Modelling of the ultrasound return from Albunex microspheres

Abstract A mathematical model that predicts the changes to the ultrasound frequency spectrum after passing through human tissue and Albunex (a registered trademark of Molecular Biosystems Inc, San Diego, CA) microspheres is proposed. Changes in backscattered intensity and mean frequency of the reflected signal can be estimated as a function of imaging geometry, ultrasound frequency, and microsphere concentration and size distribution. An important result is that the frequency shifts and the intensity variations are caused both by the microspheres in the path between the transducer and the region of interest, and by the reflection properties of the contrast agent in the region of interest. The model enables one to explain and predict clinically observed intensity effects such as the shadow effect in the right ventricle, and the fact that received intensity increases with concentration up to a certain point where it falls off. The effect of the removal of the larger microspheres in the lungs is so significant that concentrations that give positive frequency shifts with increasing concentration in the right ventricle will give negative shifts in the left ventricle. The frequency shift is only a good indicator of concentration for ultrasound frequencies below 4 MHz, while the intensity is very sensitive to the Albunex microsphere concentration and the imaged depth at the higher frequencies. It is also found that the periodic variation in heart muscle thickness during the heart cycle will significantly affect the intensity and mean frequency of the backscattered ultrasound observed in the left ventricle, and in such a way as to give a periodic variation over the heart cycle.