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

Prof. Jeff Bamber,
Institute of Cancer Research and Royal Marsden Hospital
Sutton, London, UK

Popular version of paper 3aBA1
Presented Wednesday morning, May 15, 2019
177th ASA Meeting, Louisville, KY

Photoacoustic imaging is an exciting and relatively new way to externally scan the body, to make images of internal organs, tissues and disease. It uses both sound and light, combining the two forms of energy to take advantage of the best features of both. To make this method work, the body surface is illuminated with short pulses of light. Wherever the light is absorbed, which may be at depths of many centimetres inside the body, it causes a small and brief increase in temperature. This creates a pressure disturbance that travels back to the body surface as a sound wave which can be detected by a medical ultrasound scanner and be used to make an image. It brings to medical ultrasound the ability to show optical colour and thus some of the molecules that tissues contain, or an injected “dye” or nanoparticles. This can be useful in showing how much blood there is in tumours, whether the blood is well-oxygenated, what the vascular pattern looks like, the relative concentrations of melanin, fat and water, and the concentrations and distribution of nanoparticles and anticancer drugs. Potential uses of such information span all aspects of cancer medicine, including cancer detection, diagnosis, prognosis, treatment guidance, prediction of response to treatment and monitoring of response to treatment.

This paper reviews selected previous work in photoacoustic imaging conducted at the Institute of Cancer Research or with our collaborators, and considers directions for future research. In summary, for flexible introduction as an additional type of ultrasound scanning, we built a clinical photoacoustic system by adapting a commercial ultrasound scanner. Early clinical experience using this to scan breast tumours provided images that showed similar features of the tumours as those seen in contrast magnetic resonance images but also showed that photoacoustic image quality is limited by false detail created by sound waves emitted when light is absorbed at the skin surface, which can be reflected back to the surface from deeper tissues. Ideas for reducing the impact of this false detail, which we call clutter, were therefore explored. These ideas have included computer recognition of features of the clutter so that it can be suppressed in the images, the use of injected materials called contrast agents which increase the desired signal so that the clutter is relatively less important, and deliberately moving or vibrating the tissue, which allows clutter to be distinguished from real detail because the two types of detail move in a predictably different way. The last of these approaches was particularly successful, with a technique known as localised vibration tagging (LOVIT). For example, it tripled penetration depth in breast-mimicking phantoms. Most of the clutter reduction methods are complementary in nature, and a combined approach therefore represents a worthwhile direction for the future work.

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