The development of a quantitative diagnostic method for liver fibrosis using ultrasound would be highly medically significant. In our researches, a quantitative ultrasound (QUS) method for detecting and classifying liver fibrosis on the basis of the estimation of scatterer density from the statistical analysis of echo envelopes is proposed. Fibrotic tissue is inhomogeneous; therefore, its envelope probability density function (PDF) cannot be accurately modeled by a single PDF. Additionally, some regions have variable scatterer densities. In order to detect and characterize the fibrotic liver quantitatively, the relationship between the scatterer distribution and the PDF of echo envelopes of inhomogeneous scattering media were fitted by multi-PDFs model. The validity of the theory of multi-PDFs model has been verified by computer simulations, and the analysis parameters in the simulated fibrotic tissue were successfully used to characterize liver fibrosis in clinical data sets.
Additionally, it must be devised to understand the complex interaction between ultrasound and tissue and scattering models based on tissue properties for application to definitive diagnosis and high accuracy of the QUS method. Towards this aim, speed-of-sound (SOS) and attenuation from three types of rat livers (normal, fatty, and fibrosis) were measured with a scanning acoustic microscope using transducers with center frequencies from 1-MHz to 250-MHz. Results indicated that SOS and attenuation measured with each transducer showed the following trend. Variability in SOS and attenuation values of normal liver was much smaller than other livers at any frequencies. In the fatty liver, SOS was slower and the attenuation was larger than in the normal liver. In fibrosis, SOS and attenuation had values between those of normal and fatty liver. Additionally, the relation between the pathologic state of liver and SOS and attenuation was investigated. Correlation between the ultrasound wavelength and the distribution and size of fat or fiber deposits in the liver was investigated using the corresponding stained histology photomicrograph.
Left: Liver fibrosis diagnosis on clinical ultrasound scanner with tissue density estimation.
Right: Speed-of-sound measured from 10-μm thin section of fibrosis liver by 250-MHz center frequency ultrasound.