Backscatter Quantification for the Detection of Metastatic Regions in Human Lymph Nodes

Quantitative ultrasound (QUS) methods using high-frequency ultrasound offer a means of investigating biological tissue at the microscopic level. This chapter describes high-frequency, three-dimensional (3D) QUS methods to characterize freshly dissected lymph nodes of cancer patients. 3D ultrasound radio-frequency data were acquired from lymph nodes using a 25.6-MHz center-frequency transducer. Each node was inked prior to tissue fixation to recover orientation after sectioning for 3D histological evaluation. Backscattered echo signals were processed using 3D cylindrical regions-of-interest to yield four QUS estimates associated with tissue microstructure (i.e., effective scatterer size, acoustic concentration, spectral intercept, and spectral slope). QUS estimates were computed following established methods using two scattering models. Then, the four QUS estimates were combined using linear-discriminant analysis to increase classification performance. Finally, the discriminant scores were used to compute a posteriori cancer probability. In this study, more than 400 lymph nodes acquired from more than 250 patients diagnosed with colon, breast, or gastric cancer were processed. Results indicated that metastatic and cancer-free lymph nodes of colon- and gastric-cancer patients could be well classified using these methods and that metastatic regions could potentially be detected and used to guide pathologists towards suspicious regions.