3D non-rigid motion correction using doppler-regularized optical flow for coronary ultrasound localization microscopy in beating heart

Objectives: Coronary microvascular disease, involving vessels smaller than 300 µm, contributes significantly to cardiovascular morbidity and mortality. However, direct visualization of coronary microcirculation remains challenging because current clinical imaging modalities lack the spatial and temporal resolution required to resolve the microvascular network. Ultrasound Localization Microscopy (ULM) enables imaging beyond the diffraction limit and can reveal coronary microvascular structures but is altered by large motion. This work addresses 3D non-rigid motion in coronary ULM to enable more accurate reconstruction and visualization of the coronary microvascular network. Approach: Here we developed a 3D non-rigid motion correction framework based on Optical Flow with Doppler velocity regularization to compensate for cardiac motion in 3D ULM of beating rat hearts. Main results: The method was validated in perfused ex vivo beating rat hearts imaged with ultrafast 3D ultrasound. Motion correction enhanced microbubbles tracking density and vessels delineation, enabling visualization of microvessels below 100 µm and velocity measurements with spatial resolution of at least 40 µm. Significance: This framework advances volumetric assessment of coronary microcirculation, facilitating longer acquisition windows and may support future cardiovascular research and clinical applications by overcoming motion-induced limitations in 3D coronary ULM.