Non-invasive probe

Doctors could non-invasively test the health of vital organs in critically ill patients using a technology that combines light and ultrasound.

Dr Terence Leung, of University College London’s Biomedical Optics Research Laboratory, is developing a probe to test levels of oxygen in the blood flowing through the veins. The probe would be placed on the body’s surface over a specific tissue region. It would then locate the vein draining blood from the tissue, vibrate the blood in the vessel with ultrasonic waves and shine diffused light on to it.

The probe’s optical detector would determine the wavelength of light absorbed by the blood, which in turn gives information about the colour of the blood.

‘Oxygenated blood looks bright red,’ said Leung. ‘blood with less oxygen is actually dark red.’

The technology works in a similar way to pulse oximeters, the clip-on devices that measure the oxygen saturation in a patient’s arteries, as opposed to Leung’s device that would measure oxygen levels in veins.

Arteries carry oxygenated blood from the heart to the body’s tissues. After the tissues have fed on the oxygen, veins carry the blood back to the heart.

While the oxygenation level in arteries is 98 to 100 per cent for most healthy people, the oxygen levels in veins can vary depending on which organ it has left and the health of the patient.

Leung said the brain, the most oxygen hungry organ, uses up to 30 to 35 per cent of the oxygen pumped into it. So an oxygenation reading of 65 to 70 per cent in the jugular vein, the vessel that drains blood from the brain, is normal.

‘If it is lower than that there may be some problem with the brain because it has to consume more oxygen — it may be infected,’ he said. ‘If the venous oxygen saturation in the brain is high — maybe 80 to 90 per cent — that could be even worse because the brain may be dead.’

Current methods for measuring oxygenation levels in veins are invasive and potentially dangerous. The process involves inserting a catheter into a vein near the neck to perform measurements directly on the blood.

‘Another method involves drilling a hole in the scalp and inserting a needle or catheter directly into the brain to measure the oxygen,’ said Leung. ‘It is the most accurate technique, but obviously very invasive.’

He added that these procedures carry risks such as infection and bleeding and are only carried out in patients deemed sick enough to justify the risk, such as those in intensive care.

Therefore many patients who could potentially benefit from having their venous oxygen saturation measured are precluded.

Leung said no-one has developed a non-invasive system like his before because researchers faced the challenge of finding a way to localise the light on the blood in the vein. Light, he said, has to travel through many layers of skin, fat and muscle and without his unique use of ultrasound it is difficult to target the blood in the vein.

The deeper the vein, the more difficult it is to target. Leung said pulmonary arteries — which carry deoxygenated blood from the heart to the lungs — are a case in point.

To target these, he suggests the use of microbubbles — 10 micron-sized gas filled bubbles that are injected into a vein. The bubbles would flow with the blood to the heart where they could be resonated with ultrasound and lit up.

‘Bubbles reflect light better and they could enhance the signal,’ he said.

Even though research is in its early days, Leung believes the technology will work because the acousto-optic principle was proven in the lab 15 years ago. ‘Researchers are now trying to develop it for mammography applications,’ he said.

Leung believes the non-invasive technique could have the same accuracy as pulse oximeters, which are accurate to within two per cent.

He is working with the Heart Hospital UCL Hospitals, Hamamatsu Photonics, and Homerton University Hospital on the five-year project.

For the first two years he will test the technology on material that has the same optical and acoustic properties as human tissue. He will then move on to clinical trials at some of the project partner hospitals, followed by full-scale commercialisation.

Siobhan Wagner