Nonlinear dynamics of polymer shell ultrasound contrast agents at 8–32 MHz ultrasonic excitations

The response of polymer shell ultrasound contrast agents (PSUCAs) at frequencies <; 5 MHz has been shown to be associated with shell rupture and the formation of free gas bubbles. Recently, it was shown that PSUCAs produce significant subharmonic (SH) response at higher frequencies (20-40 MHz) which is not consistent with the formation of free bubbles. Despite the high control over the manufacture of PSUCAs, there is limited knowledge on the response of PSUCAs to ultrasonic excitation, especially at higher excitation frequencies. In this work, the dynamics of individual PSUCAs in response to 30 cycle 8-32 MHz excitations with pressure amplitudes ranging ~70 kPa - 2.5MPa was studied by experimental observations using a Vevo 770 ultrasonics imaging instrument. In each case a very dilute solution of PSUCAs were used to minimize the interaction between the PSUCAs. The RF signal of the individual UCA oscillations were extracted and analyzed. The results of this study show that PSUCAs can undergo stable linear and nonlinear oscillations with substantial amplitude. 2nd and 3rd order super harmonic (SUH) oscillations excitations and 1/2, 1/3, ¼ and 1/5 order SHs were detected for the investigated frequencies. Compression only behaviors were also detected mainly resulting in linear oscillations. The generation of the compression only behavior and SH oscillations in thick shell PSUCAs may only be explainable by the Marmottant model. Results of this study show the feasibility of the substantial nonlinear oscillations in the oscillations of the PSUCAs. Further investigations are needed to be carried out to fully realize the potential of the application of PSUCAs within the field of medical ultrasound.