Only recently developed for imaging of tissue, photoacoustic tomography now is racing headlong toward clinical implementation. And one of the first areas of deployment – the “perfect niche clinical application,” said Lihong Wang, a researcher at Washington University in St. Louis who is working to develop the technique for clinical purposes – likely will be mapping of the sentinel lymph node.Biopsy of the sentinel lymph node is currently the standard practice in patients with breast cancer. With this technique, doctors inject an optical dye and radioactive colloids into the tumor and, after the contrast agents are accumulated by the sentinel lymph node, use them to pinpoint it – first determining its approximate location by detecting the emission from the radioactive colloids and then identifying its precise location surgically by tracing the accumulated optical dye. Finally, they resect it for pathological examination.Because this practice has associated morbidity, researchers have sought a noninvasive means to find the sentinel lymph node. With such a method at their disposal, Wang said, doctors could use noninvasive techniques such as fine-needle biopsy for diagnosis – thus avoiding the need for surgery in breast cancer staging.Photoacoustic tomography offers just such an alternative. Using this technique, which takes advantage of a phenomenon in which the absorption of light results in the generation of sound, researchers can perform noninvasive in vivo ultrasound imaging of tissue based upon optical absorption contrasts within the tissue. Not only does the method provide the necessary penetration depth and spatial resolution, the optical dye that Wang and co-workers use for contrast enhancement – methylene blue – already is employed by doctors. By using a dye that has been used in clinical practice, the researchers hope to reduce the amount of time it will take to get the technique into the clinic.Another, related advantage: They are building the technique on top of conventional ultrasound imaging. “We think that, by adding a photoacoustic imaging capability to ultrasound, we can facilitate physician acceptance of this new modality,” Wang said.Recent research has shown that photoacoustic imaging can facilitate mapping of the sentinel lymph node and thus contribute to staging of breast cancer. Shown here are images of a sentinel lymph node in a small animal before (a) and after (b) injection of an optical dye. Reprinted with permission of the Journal of Biomedical Optics.The investigators reported a pilot study in the Sept. 15, 2008, issue of Journal of Biomedical Optics. Here they demonstrated the clinical feasibility of the technique by imaging 20- and 31-mm-deep sentinel lymph nodes in a rat model in two and three dimensions, respectively.The same month, they received funding from the National Cancer Institute in Bethesda, Md., to develop the technology for clinical application. They are one of four centers nationwide that belong to the Institute’s Network for Translational Research: Optical Imaging, which has been charged with developing new methods for cancer detection, diagnosis and treatment, and for monitoring response to therapy.During the five-year grant period, they plan to work with Philips Research USA – one of the three major manufacturers of ultrasound systems in the US – to design and build a photoacoustic tomography system. (This collaboration also will help to shorten the development process, Wang noted.) Once they have a prototype, they will test it in the lab to determine whether they can accurately identify the sentinel lymph node and then move it into Washington University Medical School to begin human studies.