about a foot, a crucial improvement.'

Health experts say that increased hours of TV watching, a decline in numbers walking to work or school, and decreases in consumption of fresh fruit and vegetables mean that more than one in five Britons are clinically obese, a figure that is expected to reach one in four by the end of the decade.

Apart from the strain put on a person's heart and the increased risk of triggering diabetes, this jump in obesity causes several headaches for the medical profession. For example, high levels of fat around patients' necks pose a danger during operations because this layer increases pressure on the windpipe while the patient is anaesthetised, leading to risks of choking and asphyxiation.

However, it is the problem of ultrasound use that causes most concern. 'We had various discussions with doctors and realised there was a real need to improve scanning depth,' added Gooding. Scientists at QinetiQ had recently made improvements in the performance of naval mine detectors using a technique called wind-band sonar. Mine detectors work by transmitting a pulse of low frequency sound and analysing the resulting echoes. Ultrasound scanners exploit the same principle, though they use high frequency pulses.

However, there is a limit to the accuracy of sound scanning due to the scattering of returning echoes and other effects. Scientists at QinetiQ have found techniques to improve this image resolution, using computer software, and have transferred these from their advanced sonar equipment to medical scanners. A prototype has been constructed and the company is in discussions with manufacturers in a bid to begin commercial production. 'We hope to be ready for production in a year,' added Gooding.

· Additional research by Neesh Asghar