Temperature dependence of ultrasonic enhancement with a site-targeted contrast agent

Molecular imaging contrast agents specifically detect the biochemical "signatures" of disease before anatomical manifestations are apparent. Sensitive and specific localization of fibrin both in vivo and in vitro has been demonstrated with the use of a ligand-directed liquid perfluorocarbon nanoparticle. Since the acoustic properties of perfluorocarbons are known to vary with temperature, it was hypothesized that temperature could be used to augment the magnitude of enhancement imparted by targeted nanoparticles. Accordingly, the acoustic backscatter of two different substrates, nitrocellulose membrane and human plasma clot, targeted by the nanoparticles was measured at temperatures ranging from 27 degrees to 47 degrees C in 5 degrees C increments. Classic avidin-biotin interactions were utilized to couple biotinylated nanoparticles to avidin-conjugated nitrocellulose membranes. Ultrasonic contrast enhancement of the nitrocellulose membrane at 25 MHz, measured by acoustic microscopy, increased from 2.0+/-0.3 dB at 27 degrees C to 3.7+/-0.4 at 47 degrees C. In a similar experiment, antifibrin nanoparticles bound to human plasma clots also exhibited temperature-dependent ultrasonic signal enhancement ranging from 13.9+/-1.5 dB at 27 degrees C to 18.1+/-1.5 dB at 47 degrees C. The increase in ultrasonic contrast enhancement measured was well described by a simple, acoustic transmission line model with temperature-dependent impedance. These results suggest that temperature-dependent changes in acoustic backscatter may be used to further differentiate tissues targeted with site-specific nanoparticles from surrounding normal soft tissues.