Tissue Parameter Measurement and Imaging

Abstract

The attenuation and velocity of ultrasound in tissues are often measured in attempts to uncover an experimental basis for some tissue characterisation methods. A knowledge of these parameters is essential also to describe fully the propagation of ultrasound waves in tissues, and to correct for certain image artefacts. However, even a cursory glance at the literature will reveal that published values for the observed attenuation, in particular, show a wide discrepancy. Much of this is known to result from, for example, differences in sample preparation and handling, but it is equally true that little attention has been paid to the influence that the experimental techniques themselves have on the measured parameter values. Methods for assessing “in vivo” attenuation and velocity are still in their infancy, and experiments with tissues are generally conducted with in vitro specimens. Such water-tank experiments usually rely on the interpretation of pulse transmission measurements in order to arrive at the desired propagation parameters. It is to this type of experiment that we will confine ourselves here. The analysis and interpretation of the measurements are made difficult by the twin circumstances of diffraction and dispersion. The former arises from the inevitably finite aperture size of the pulse generating transducer, and the latter arises from the relatively poor current understanding of the behaviour of propagating pulses within tissue-like dispersive media. A further complication is that the receiving transducers in most experiments are of the phase-sensitive type, but this is only briefly alluded to here.