A scanner that measures particles of light as they travel through the head is helping doctors to assess whether very premature babies are at risk of brain damage.

The £1m machine, which uses a completely new form of optical imaging, has been compared to the early breakthroughs made in ultrasound scanning, which transformed doctors' ability to see disease in the body's soft tissues and a baby's development in the womb.

By sending beams of infrared light through the head of the infant in their first few days of life, the scanner uses a completely new way of creating three-dimensional images of the whole brain.

Blood absorbs light, so by measuring particles of light as they shoot through the brain doctors can see how much is absorbed and assess whether or not the baby has suffered any kind of haemorrhage. Researchers have been working with doctors at the intensive care unit of University College London Hospitals to carry out the first scans of very premature infants in order to see if the baby has suffered an injury after birth, when there is a danger that blood vessels might leak into the undeveloped brain.

There are currently two ways of performing brain scans on babies. Magnetic resonance imaging (MRI) can show brain function, but the babies often have to be sedated before being taken off to the scanner, which carries its own risks. The other technique, ultrasound, can reveal the anatomy of the brain, but does not show how it is functioning in terms of its oxygen supply or blood flow.

The new technique, known as optical tomography, gives doctors 3D images of how much oxygen is in the blood that is getting to the brain. For the first time they can also see much deeper into the 'white matter', which contains the more primitive functions of the brain, than is possible with conventional techniques which simply show the grey matter closer to the skull.

The babies are fitted with a tiny, foam-filled helmet out of which protrude dozens of different wires containing the optical fibres that are connected to a laser. The helmet, which incorporates 32 light detectors and 32 kinds of low-intensity light, sends short flashes of infrared light across the brain. The flashes are of much lower intensity than light on an average sunny day in Britain, so no harm can be done.

The machine has been nicknamed Monstir, a catchier version of its full title - 'Multi-channel Opto-electronic Near-infrared System for Time-resolved Image Reconstruction'. The time that it takes the light to arrive - and the amount of light absorbed - is measured in nanoseconds (billionths of a second). The whole scan can be performed in eight minutes, and often the baby sleeps through the procedure.

Dr Topun Austin, a clinical research fellow in neonatology at UCL, said: 'Babies born very early, for example at 24 weeks, have parts of the brain which are not fully formed and are very fragile. Any changes in their blood pressure can cause the vessels to bleed and that can cause disability. What this technique allows us to do is see at an early stage if there are deeper parts of the brain which are becoming deoxygenated. Currently it takes us several weeks before we can see if injuries have occurred, which means it is too late to do much about it.'

Dr Adam Gibson, medical physicist and research fellow at UCL, explained: 'Currently there is no alternative method which can be used safely and continuously on babies in intensive care.

'We are at the early stages of seeing what it can do, but the trials have proved that it does work and that we can see the activity of the young brain as never before.'

Funded by grants from the Engineering and Physical Sciences Research Council (EPSRC) and the Wellcome Trust, Monstir currently lives up to its name by being the size of a fridge-freezer, but Gibson hopes it will be possible to reduce the size and improve the speed of the operation. 'This technology has the potential to produce high-quality images by the side of the baby's cot, and perhaps it will be commercially available within the next few years.'