Image formation methods in quantitative acoustic microscopy

Jonathan Mamou, Daniel Rohrbach

Abstract: Quantitative acoustic microscopy (QAM) is an imaging modality which uses very-high-frequency ultrasound (i.e., >200 MHz) to form two-dimensional (2D) quantitative images of acoustical and mechanical properties of soft tissues with microscopic resolution (i.e., better than 8 μm). The key component of a QAM system is the ultrasound transducer which must be broadband, have a very small F-number (i.e., <; 1.2), and good sensitivity. In this study, two QAM systems based on a 250-MHz and a 500-MHz transducer are presented, yielding 2D quantitative images at spatial resolution of 7 μm and 4 μm respectively. Thin tissue sections obtained using a microtome or cryotome are raster scanned with precise motors and pulse-echo RF signals are digitized. Inverse models are then used to process each RF signal individually to estimate acoustic impedance, speed of sound, and acoustic attenuation as well as derived parameters such as bulk modulus, mass density, and compressibility. To illustrate the QAM technology and signal processing algorithms, images from cancerous human lymph nodes and ophthalmologic samples are presented and coregistered with histology photomicrographs.