Methods and devices for characterizing tissues and the like utilize signals obtained from ultrasonic backscatter processes. The method accounts for frequency-dependent attenuation, spatially-varying media statistics, arbitrary beam geometries, and arbitrary pulse shapes. Statistical analysis is employed to estimate the scatterer number density (SND) of tissues. The method for estimating the scatterer number density incorporates measurements of both the statistical moments of the backscattered signals and the point spread function of the acoustic system.

Ultrasonic tissue characterization

Accurate characterization of tissue pathologies using ultrasonic attenuation is strongly dependent on the accuracy of the algorithm that is used to obtain the attenuation coefficient estimates. In this paper, computer simulations were used to compare the accuracy and the precision of the three methods that are commonly used to estimate the local ultrasonic attenuation within a region of interest (ROI) in tissue; namely, the spectral log difference method, the spectral difference method, and the hybrid method. The effects of the inhomgeneities within the ROI on the accuracy of the three algorithms were studied, and the optimal ROI size (the number of independent echoes laterally and the number of pulse lengths axially) was quantified for each method. The three algorithms were tested for when the ROI was homogeneous, the ROI had variations in scatterer number density, and the ROI had variations in effective scatterer size. The results showed that when the ROI was homogeneous, the spectral difference method had the highest accuracy and precision followed by the spectral log difference method and the hybrid method, respectively. Also, when the scatterer number density varied, the spectral difference method completely failed, while the log difference method and hybrid method still gave good results. Lastly, when the scatterer size varied, all of the methods failed.

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A theoretical comparison of attenuation measurement techniques from backscattered ultrasound echoes

An algorithm for nonparametric estimation of 1D ultrasound pulses in echo sequences from human tissues is derived. The technique is a variation of the homomorphic filtering technique using the real cepstrum, and the underlying basis of the method is explained. The algorithm exploits a priori knowledge about the structure of RF line echo data and can employ a number of adjacent RF lines from an image. The prime application of the algorithm is to yield a pulse suitable for deconvolution algorithms. This will enable these algorithms to properly take into account the frequency dependence of the attenuation and its variation within a patient and among patients. It is also possible to use the estimated pulse for attenuation estimation, and the consistency of the assumptions underlying the proposed technique is demonstrated by its ability to recover low variance attenuation estimates in the normal liver from in vivo pulse-echo data. Estimates are given for 8 different patients. >

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Nonparametric estimation of ultrasound pulses

Ultrasonic attenuation coefficients of liver have been derived from echoes received by a modified commercial B-scan imaging instrument Values have been measured from selected regions within liver scans of 59 individuals, of which 15 cases were presumed normal (based on medical histories), and the remainder were involved with diffuse liver disease such as alcoholic cirrhosis, chemotherapy toxicity, chronic hepatitis, and liver metastases Medical histories on most individuals include the results of serum liver function enzymes, conventional B-scan examinations, and exposure to drugs and alcohol The results of CT abdominal scans (N = 13) and/or liver biopsy (N = 12) were also available The results show that normal attenuation values for human liver are 0054 +/- 0009 Np/cm-MHz (047 dB/cm-MHz) with a frequency dependence of fn, where n = 105 +/- 025, in agreement with in vitro studies of mammalian liver In diffuse liver disease, no relationship was found between the attenuation coefficient and the results of CT or conventional ultrasonic examination A trend towards higher attenuation with increased fibrosis and fat, as graded from liver biopsies, was noted, but the results were generally not statistically significant However, a significant correlation was found between high values of attenuation and abnormal liver function tests High attenuation is also found with ingestion of alcohol, chemotherapeutic agents, and steroids, all of which may affect liver composition

In-vivo measurements of ultrasound attenuation in normal or diseased liver

The attenuation of an ultrasound pulse within tissue can be estimated from either the amplitude decay or the frequency downshift of returning echoes. The authors compare the results of both analyses as applied to ultrasound B scan echoes from the livers of 49 individuals. The amplitude decay of the backscattered signal Fourier components with depth was used to calculate attenuation coefficients. In addition, the frequency downshift of the same backscattered signals was estimated using both zero-crossing and spectral centroid methods. The results show that the frequency-domain estimators yield consistently higher attenuation coefficients, with higher variability compared to the amplitude decay method. Explanations for the apparent bias and variability of the frequency-shift estimators include both the assumptions regarding tissue and signal which may not be met in practice and the effects of low-frequency electronic noise on spectral estimates. >

Comparison of techniques for in vivo attenuation measurements

Invited Paper Abstruct-Attenuation is an important parameter in the description prehensions of the physical and computational foundations of the propagation of an ultrasonic pulse in tissue. Since attenuation of the technioues. has been shown to be influenced by tissue state and pathology it also an important factor in the development of quantitative techniques for the measurement of ultrasonic tissue parameters and/or the display of ultrasonic tissue signatures (ultrasonic tissue characterization). Although attenuation is intrinsically defined in terms of a transmission measurement, it can be estimated from an analysis of the backscattered echo waveform. The development of such estimates is fundamentally a problem for digital signal processing. Recent advances in ultrasound instrumentation and computing technology have made the development of attenuation estimation from backscattering practical, thereby suggesting the feasibility of in vivo measurement of attenuation, and creating a popular and important area for research and development. Here we provide a perspective of the estimation of attenuation from pulse-echo signals, and overview the various attenuation estimation techniques proposed to date.

S. Leeman

Papers

Perspectives on Attenuation Estimation from Pulse-Echo Signals