Most conventional optical imaging techniques are not suitable for imaging thick living tissue because light is strongly scattered by biological matter, which leads to poor spatial resolution. Photoacoustic imaging overcomes this problem by measuring the ultrasound produced by photons that are sent into a sample. The technique works because tissue heats up when it absorbs photons, causing it to expand. Pressure waves propagate out from the expanding structures and these can be detected by an ultrasound transducer.

Efficient though it is, the technique still requires contrast agents because most diseases do not show natural photoacoustic contrast. Although a number of such agents have been developed, most cannot target a diseased site in living tissue.

Excellent contrast agents
Now, Sanjiv Gambhir and colleagues have shown that single-walled carbon nanotubes conjugated with cyclic Arg-Gly-Asp (so-called RGD) peptides are excellent contrast agents for photoacoustic imaging of tumours – multiplying by eight the signal produced from tumours in mice.

Tumours need blood vessels to grow and these contain receptors (called integrins) that the RGD binds to, explains Gambhir. "Since the RGD is attached to the nanotubes, they end up 'seeking out' newly growing blood vessels in tumours," he told "This technique allows tumours to be detected early on."

The carbon nanotubes make good contrast agents because they absorb light well and then produce heat. As well as tumour detection, the technique might also come in useful for therapies that block blood supply to tumours and so could be used to monitor their spread.

The team is now looking at other photoacoustic contrast agents, such as gold nanoparticles, which may be more sensitive than nanotubes. "We are also targeting other processes in cancer as well as different diseases," revealed Gambhir.

The work was published in Nature Nanotechnology.