Amplitude modulation and baseband delay-multiply-and-sum beamforming for improved vessel visualization with volumetric contrast-enhanced ultrasound

Contrast-enhanced ultrasound (CEUS) imaging is performed clinically and preclinically to explore the changes in microvascular blood flow. Nonlinear imaging techniques such as amplitude modulation (AM) can improve the contrast-to-tissue ratio (CTR) of microbubble (MB) contrast agents. In order to visualize blood vessels in a volume space, volumetric CEUS can be performed with matrix array probes. However, these probes have limitations such as high side lobe levels and poor signal-to-noise ratios (SNR). Although delay-and-sum (DAS) beamforming is the most widely used beamforming method, these issues can be mitigated by alternative strategies such as baseband delay-multiply-and-sum (BBDMAS) beamforming. We therefore conducted this research to determine if AM imaging and BBDMAS beamforming provide an advantage in volumetric CEUS over B-mode US imaging and DAS beamforming. In vitro and in vivo experiments were performed. For the in vitro experiment, MBs were circulated through a tissue-mimicking phantom with a 1.5 mm vessel lumen, and the phantom was imaged with a Vantage 256 research scanner (Verasonics Inc.) equipped with a 2-dimensional (2-D) matrix array probe (Vermon). The phantom data was processed into a series of B-mode US, AM-DAS, and AM-BBDMAS volumes, and temporal average intensity volumes were computed for each method. The CTR of the vessel was found to be 9.1, 14.7, and 20.4 dB for these methods, respectively. For the in vivo experiment, the hindlimb of a healthy rat was perfused with MBs and imaged with AM. The data was beamformed with BBDMAS and additionally processed using 4-dimensional (4-D) spatiotemporal nonlocal means (4D-STNLM) denoising. This combined processing over our acquisition enabled us to have a dynamic range of 65 dB for a clear visualization of the blood vessels. Our results demonstrate that the AM-BBDMAS technique and 4D-STNLM denoising improve the CTR and visual appearance of vessels in volumetric CEUS.