Pavel Matousek, at the Rutherford Appleton Laboratory, UK, describes emerging spectroscopic techniques that promise to change cancer and bone disease diagnosis.

Over the past two years we have witnessed the emergence of chemically specific analytical tools that could revolutionise the way various diseases are diagnosed. The same tools could also have a profound impact on how pharmaceutical product quality is assessed and hidden drugs and explosives are detected. The methods are based on Raman spectroscopy - a powerful vibrational spectroscopy technique.

Raman spectroscopy provides detailed chemical information from the inelastic scattering of photons as they hit molecules in a sample. The scattered photons red shift from their original colour and the pattern of the red shifts can be used to identify the sample's chemical composition. Yet, until recently, Raman spectroscopy could be applied to only shallow depths in tissue; access to deeper layers has been prevented by their strong diffuse photon scattering.

Recently, several new methods have been developed that have allowed the penetration depth to be extended substantially. A technique holding a particular promise, because of its efficacy and simplicity, is spatially offset Raman spectroscopy (SORS). Unlike conventional methods, which rely on sharp image formation from the probed object to provide depth resolution, the SORS approach uses the diffuse component of light.

The concept is based on collecting Raman spectra from regions away from the illumination point on the sample surface. Each laterally-offset Raman spectrum contains different relative contributions from sample layers at different depths. This difference is brought about by a wider lateral diffusion of photons emerging from greater depths - these photons have to travel larger distances and on their way they diffuse sideways to a greater extent than the photons originating from shallower layers. For a two-layer system, a simple scaled subtraction of two spectra obtained at different spatial offsets can be used to produce pure Raman spectra of individual layers. If there are more than two layers, as is often the case with biological samples, then multivariate data analysis of a more extensive data set is needed.

Since its first demonstration, the SORS concept has been used in various areas, for example in biomedical applications. Several groups have shown that the technique has a potential for non-invasive diagnosis of a range of bone conditions, such as brittle bone disease or osteoporosis. In contrast with the conventional x-ray analysis, SORS enables both the mineral and protein components to be characterised, offering substantially richer information on the condition of the analysed bones.

Variants of SORS have also shown potential in the non-invasive detection and chemical composition analysis of calcifications buried deeply within breast tissue. As different types of calcifications apparently associate with different types of cancer lesions, benign and malignant, the techniques could potentially enhance the diagnostic potential of conventional mammography and help reduce the number of biopsies. Since reports suggest that 70-90% of needle biopsies uncover benign lesions, many of these invasive procedures could be avoided.

Examples of other applications include the quality control of pharmaceutical products, the non-invasive probing of counterfeit drugs through white plastic bottles and blister packs and the detection of powder explosives inside envelopes or plastic containers.

These exciting developments come at a time when Raman spectroscopy is completing its transformation from laboratory technique to practical analytical tool; a journey driven by recent advances in laser and detection technologies. Further developments of many of these new applications promise to exert a profound influence over our daily lives in the not too distant future.

Read Pavel Matousek's review 'Deep non-invasive Raman spectroscopy of living tissue and powders' in the August issue of Chemical Society Reviews.