Label-free Detection of Lymph Node Metastases with US-guided Functional Photoacoustic Imaging

To determine the ability of ultrasonography (US)-guided spectroscopic photoacoustic (sPA) imaging to depict changes in blood oxygen saturation (SO2) in metastatic lymph nodes of a mouse model of oral cancer.

All studies were performed by following protocols approved by the institutional animal care and use committee at the University of Texas at Austin. Coregistered US and photoacoustic images were acquired spanning volumes containing a total of 31 lymph nodes in 17 female nu/nu mice. The mice were either healthy (three mice, five nodes) or bearing a primary tumor consisting of luciferase-labeled FaDu cells (14 mice, 26 nodes). Ten photoacoustic images acquired with optical wavelengths spanning from 680 to 860 nm were spectrally unmixed by using a linear least-squares method to obtain sPA images. After imaging, histologic analysis enabled confirmation of the presence of micrometastases. Generalized estimating equations were used to compare metastatic and normal lymph nodes, with a P value of .05 taken to indicate a significant difference. Sensitivity and specificity were determined with a receiver operator characteristic curve constructed from the background-subtracted SO2 values.

Metastatic lymph nodes (n = 7) exhibited a significantly (P = .018) lower spatially averaged background-subtracted SO2 (mean, 5.4% ± 3.5 [standard error]) when compared with lymph nodes without metastases (mean, 13.7% ± 1.3; n = 24). This effect was observed throughout the entire volume of the nodes rather than being limited to the metastatic foci. The change in SO2, which was inversely related to the size of the metastasis, was detectable in metastases as small as 2.6 × 10(-3) mm(3).

The results show that US-guided sPA imaging is capable of depicting changes in SO2 in lymph nodes that were correlated with metastatic invasion.