FREQUENCY AND CONCENTRATION-DEPENDENCE OF THE BACKSCATTER COEFFICIENTOF THE ULTRASOUND CONTRAST AGENT ALBUNEX(R)

A broadband ultrasonic measurement system has been utilized to charact erize the concentration and frequency dependence of in vitro suspensio ns of Albunex(R) microspheres at concentrations ranging from 1.7 X 10( 5) to 2.1 X 10(7) microspheres/ml and over a bandwidth of 1-16 MHz. Th e apparent backscattered power (not compensated for effects due to att enuation) was shown to increase with dose for lower concentrations of microspheres, but then to decrease rapidly with increasing concentrati on where attenuation effects become significant. Measurements of signa l loss demonstrated that the attenuation grew exponentially with incre asing concentration, so that a doubling of the number of microspheres led to a doubling of the value of the attenuation coefficient measured in dB/cm. This relationship was demonstrated over the entire system b andwidth. Compensation of the apparent backscattered power for the att enuation yielded the backscatter transfer function. This quantity was shown to be linearly proportional to concentration, so that a doubling of the number of microspheres led to a 3-dB increase in the backscatt er transfer function. A broadband data reduction technique was used to further reduce the data to backscatter coefficient, an absolute param eter describing the intrinsic scattering efficiency of the Albunex(R) microsphere suspensions. The backscatter coefficient was shown to be l inearly proportional to microsphere concentration at all concentration s investigated and over all the usable bandwidth. This suggests that, with appropriate compensation for attenuation and equipment parameters , perfusion or flow quantification techniques which assume a linear de pendence of backscatter with contrast agent concentration should be ap plicable. The backscatter coefficient exhibits a rapid rise with frequ ency below 3 MHz, and appears to approach a frequency independent limi t above 3 MHz. The relationships of the attenuation coefficient and ba ckscatter transfer function to concentration were generally consistent with predictions from a simple scattering model. These relationships appear to be valid within the usable bandwidth of our measurement syst em for all concentrations investigated. (C) 1998 Acoustical Society of America. [S0001-4966(98)05508-8]