Cross Amplitude Modulation and Compound Amplitude Modulation for Nonlinear Contrast-Enhanced Ultrasound Imaging of Nanobubbles

In nonlinear contrast-enhanced ultrasound (CEUS) imaging, nanobubbles (NBs) offer a promising alternative for enhanced visualization of microvascular structures and molecular imaging. This study explores two amplitude-modulated (AM) techniques-cross amplitude modulation (xAM) and compound amplitude modulation (cAM)-to enhance the capabilities of NB-mediated CEUS imaging. Both methods were tested on the Vevo F2 ultrasound imaging system (Fujifilm VisualSonics Inc.) using the Vevo Advanced Data Acquisition (VADA) mode, allowing full customization of pulse sequences. The xAM technique utilized a three-event pulse sequence that transmits cross-propagating plane-wave beams from dual apertures. This method isolated nonlinear scattered waves from NBs, reducing background noise and enhancing image quality. In contrast, cAM achieved a high frame rate of 706 Hz, a valuable feature for tracking the NB vascular flow dynamics. cAM combined plane-wave compounding with amplitude modulation, transmitting two events (half- and full-amplitude), achieving high frame rates for velocity imaging at the expense of image quality. NBs at a concentration of 10⁹ NBs/mL, intended to mimic estimated in vivo post-injection concentrations, were injected into custom-built tissue-mimicking vessel phantoms. Experiments demonstrated that xAM significantly improved the contrast-to-noise ratio (CNR) and contrast-to-tissue ratio (CTR) by over 10 times compared to B-mode imaging, especially at larger steering angles. Conversely, cAM's CNR and CTR were at least 50% lower than that of xAM, but it achieved a frame rate over 100 times faster than xAM. These results suggest xAM can enhance imaging clarity, while cAM offers high frame rates for velocity imaging, providing an imaging framework for preclinical and clinical applications.