Fine-resolution maps of acoustic properties at 250 MHz of fresh samples and unstained fixed 12-µm thin sections from cancerous human lymph nodes

Scanning acoustic microscopy (SAM) at 250 MHz permits measuring the acoustic properties of tissue microstructure with a spatial resolution of 7 μm. However, only limited data exist for sentinel lymph nodes at these scales. 250-MHz SAM was performed on four fixed and one fresh lymphnode samples. 12-μm sections of deparaffinized, fixed tissue and fresh half nodes were scanned using a custom-built acoustic microscope. 2D SAM maps of speed of sound (c), attenuation (a), and acoustic impedance (Z) were generated using custom signal processing algorithms. Scanned samples then were stained using hematoxylin and eosin and imaged by light microscopy. The spatial resolution and contrast of SAM maps were sufficient to distinguish among tissue regions consisting of lymphocytes, fat cells, and fibrous tissue. Average properties for lymphocyte-rich tissue were c = 1575 ± 57 m/s, a = 8.27 ± 2.45 dB/MHz/cm, and Z = 1.59 ± 0.14 Mrayl. We found a significant increase (p<;0.05, ANOVA) of 69.0 ± 0.5 m/s for c and 1.2 ± 0.1 dB/MHz/cm for a in fibrous connective tissue compared to lymphocyte-rich tissue. The study demonstrates that fine-resolution maps of acoustic properties of lymph nodes can be generated at 250 MHz. The data will serve as a basis for developing new lymph-node-specific ultrasound-scattering models to improve current quantitative ultrasound approaches for detecting metastatic regions in freshly-excised sentinel lymph nodes from breast-cancer patients.