Contrast enhanced ultrasound imaging using adaptive third-order Volterra filter

Abstract

We have previously demonstrated the ability of the post-beamforming Volterra filter (VF) to separate linear and nonlinear echo components in contrast-enhanced ultrasound (CEUS) application, including imaging tumor perfusion in vivo. Compared to pulse inversion, the quadratic and cubic components from a 3rd-order VF (ThOVF) produce the same or higher level of contrast without sacrificing bandwidth or frame rate. However, CEUS imaging of small vessels with high level of sensitivity and specificity remains a challenge using clinical probes. Imaging such fine structures with highly focused beams may improve the sensitivity and specificity of CEUS in some applications. We have used a concave 64-element, 3.5-MHz array (40-mm roc) and also a linear diagnostic array probe (HST 15-8), in cross sectional imaging a 200 µm cellulose tube embedded in a tissue-mimicking phantom. An infusion syringe pump was used to pump a saline solution with small concentration contrast agents or cellulose linear scatters through the cellulose tube. Different power settings were used when collecting the data. Pulse inversion data was acquired using positive and negative transmit pulses for each image line. An adaptive third order VF was applied to the beamformed data to separate the linear, quadratic, and cubic signal components. The quadratic and cubic components from the ThoVF were compared to the corresponding PI component with and without the UCA. The PSDs of the quadratic and cubic components of the ThOVF and the PI signal with and without UCA, all three signals show enhancement due to the presence of UCA in different frequency bands. The results also show that higher power setting result in larger value of temporal perfusion index (TPI) and better imaging quality. And compared with the linear array, concave dual-mode Ultrasound array (DMUA) has lower noise level and better enhancement because of the highly focused imaging beam matched to the target microvessel.