Lasers look inside human bones

20 Jul 2009

Spatially offset Raman spectroscopy could help diagnose bone disease.

Scientists in the UK are set to carry out hospital tests on a new laser technique that could ultimately lead to rapid and reliable detection of bone disorders such as brittle bone disease. The method includes a novel version of Raman spectroscopy – routinely used by physicists and chemists – and could give medics the most detailed characterisation yet of bones inside the human body, according to researchers based at the Central Laser Facility in the UK.

Bare bones

The strength of bones comes from two main components: the mineral content, such as calcium and crystalline mineral salts; and a protein-based substance known as collagen. There are various medical conditions linked with these substances including brittle bone disease where defective bones result from a deficiency of "type-I" collagen.

To detect these diseases and to monitor a patient's response to treatment, doctors require non-invasive ways of seeing inside bones in the body. At present the two favoured approaches are X-ray spectroscopy and ultrasound, but both of these techniques struggle to see the collagen. As a result many cases of bone disease can go undetected.

Elusive proteins

In recent years, a different approach has been suggested that could detect both mineral content and collagen. Raman spectroscopy relies on inelastic scattering of light as it interacts with matter. In the classic experimental set-up a laser is shone on a sample, which then scatters some photons at a slightly lower frequency. Measuring this change in frequency can determine the identity of the chemical under study.

Already, medical physicists have successfully applied Raman spectroscopy to tissues extracted from the body, but a problem arises when you attempt to observe bones that are still in vivo. Raman signals from surface layers tend to dominate those signals from under the skin of a patient, which are weakened significantly as they scatter sideways. These bone signals are then drowned out even further by the strong fluorescence in the melanin content of skin.

To overcome this problem, Pavel Matousek at the Central Laser Facility and his colleagues have bedecked Raman spectroscopy with an innovative new geometry. The scientists have reduced the dominance of the skins' signal by creating a distance between the illumination and collection points of the Raman process. They also employ a ringed detector to ensure that all signals are captured effectively (Analyst 134 1058).

Souped-up spectroscopy

This technique, known as spatially offset Raman spectroscopy (SORS), has already been used by an independent group in the US to determine the ratio of phosphate to carbonate – a potential indicator of osteoporosis – in a chicken's shin bone. Matousek and his team now plan to take the next step by testing out SORS on human subjects at the Royal National Orthopaedic Hospital (RNOH) in Middlesex, UK.

Michael Morris, a spectroscopy researcher at the University of Michigan is optimistic about the new development. "Matousek is well-known for his excellent pioneering work in developing spatially offset Raman spectroscopy." However, Morris also anticipates one of the main challenges ahead. "Confirmation by independent investigators is needed before any new finding can be accepted by clinicians."

Matousek and his team will initially use their technique to test for brittle bone disease but they hope this will lead on to the investigation of other bone conditions such as osteoporosis, which presently goes undetected in three to four out of ten cases. "This project has the potential to provide a means of detection and confirmation of some of the rare bone disorders that are still difficult to diagnose," said Richard Keen, a consultant rheumatologist at RNOH.