Quantitative ultrasound parameters in ex-vivo fibrotic rabbit livers: Liver stiffness and tissue microstructure estimation

Liver fibrosis is the common result of chronic liver disease. Diagnosis and grading liver fibrosis for patient management is mainly based on blood tests and hepatic puncture-biopsy, which is particularly invasive. Quantitative UltraSound (QUS) techniques provide insight into tissue microstructure and are based on the frequency-based analysis of the signals from biological tissues. This study aims to explore the diagnostic performance of QUS technique in high frequency (10-40 MHz) for detection of liver fibrosis. The changes in QUS parameters of healthy and fibrotic livers were investigated and were compared to the changes in liver stiffness using shear wave elastography (SWE). Liver fibrosis was induced by subcutaneous injection of CCI4 in the neck of rabbits. Attenuation and backscatter coefficients (BSCs) were estimated using the spectral difference method and the reference phantom method, respectively. Effective scatterer diameter (ESD) and effective acoustic concentration (EAC) were estimated by fitting the measured BSC to an estimated BSC calculated with a spherical Gaussian model. Finally, histopathological analysis of rabbit livers was performed using picrosirius red staining in order to rank the livers according to the Ishak fibrosis staging scale. Rabbit livers were classified on grade 0 (N=3), 2 (N=1), 3 (N=2), 4 (N=3) and 5 (N=2) using histological analysis. Mean attenuation at 40 MHz was higher in fibrotic livers: 0.83 dB/cm/MHz for grades 2 and 3, 0.90 dB/cm/MHz for grades 4 and 5, when compared to 0.59 dB/cm/MHz for healthy livers. Mean ESD estimated in the 10-20 MHz frequency bandwidth was observed to be 24 mu m, 31 mu m and 34 mu m for the healthy liver, for grades 2 and 3, and for grades 4 and 5, respectively. Results showed that the elastic (Young) modulus values were on the order of several thousand of Pascals. The elastic modulus generally increased with the fibrosis grade (scores 2 to 5).