In vivo carbon nanotube-enhanced non-invasive photoacoustic mapping of the sentinel lymph node

Sentinel lymph node biopsy (SLNB), a less invasive alternative to axillary lymph node dissection (ALND), has become the standard of care for patients with clinically node-negative breast cancer. In SLNB, lymphatic mapping with radio-labeled sulfur colloid and/or blue dye helps identify the sentinel lymph node (SLN), which is most likely to contain metastatic breast cancer. Even though SLNB, using both methylene blue and radioactive tracers, has a high identification rate, it still relies on an invasive surgical procedure, with associated morbidity. In this study, we have demonstrated a non-invasive single-walled carbon nanotube (SWNT)-enhanced photoacoustic (PA) identification of SLN in a rat model. We have successfully imaged the SLN in vivo by PA imaging (793 nm laser source, 5 MHz ultrasonic detector) with high contrast-to-noise ratio (=89) and good resolution (~500 µm). The SWNTs also show a wideband optical absorption, generating PA signals over an excitation wavelength range of 740–820 nm. Thus, by varying the incident light wavelength to the near infrared region, where biological tissues (hemoglobin, tissue pigments, lipids and water) show low light absorption, the imaging depth is maximized. In the future, functionalization of the SWNTs with targeting groups should allow the molecular imaging of breast cancer.