Interoduction: Ultrasound images have often low contrast due to small differences in acoustic impedance between different tissues. Air or gas microbubbles that surrounded by membrane are most of the contrast agents in ultrasound imaging. Problems such as instability in sound pressure and inability in penetrating from the blood vessel into body tissues limited the use of microbubbles into the intravascular space. Due to the potential benefits of nanomaterials, extensive research about developing nanoparticle-based contrast agents is done recently. In this work, we investigated acoustic properties of gold nanoparticles coated with silica (Au@SiO2) to use it as an ultrasound contrast agent.
 
Materials and Methods: In this study, the size of Au@SiO2 nanoparticles was measured by DLS and TEM. Agarose gel samples containing different concentrations of nanoparticles were made. Samples were placed in the Plexiglas container involved deionized water and B mode ultrasound imaging of samples obtained at three different frequencies (6, 8 and 10 MHz). Images were analyzed by MATLAB program. Average of gray Scales and backscatter amplitude were obtained for each image. Effect of increasing concentration of nanoparticles on average of gray Scales and backscatter at three frequencies was measured. Then stability of nanoparticles were examined during insonication.
Results: size and morphology of Au@SiO2 nanoparticles was, 18.67 nm by TEM and 25.8 nm, by DLS. Image brightness levels increased in the samples containing nanoparticles compared to the net agarose gel. brightness level increased with particles number concentration. In three frequencies that we used, in constant concentration with increasing frequency, image gray levels will be increased. Quantitative results of the regression analysis for assessment of backscatter indicated that there is linear relationship between increasing backscatter signal and particles concentration at 8 and 10 MHz, but at 6 MHz didn’t observed association between this two values. Also didn’t observed significant change in particle size distribution after 30 minutes exposure by ultrasound.
Conclusion: Our results show that appropriate concentration of Au@SiO2 nanoparticles have ability to improve effective contrast. Proper particle size distribution caused these particles to penetrate from pores of vessels of tumor tissue and accumulate in cancer cells. more stability of these particles against ultrasound waves compared to Microbubbles caused their to be usefulness for long-term ultrasound imaging and guide and monitor treatment.