Acoustic characteristics of fatty and fibrotic liver measured by an 80-MHz and 250 MHz scanning acoustic microscopy

Ultrasound imaging is ideally suited for early-stage assessments of liver steatosis and fibrosis, but conventional ultrasound B-mode images do not display quantitative tissue information because conventional ultrasound formation does not incorporate modeling of the complex interactions between ultrasound and liver tissue in normal and diseased states. In this study, two normal, two fatty, and two fibrotic rat livers were harvested, fixed, and embedded in paraffin. For each specimen, a single, central, 10-μm thin section was placed on a microscope slide and scanned in two dimensions (2-D) using a modified scanning acoustic microscope (AMS-50SI; Honda Elec.) incorporating transducers operating at 80- and 250-MHz center frequencies. RF echo signals were digitized with 8-bit precision at a sampling frequency of 2-GHz. 2-D quantitative images of speed of sound (SOS) and attenuation were obtained (2.4 mm × 2.4 mm and 0.6 mm × 0.6 mm using the 80 and 250-MHz transducers, respectively) All images contained 300 × 300 pixels. The SOS and the attenuation values were calculated by averaging all values within 11 regions of interest (ROIs) from each 2-D image of all six livers. At 250-MHz, the SOS and attenuation values of normal, fatty, and fibrotic livers were 1622±32, 1591±20, 1700±44 m/s and 5.70±0.62, 8.38±0.51, 7.90±1.00 dB/mm respectively. The differences in SOS and attenuation values among liver types were greater at 250-MHz than at 80-MHz because of the improved spatial resolution, which allowed more-optimal placement of ROIs to contain only fatty or fibrotic tissue.