Volkmar Uhlendorf

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.<<ETX>>

Acoustic behaviour of current ultrasound contrast agents.

A general law gives the approximate change in signal level obtained in a particular imaging mode when a suitable contrast agent is added. It also shows that reduction of background signals is essential to overcome limitations found mainly in conventional (linear) ultrasound contrast imaging. Contrast agents contain stabilized microbubbles with very helpful non-linear properties. Acoustic methods for non-destructive and destructive testing of microbubbles are briefly discussed. In the main part, the linear and non-linear acoustic behaviour of various types of contrast agent are described. The latter is useful for new applications in diagnostic ultrasound.

Imaging of Spatial Distribution and Flow of Microbubbles Using Nonlinear Acoustic Properties

Today, medical ultrasound images can reveal subtle details of anatomy or blood flow with high spatial and temporal resolution. This has been made possible by the rapid progress in acoustical imaging during the last decades. Various ultrasound contrast agents are now becoming additional tools which allow special problems in imaging and functional diagnostics to be solved. They have a number of applications with state-of-the-art imaging techniques since most agents became sufficiently adapted to “conventional” imaging modes during their development. A certain class of contrast agents (first and foremost SHU 508 A, i.e. Levovist®) shows a very marked response at the second harmonic of the transmitted frequency f0, which is now used for selective detection in Harmonic Imaging — a class of imaging modes specially adapted to contrast media.1

Nonlinear properties of microbubbles and applications to medical ultrasound imaging

Gas bubbles smaller than 10 μm dissolve within milliseconds even in gas‐saturated liquids, but coatings, etc. can prevent dissolution. These stabilized microbubbles serve as transpulmonary contrast agents. When only their linear properties are employed, physical laws limit the diagnostic potential to detection in large diameter vessels. Nonlinear acoustic properties of contrast agents permit very important additional applications. Nonlinearity arises from bubble pulsations, shell properties, finite amplitude waves, and electronic hardware. The first two sources can dominate, allowing new diagnostic imaging modes sufficiently sensitive to detect isolated microbubbles in vivo. Harmonic imaging modes detect 2nd harmonics of the transmit frequency, mainly from microbubbles. Acoustic Emission modes destroy bubble shells by one pulse of moderate amplitude. Consequently, the free bubbles respond strongly to this and other pulses before dissolving. Observed lifetimes of 1–20 ms are enough for harmonic or conventi...