Light and sound illuminates cancer

According to a recent survey from Cancer Research UK1 the threat of cancer is one of our greatest fears, with one in five of us in the UK more frightened of the “big C” of than of debt, knife crime, Alzheimer’s disease or losing our job. But this dread defies the reality – cancer is far from the death sentence it used to be. Thanks to better diagnosis, screening programmes, continually improving treatments and on-going research, long-term survival has doubled since the 1970s. Adding to the array of new diagnostics under research, is a pioneering medical imaging instrument developed by scientists from University College London’s Medical Physics and Bioengineering Department and Cancer Institute. Winning the 2010 IPEM Robert’s Prize for their work to date,2 the researchers have developed an innovative approach to the implementation of the photoacoustic effect – a phenomenon based on laser generated ultrasound that provides high-resolution structural and functional images of soft tissue. A relatively new clinical diagnostic tool, photoacoustic imaging (PA) has implications for the non-invasive study and treatment of certain cancers, in particular skin tumours such as malignant melanomas, and breast cancer. PA exploits the advantages of both optical and acoustic imaging techniques while avoiding their limitations. Despite its inherent promise and wide range of potential applications there are, however, significant challenges to its practical implementation. The unique optical sensing technology at the heart of the UCL instrument has been developed to overcome these.

Photoacoustic imaging

Conventional medical ultrasound produces reflections from different anatomical features by firing pulses of ultrasound into the tissues of interest. The image is produced by measuring the time these reflections take to rise to the surface, rather like radar. Indeed, Professor Paul Beard, from UCL’s Department of Medical Physics and head of the College’s photoacoustic imaging group, describes PA as “similar in a sense”. “The basis of PA is that the ultrasound we use to image tissue is first generated with light,” he said. Short pulses of laser light are fired onto tissue using a wavelength or colour of light in the near infra-red part of the spectrum that can penetrate as much as several centimetres. Absorption of the light produces a miniscule rise in temperature of around 0.1°C, which via rapid thermoelastic expansion, produces acoustic waves. Because haemoglobin is such a strong absorber of light, these sound waves are largely emitted from the blood vessels. An array of detectors, “like an array of microphones” but incorporating very high frequency ultrasound transducers, detect the time taken from firing of the laser to the arrival of the acoustic pulses at the surface, in a similar process to normal medical ultrasound. However, because the sound waves in the tissue are generated using light, rather than producing “reflections”, PA produces images of a completely different physical property. Conventional ultrasound images depend on the mechanical properties of different tissue features, such as density, elasticity and so on, so that a small blood vessel, for example, surrounded by connective tissue or fat, which have mechanical properties similar to those of the vessel wall will not produce a clear image, or even be seen at all. PA images instead depend on the optical absorption of the tissue rather than its mechanical or elastic properties. “This is why PA is so good at imaging blood,” says Prof. Beard. “Blood absorbs light to produce a relatively large signal to produce much higher contrast images of small blood vessels than are achievable with conventional ultrasound. We can use PA to create three-dimensional images of blood vessel networks.”