Fundamental correlation lengths of coherent speckle in medical ultrasonic images

Abstract

Refinements to previous analyses of the natural correlation lengths within simple images and between images to be compounded are presented. Comparison of theoretical and experimental results show very good agreement for the case of Rayleigh scattering media: the correlation length within a simple image is comparable to the resolution cell size; the correlation length between images to be spatially compounded is comparable to, but smaller than, the transducer or array aperture; and the correlation length between images to be frequency compounded becomes a frequency comparable to their bandwidth. Complications arising from the presence of specular scattering or due to the presence of just a few scatterers are considered. It is shown that straightforward solutions exist for either one of these problems taken by itself. When they occur in combination, calibration techniques may lead to unambiguous identification of the contributions to the scattering from diffuse or incoherent scattering and from specular or coherent scattering, and to estimation of the density of diffuse scatterers.

Subsample interpolation strategies for sensorless freehand 3D ultrasound

“…Referring to Figure 2, this involves fitting some function f (d l ) to the available samples of ρ, then determining d * l and ρ * from the peak of f (d l ). The most natural choice for f (d l ) is the true shape of a lateral decorrelation curve, which is approximately Gaussian when imaging fully developed speckle at the beam's focus and correlating the intensity or amplitude envelope of the RF echo signal [14,17,18]. Away from the focus, there are side-lobes in the point spread function and a Gaussian curve is no longer appropriate.…”

Speckle classification for sensorless freehand 3-D ultrasound

“…Due to the coloured nature of the received RF echo, it is possible to estimate the local shape of the resolution cell via the spatial decorrelation of the RF echo intensity. For scans comprising fully developed speckle, there is a direct relationship between the resolution cell's dimensions and the decorrelation functions in the principal scanning directions [2,5,31,32]. We therefore measured decorrelation functions in the elevational and axial directions by scanning a speckle phantom with Rayleigh backscatter and uniform attenuation of 0.4 dB/cm/MHz 4 [17,19].…”

Improvements in elastographic contrast-to-noise ratio using spatial-angular compounding

“…Local strain estimates around the same region-of-interest (ROI) acquired from different insonification angles are averaged to obtain a compound elastogram. This technique utilizes the same concept to reduce speckle noise as in conventional US imaging with angular compounding of the B-mode signals referred to as sono-CT (Burckhardt 1978;Trahey et al 1986;O'Donnell and Silverstein 1988;Wagner et al 1988;Ping 1997;Entrekin et al 2001;Tanter et al 2002). However, because strain is a tensor, strain estimates obtained from different angular views have to be appropriately weighted before compounding (Techavipoo et al 2004b).…”

“…Another advantage with spatial angular compounding is that the spatial resolution in the compounded images is not significantly degraded by the compounding process because the compounded signals arise from the same spatial location, but at different insonification angles. The variation in the insonification angle provides sufficient decorrelation to enable the improvement in the SNR e of the averaged strain estimate (Trahey et al 1986;O'Donnell and Silverstein 1988;Wagner et al 1988). …”