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 particl
es encapsulated in a shell. The shell makes the acoustic properties of
these particles differ from that of gas-bubbles. It is shown how acou
stic behaviour of such particles can be explained from their size and
compressibility. The particle bulk modulus K was described with the Ke
lvin-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. A
pplication of this model to an experimental contrast agent from Nycome
d Imaging gave as a result K-0=2.2 MPa, mu=0.080Pa . s.