ABSTRACT
This paper is devoted to developing a scattering model for coherent imaging systems. It describes the statistical properties of the image intensity due to the distribution of scatterers on a subresolution scale. The roughness of the scatterers' strength, the randomness of the spatial architecture, and the birth-death process of the scattering centers are considered. Two parameters of the image intensity are examined for the characterization of the scattering medium: 1) the signal-to-noise ratio (SNR), which permits the distinguishing of sub-Rayleigh (SNR<1) from super-Rayleigh (SNR>1) conditions and 2) the correlation coefficient. These parameters permit the estimation of the roughness and randomness degrees of the structure. The model is then extended to account for specular effects and applied to sea clutter. A quantitative treatment of the spatial and temporal correlation characteristics, based on a physical understanding of the underlying process, is derived. Experimental results are presented to validate the model.
ABSTRACT
A novel programmable delay line system (Patent 67595-A/89 from the Ministero dell'Università e della Ricerca Scientifica e Tecnologica, Italy) for electronic control of delay profile in ultrasound real-time imaging systems is described. The line consists of a number of LC filters. The capacitance is obtained by using varicap diodes, so that electronic control over the desired depth is obtained by changing the diode inverse polarization voltage. This delay line exhibits a dramatic simplification and a broader time delay excursion than alternative techniques. Based on computer simulations, a delay line system was designed and integrated into an annular transducer-based echographic apparatus.
Subject(s)
Computer Systems , Transducers , UltrasonicsABSTRACT
A model of the echo formation process from tissues is suggested, in order to relate microstructural features to ultrasonic spectral signatures. The tissue is modeled as a collection of ideal randomly distributed scatterers, filtered by a time-invariant system representing the measurement apparatus and the average properties of the scattering medium, to obtain the backscattered signal. The gamma distribution has been assumed to describe the process, because it offers a flexible approach to approximating the parametric regularity of the scatterers. According to the model the spectral characteristics of the backscattered signal are strictly correlated with the spatial architecture and can be related to the gamma distribution parameters, i.e. interdistance and order. The model has been tested by simulating practical situations, in order to prove its validity and to test the procedure for estimating model parameters from actual data. The correspondence of experimental results obtained from tissues of different degrees of regularity with the simulated results confirms the effectiveness of the model for tissue characterization studies.
ABSTRACT
A method of image restoration for ultrasonic B-scan images has been proposed that need no a priori knowledge on the PSF (point spread function) of the imaging system and is feasible for in vivo applications. The entire system's response, including the interposed medium and possible transducer defects, is estimated from the degraded image itself with a few simple operations. The ultrasonic image is restored based only on a knowledge of the estimated PSF and on the spectral characteristics of the resultant echo signal. The proposed method does not modify the phase relations between echoes from multiple scatterers since the restoration filter is phaseless and the display operation does not involve nonlinear detection. The effectiveness of the restoration filter was tested on simulated ultrasonic images in the absence and in the presence of interposed tissue. Then the filter was tested on a phantom made of scatterers randomly distributed in nonattenuating gel with and without an interposed medium whose attenuation linearly increases with frequency. A good correspondence between simulations and experimental results was found: both tests show an exceptional improvement of image resolution.
ABSTRACT
A model of ultrasonic image formation has been developed for an electronically focused linear-array transducer. An analytical expression for the point-spread function of the image degradation process is obtained. It consists of two terms: one accounts for the electromechanical properties of the transducer; the other describes the diffraction phenomena associated with the wave propagation. To reduce the computational complexity in image restoration, a piecewise-invariant form for the point-spread function has been suggested. Experimental results prove the validity of the model and the feasibility of the reconstruction process with a few shift-invariant filters.
