Nonlinear Imaging of Microbubble Contrast Agent Using the Volterra Filter: In Vivo Results

Area of Science:

• Ultrasound imaging
• Biomedical engineering
• Nonlinear acoustics

Background:

• Microbubble ultrasound contrast agents (UCAs) are crucial for visualizing microvessel dynamics.
• Traditional ultrasound imaging methods face limitations in resolving nonlinear UCA behavior.
• Accurate imaging of UCA activity is vital for diagnosing conditions like cancer and ischemia.

Purpose of the Study:

• To present a novel nonlinear filtering approach for imaging UCA dynamics.
• To evaluate the performance of the adaptive third-order Volterra filter (TVF) in separating nonlinear components.
• To assess the sensitivity and specificity of the proposed method compared to conventional techniques.

Main Methods:

• Utilized an adaptive third-order Volterra filter (TVF) to process beamformed pulse-echo ultrasound data.
• Separated linear, quadratic (QB-mode), and cubic (CB-mode) components from echo signals.
• Quantified contrast enhancement using contrast-to-tissue ratio (CTR) in phantom studies.
• Evaluated echogenicity changes in vivo within regions of varying perfusion with and without UCAs.

Main Results:

• QB-mode and CB-mode images demonstrated higher mean CTR values than standard B-mode, indicating improved sensitivity.
• The TVF components offered comparable or superior mean CTR values to pulse inversion (PI) with enhanced spatial specificity.
• In vivo studies showed that nonlinear TVF components significantly increased echogenicity in heterogeneous perfusion regions, consistent with in vitro findings.

Conclusions:

• The adaptive TVF provides an effective tool for imaging UCA activity, particularly in complex vascular environments.
• This nonlinear filtering approach enhances the visualization of microvessel dynamics in conditions with heterogeneous perfusion, such as tumors and ischemic tissues.
• The method preserves axial resolution and signal-to-noise ratio (SNR) by enabling broadband pulse transmission.