Information obtained from a new application of photoacoustic tomography (PAT) is worth its weight in gold to breast cancer patients.
For the first time, Lihong Wang, Ph.D., Gene K. Beare Distinguished Professor in the Department of Biomedical Engineering, with a joint appointment in Radiology, and Younan Xia, Ph.D., James M. McKelvey Professor in Biomedical Engineering, with a joint appointment in chemistry in Arts & Sciences, both at Washington University in St. Louis, have used gold nanocages to map sentinel lymph nodes (SLN) in a rat noninvasively using PAT. Wang's lab is the largest PAT lab in the world credited with the invention of super-depth photoacoustic microscopy, and Xia's lab invented the gold nanocages.
Their work can minimize invasive surgical lymph node biopsy procedures to determine if breast cancer has metastasized and reduce the patient's exposure to radioactivity. The nanocages also have the potential to serve as an alternative to chemotherapy to kill targeted cancers by heating them up.
From heat, to expansion, to sound and an image
PAT blends optical and ultrasonic imaging to give high-resolution images of the body that contain information about physiology or tissue function. Molecules already present in the body (endogenous molecules, as opposed to exogenous ones that are from outside the body), such as melanin, hemoglobin, or lipids, can be used as endogenous contrast agents for imaging. When light is shone on the tissue, the contrast agent absorbs the light, converts it to heat, and expands. This expansion is detected as sound and decoded into an image.
"Using pure optical imaging, it is hard to look deep into tissues at high resolution because light scatters. The useful photons run out of juice within 1mm," Wang explained. "PAT improves tissue transparency by 2-3 orders of magnitude because sound scatters less than light. This allows us to see through the tissue by listening to the sound."
Exogenous contrast agents, like the gold nanocages developed by Xia's group, can be used to image parts of the body that even contain endogenous contrast agents. These nanocages are especially attractive because their properties can be tuned to give optimal contrast and gold is non-toxic.