Finding cancers early allows for more effective treatment with the least side effects, so finding better ways to detect cancers is an important part of the fight. A new technique may help us “hear” where cancers are, allowing doctors to diagnose cancers more precisely than is currently possible. The technique, which is called photoacoustic tomography, takes pictures of sound waves that come from tissues when laser light is shined on the tissue. This is possible because different parts of the body absorb different amounts of light. When light is absorbed, it raises the temperature of the tissue, and the temperature change creates a sound wave which can travel to the skin. By recording the sounds at multiple locations, images of the tissues can be created.
3D photoacoustic imaging of melanoma3D photoacoustic imaging of melanoma

The method is more effective than just using light. When light passes through tissue, it is scattered, and the scattering blurs images from light reflected off tissue. The effect is dramatic, because sound waves pass through tissue with about 10,000 times less scattering, so the sound waves coming from tissues are much less blurry. This means that tissues can be imaged with good detail up to 10 centimeters deep -rather than a few millimeters using just light.
Photoacoustic tomography is very flexible, as almost all molecules absorb light at some wavelengths. By changing the wavelength of light used to excite the tissue, different molecules can be heated and detected. For instance, DNA and RNA absorb specific wavelengths of ultraviolet light, so by shining UV light on tissue, they can be imaged. This can be used to identify nuclei with abnormal chromosomes, a common defect seen in cancer cells. Hemoglobin is also easy to image, which can identify blood vessel formation around tumors. In fact, the flow of blood can be imaged. Likewise, the level of oxygen can be measured, which can indicate regions with hypoxia which are found at the center of tumors and regions with heightened metabolism, such a quickly growing tumors. Tissues that are hard to image can also be imaged by introducing a dye that changes the light absorption. For instance, nanoparticles designed to stick to cancer cells can increase the contrast of the cells and make them easier to detect.

The technique is now being used to image skin cancer, detect breast and prostate cancer, and to follow the response of tumors to treatments. As the technology gets better, hearing the “echoes” off of tumors may become one of the best ways to find and monitor them!

Photoacoustic Tomography: In Vivo Imaging from Organelles to Organs. Lihong V. Wang and Song Hu Science 23 March 2012: 1458-1462