Journal of Ultrasound in Medicine, Vol 9, Issue 4 215-229, Copyright © 1990 by American Institute of Ultrasound in Medicine
Texture in tissue echograms. Speckle or information?
J. M. Thijssen and B. J. Oosterveld
Biophysics Laboratory, Institute of Ophthalmology, Academic Hospital, Nijmegen, The Netherlands.
Models of biological tissues are described in terms of acoustic parameters
and of structure. Beam formation is discussed for continuous wave and
pulsed modes of transducer operation and the concept of the point spread
function (PSF) is introduced. The PSF is equivalent to the resolution cell,
or the sampling volume, of echographic equipment. The generation of
echograms from parenchymal tissues is described in terms of speckle
formation due to interference at reception on the transducer. The speckle
dimensions are quantitatively compared to the sampling volume of the
employed transducer. It is shown that for fully developed speckle the
tissue characteristics are exclusively reflected in the mean echolevel and
not in the speckle size. The speckle size is, however, greatly dependent on
the bandwidth, the frequency, and the geometry of the employed transducer.
The attenuation by the insonated tissue yields a depth-dependent increase
of mainly the lateral speckle size, in addition to the depth dependence
caused by the beam formation. If the number density of scattering sites
within the tissue is relatively low, the speckle characteristics are
dependent on this density and, hence, tissue characterization is feasible
if these characteristics are analyzed by statistical methods. These methods
are gray level histogram analysis and the estimation of the autocorrelation
function, ie, first and second order statistics, respectively. Structural
order in tissues can be quantified by autocorrelation analysis and clinical
studies on diffuse liver diseases support this conclusion. The effects of
pre- and postprocessing on the detectability of focal lesions are outlined.
The impact of multifocus systems and of the acquisition of radio frequency
echograms on further developments of clinical echography is discussed.
