Physics of ultrasound contrast imaging: scattering in the linear range

A simple model for B-mode image formation in diagnostic ultrasound is presented. It is used to give a general description of the effects, which scattering ultrasound contrast agents have on B- or M-mode images, as long as linear propagation of ultrasound is prevailing. The results of the model calculations are illustrated for the case of a homogeneous medium. It turns out that acoustical shadowing is limiting the maximum attainable backscatter enhancement. This becomes obvious when the model is also applied to the slightly more complicated case of cardiac and especially myocardial (or tissue) contrast. Conditions for optimum enhancement are derived for a number of typical diagnostic situations. Some possible pitfalls, which may prevent good results, are also called to attention. Microbubble suspensions are discussed as an important special example for contrast agents. In this case, the limits of the linear range can be estimated from the acoustically driven pulsations of a microbubble in a viscous fluid. The properties of contrast agents in the linear range described here should also provide a basis for later discussion of their nonlinear properties, which may help to overcome limitations imposed on tissue contrast in the linear range.<>