In Vivo Murine Lymph Node Imaging With Multispectral Photoacoustic Technology And Clinical Contrast Agents

Here we describe the use of a commercially available photoacoustic (PA) imaging system (Vevo LAZR, VisualSonics, Toronto) to detect methylene blue (MB) and indocyanine green (ICG) in the left and right iliac lymph nodes after injection into the synovial space of the left and right knee joints. The PA imaging system generated light from a tunable laser (680 – 970 nm) which was delivered through fiber optic bundles integrated into a linear array transducer (LZ‐250, fc = 21 MHz), driven by a linear stepper motor for 3D imaging. Ultrasound image‐guided injections of 25ul injections of MB (10 mg/ml) and ICG (5 mg/ml) were performed by inserting a 30 gauge needle inserted into the synovial space of the left and right knee, respectively. Two‐dimensional regions of interest surrounding the left and right iliac lymph nodes and the abdominal aorta provided dye and blood absorption spectra. Comparison of pre‐injection and post‐injection multiplexed images clearly demonstrated ICG and MB in the left and right iliac lymph nodes, respectively. The absorbance spectra of the dyes demonstrated characteristic results with clear differences apparent for MB (peak at 680nm), ICG (peak at 800nm) and typical blood spectra. Photoacoustic imaging thus can be used to identify sentinel lymph nodes. With dyes targeted to biomarkers for cancer cells, the method could also be used to identify metastases there and elsewhere. Photoacoustic imaging should also be useful in the study of lymphatic function in other disease processes such as lymphedema, arthritis, and parasitic infections. All research was conducted and funded by VisualSonics, Inc.