Photoacoustic imaging in cancer medicine and research: Systems, results and future directions

Abstract

This paper reviews selected previous work in photoacoustic imaging conducted at the Institute of Cancer Research or with collaborators and considers directions for future research. For flexible introduction as an additional mode to clinical freehand ultrasound scanning, a multispectral epiphotoacoustic system was built by adapting a Zonare z.oneUltra™. Early clinical findings from photoacoustic imaging of breast tumours were consistent with contrast MRI although also provided evidence that photoacoustic image interpretation and penetration were limited by clutter generated by photoacoustic emissions from regions at or near the skin surface. Schemes for improving photoacoustic signal-to-clutter ratio were therefore explored. These have included deformation compensated averaging, spatial coherence processing, photoacoustic contrast agents, and localised vibration tagging (LOVIT). LOVIT (implemented using acoustic radiation force) was particularly successful, tripling penetration depth in breast-mimicking phantoms. It has also been implemented on a Verasonics™ platform and extended using a 3 + 3 interleaved comb-push method, which further reduces the clutter and improves the frame rate. Multidimensional and other extensions to LOVIT remain possible. Meanwhile, work at other institutions has explored various propagation model-based schemes. Most of the clutter reduction methods are complementary in nature, and a combined approach therefore represents a worthwhile direction for the future work.