Acoustic properties of ultrasonic contrast agents

Diameters of particles used as ultrasound contrast agents are limited by capillary diameters to about 8 mu m, much smaller than the acoustic wavelength used in diagnostic ultrasound. Strong acoustic backscatter from such small particles is achieved by using gas-containing particles encapsulated in a shell. The shell makes the acoustic properties of these particles differ from that of gas-bubbles. It is shown how acoustic behaviour of such particles can be explained from their size and compressibility. The particle bulk modulus K was described with the Kelvin-Voigt model for visco-elastic solids, K=K-0+i omega mu, where the parameters K-0 and mu were found from acoustic attenuation spectra. Application of this model to an experimental contrast agent from Nycomed Imaging gave as a result K-0=2.2 MPa, mu=0.080Pa . s.