MODELING OF THE ULTRASOUND RETURN FROM ALBUNEX MICROSPHERES

A mathematical model that predicts the changes to the ultrasound frequ ency spectrum after passing through human tissue and Albunex (a regist ered trademark of Molecular Biosystems Inc, San Diego, CA) microsphere s is proposed. Changes in backscattered intensity and mean frequency o f the reflected signal can be estimated as a function of imaging geome try, ultrasound frequency, and microsphere concentration and size dist ribution. An important result is that the frequency shifts and the int ensity variations are caused both by the microspheres in the path betw een the transducer and the region of interest, and by the reflection p roperties of the contrast agent in the region of interest. The model e nables one to explain and predict clinically observed intensity effect s 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 microspher es in the lungs is so significant that concentrations that give positi ve frequency shifts with increasing concentration in the right ventric le will give negative shifts in the left ventricle. The frequency shif t is only a good indicator of concentration for ultrasound frequencies below 4 MHz, while the intensity is very sensitive to the Albunex mic rosphere concentration and the imaged depth at the higher frequencies. It is also found that the periodic variation in heart muscle thicknes s during the heart cycle will significantly affect the intensity and m ean frequency of the backscattered ultrasound observed in the left ven tricle, and in such a way as to give a periodic variation over the hea rt cycle.