Metals absorb light using plasmon resonance (the collective motion of conduction electrons) and in metal nanoparticles this effect leads to very efficient light absorption. For medical imaging applications, the resonance wavelength can be tuned to within the tissue-transparent window of 700–1000 nm. This allows the light to penetrate more deeply into the tissue for better imaging.

In the new technique, Ashkenazi and colleagues employed elongated gold nanorods that are 15 nm in diameter and 45 nm long. These rods have plasmon resonance in the near-infrared region of around 750 nm. The researchers conjugated the nanomaterials with antibody ligands specific to the HER2 receptor, which is overexpressed in certain prostate and breast cancers.

The Michigan team verified the binding efficiency of the conjugated particle to prostate cells (in the LNCaP cell line) using a home-built photoacoustic microscope that had a resolution of 50 µm. The photoacoustic technique converts light into sound and works by using laser light to illuminate tissue, which instantly heats up, causing it to expand rapidly. Sound pressure waves propagate out from the expanding structures, which are then detected by an ultrasound transducer at high resolution.

The researchers found that the conjugated particles bound strongly to prostate cells compared with non-conjugated ones. They also tested how photoacoustic images formed on a tissue-mimicking phantom and showed that sub-millimetre resolution at a tissue depth of 2 cm is possible.

To take the technique a step further, the scientists then imaged the nanoparticles in a human cadaver prostate using the photoacoustic clichés overlaid on images obtained from conventional ultrasound techniques. Combining images from these two methods means that both the anatomical details of the tissue and the distribution of nanoparticles can be obtained (see figure). This approach could allow for better and earlier screening of prostate cancer, says Ashkenazi.

"In a real medical setting, gold nanoparticles would be injected into the blood, where they would then accumulate in cancer tissue," he told "Combined ultrasound and photoacoustic imaging gives the best and most complete information – ultrasound provides structural images and photoacoustics provide nanoparticle distribution, reflecting cell expression as targeted by the specific ligand employed."

But the story does not end there. By using different antibody ligands, different cell expressions could be tested for better and more accurate disease characterisation, says Ashkenazi. "Compared to biopsy where only limited and randomly selected samples are examined, our method would provide full analysis of the complete organs."

What is more, the same nanoparticles could even be used in photothermal therapy to kill cancer cells, he continues. "The highly selective plasmon resonance means that heat is only absorbed by cancer tissue, leaving the healthy surrounding tissue intact."

The researchers will now study nanoparticle cancer targeting in small animals using their technique. They also plan to develop real-time ultrasound/photoacoustic imaging for human diagnostics.

The work was published in J. Appl. Phys.