In this article, a model for the amplitude statistics of the backscattered ultrasonic signal is presented. We propose to view a tissue as being composed of a large number of small units, each having slightly different characteristics. This variability within the tissue is expressed by fluctuations in the parameters of the local probability distribution function (PDF). Based on analogous expressions derived for radio propagation and optical scintillations, the local PDF is modulated by a lognormal distribution of the local standard deviation. Integrating the local contributions yields the amplitude PDF for the entire tissue. We show that the local entropy is a normal variable, since for four different local PDFs it is linearly related to the logarithm of the local standard deviation. When the local entropy histogram exhibits distinct and multiple peaks, the local entropy distribution can be used for region segmentation. This fact is demonstrated for ultrasonic images of ovarian cysts.

The distribution of the local entropy in ultrasound images

Effects of adaptive speckle filtering on the diagnostic accuracy of ultrasound in vivo tissue characterization

The ultrasound images taken on homogeneous and isotropic tissue mimicking phantoms cannot be directly compared to the images taken from inhomogeneous and anisotropic tissues. For this purpose special comparison and measuring methods for the recognition and quantitative embracement of the propagation phenomena in inhomogeneous media were developed. Propagation phenomena appear in many cases as special attenuation issues or as characteristic frequency dependent back scattering. By severe speed of sound inhomogeneities appears serious reduction of image contrast accompanied by reverberations and multiple scattering.

Differences of Ultrasound Propagation in Tissue and Tissue Mimicking Materials

The detection of sources responsible for coherent components in ultrasound signals, has been a difficult task. In this work, we explore the idea of using phase coherence as a measure of the level of structured regularity present in the scattering medium. If the scattering sites are located randomly, then the reflected signal should be incoherent. This is the case of purely diffuse scattering. If, however, there is structure in the scattering medium, then the reflections from those sites will have some non-random phase relationship. In this work, the phase distribution is characterized as follows. For each demodulation frequency, we plot the power as a function of phase. This is computed for all frequencies in the usable bandwidth of the transducer. For each frequency, the power is uniformly distributed across phase from 0 to 2 (pi) for a purely incoherent signal. Systematic deviations from the uniform distribution may indicate the presence of coherent scattering components. This approach was first verified using simulation data, then applied to two sets of clinical ultrasound data. We have achieved good classification performance (area under the ROC curve, Az equals 0.86 +/- 0.04) using two features extracted from this analysis of phase.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Using phase information to characterize coherent scattering from regular structures in ultrasound signals

The backscattered ultrasonic signal from soft tissue can be considered to consist of two parts: a spatially fluctuating component due to coherent scattering from randomly distributed scattering centers and a relatively constant component related to structural ordering. The relationship between these, which has been termed the “beam ratio parameter” influences the speckle pattern of an ultrasound image. First-order image statistics for this special speckle pattern are characterized by the Rician distribution. Changes of second-order statistics have also been derived as a function of the beam ratio parameter. A new method for the estimation of the beam ratio parameter from the intensity distribution of the ultrasonic signal is described in this paper. This method was used to measure the parameter in ultrasonic images from phantoms and liver tissue yielding values of about 0.1. The measured changes of the second-order statistics were not correlated to this parameter. The results indicate that the relatively constant intensity component related to structural ordering has only a small influence on the speckle pattern.

New method of estimating the ultrasonic beam ratio parameter for characterizing sound propagation in tissue.

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New method of estimating the ultrasonic beam ratio parameter for characterizing sound propagation in tissue.