Open-chest cardiac ultrasound-mediated imaging with a vacuum coupler

Background: A fundamental obstacle for the preclinical development of ultrasound-(US) mediated cardiac imaging remains cardiac motion, which limits interframe correlation during extended acquisition periods. Purpose: To address this need, we present the design and implementation of a 3D-printed vacuum coupler that stabilizes a US transducer on the epicardial surface of the heart for feasibility assessment and development of advanced, cardiac, US-mediated imaging approaches. Methods: The vacuum coupler was 3D printed with biocompatible resins and secured with a standard intraoperative suction aspirator. US-mediated imaging (i.e., B-mode and photoacoustic [PA] imaging) was performed in an open-chest porcine model with and without the vacuum coupler. Based on inter-frame displacement tracking and cross-correlation (CC) coefficients, changes in frame motion and stability were compared for each imaging mode/configuration through a prolonged (∼1 min) acquisition, while the impact on PA-based SO₂ accuracy was assessed. Results: When compared to conventional handheld imaging, stand-off imaging, and coupler without suction, epicardial imaging with the vacuum coupler and suction applied led to a significantly reduced mean axial displacement of 0.15 mm versus 0.89, 0.49, & 0.49 mm, respectively (p-values ≤ 8.65e-7). Comparing the coupler without suction to that with suction applied, physiologically unrealistic SO₂ estimates reduced from 1.72 to 0.81%, respectively, and lateral interframe displacement reduced from 4.58 to 2.01 mm, respectively (p-value = 5.07e-23). Overall, reduced cardiac tissue motion and increased interframe CC coefficient (baseline = 0.43 vs. coupler with suction = 0.80) allow for more accurate PA unmixing. Conclusions: Epicardial US-mediated imaging with a vacuum coupler reduces cardiac motion artifact, providing a consistent sampling of an intended region of interest (ROI) over multiple cardiac cycles. This could help facilitate the development of advanced US-mediated imaging, which is often hindered by cardiac motion. Stable implementation of these imaging techniques could allow for intra-operative assessments of local cardiac perfusion as well as tissue characterization.