500-MHz quantitative acoustic microscopy imaging of unstained fixed 6-µm thin sections from cancerous human lymph nodes

Quantitative acoustic microscopy (QAM) at 500 MHz permits measuring acoustic properties, such as speed of sound (SOS), attenuation (A) and acoustic impedance (Z), of tissue microstructure with a spatial resolution of 4 µm. Although high-frequency QAM has been shown to be a suitable tool for measuring the acoustic properties of several soft tissues, very few studies have been performed at 500 MHz, and none in cancerous lymph nodes (LNs). However, data at such fine resolutions are essential to improve our understanding of ultrasound scattering at lower frequencies (25 MHz) from LNs to detect clinically significant micrometastases. In this study, quantitative acoustic microscopy (QAM) at 500 MHz was performed to obtain 2D maps of speed of sound (c), attenuation (A), acoustic impedance (Z), and other derived acoustical parameters of nodal tissue microstructure with a spatial resolution of 4 µm. Thin section (i.e., 6-µm) of lymph nodes were scanned using a custom-built QAM system based on a novel F-1.08, 500-MHz transducer. The system digitized radio-frequency (RF) signals at 2.5 GHz with 12-bit accuracy.2D QAM maps of the acoustic parameters were obtained using custom a signal-processing algorithm. Following QAM scanning, the samples were stained using hematoxylin and eosin and imaged by light microscopy. The study illustrates that fine-resolution maps of acoustic properties of lymph nodes can be obtained and can provide previously unavailable information. Future studies will investigate the use of 2DZMs to improve the model of ultrasound scattering at 26 MHz. The new lymphnode-specific, ultrasound-scattering models could improve sensitivity and specificity of current QUS approaches for detecting metastatic regions in freshly excised lymph nodes from cancer patients.