Compact programmable transmit scheme for contrast imaging using nonlinear difference-frequency ultrasound signals

The nonlinear interaction between two acoustic waves at different primary frequencies generates a signal with a frequency equal to the difference between these primary frequencies. This signal is proportional to the nonlinear elasticity of the material being insonified; therefore, this signal can be used for elastographic contrast imaging. Previous studies on vibro-acoustography and difference-frequency ultrasound (dfUS) have used multiple separate low-element-count transducers for imaging, thereby increasing the bulk and complexity of the imaging system and necessitating mechanical steering and scanning. To address these limitations, we propose, demonstrate, and evaluate a novel array-based approach to difference-frequency-based ultrasound imaging. This 64-element ring array was constructed using a commercial 32×32 2D matrix array. This array-based scheme for dfUS imaging is customizable, programmable, and capable of electronic steering and scanning. The scheme generated traditional linear ultrasound and dfUS images of synthetic agarose and ex vivo phantoms. Statistical analysis of the results shows dfUS provides a 14.8 dB higher signal-to-noise ratio, 58.5% increased sensitivity, and 74.8 dB higher contrast compared to traditional linear ultrasound. In addition, a low mechanical index of 0.0248 was used in this preliminary study, thereby presenting opportunities for using transmit signals with increased strength in the future.