Characterizing the subharmonic response of four new microbubble formulations compared with three commercially-available ultrasound contrast agents

Subharmonic imaging (i.e., receiving signal at half the transmitting frequency) is a promising technique to detect ultrasound contrast agents in the circulation. The aim of this study was to characterize the subharmonic enhancement of four new microbubble formulations (MBF-1 to 4), compared with three commercially-available contrast agents (Definity, Sonazoid and SonoVue), and to compare this subharmonic enhancement with the fundamental response. The new microbubble formulations consisted of either C3F8 or 2% C
₃
F
₈
and 98% N
₂
as the inner gas, and had different shell configurations containing polyethylene glycol 4000. Equal concentrations of each reconstituted contrast agent were investigated in vitro using a flow phantom. Radiofrequency data were acquired at four transmitting frequencies (2.5, 3.0, 3.5 and 4.0 MHz), four pulse-inversion configurations (1-4 cycles/pulse) and four scanner acoustic output levels (25, 52, 77 and 100%). Enhancement was derived as the relative increase in signal amplitude after contrast was added. Subharmonic enhancement of the new microbubble formulations were generally less affected by transmitting frequency, pulse length and acoustic output level than the commercially-available contrast agents (p <; 0.05 was considered statistically significant for main and interaction effects). Mean subharmonic enhancement over all transmitting conditions for the new microbubble formulations (MBF-1 8.4 ± 4.0; MBF-2 8.5 ± 3.8; MBF-3 6.5±3.1; MBF-4 7.5±3.2 dB) were within the range exhibited by the commercially-available contrast agents (Definity 6.1±3.4; Sonazoid 7.0±3.4; SonoVue 10.7±3.2 dB). The subharmonic enhancement was similar to or higher than the fundamental response across all contrast agents investigated. Subharmonic imaging may be applied to detect the new microbubble formulations in future work.