3D super-resolution ultrasound imaging of rabbit lymph node vasculature in vivo using microbubbles

Background: Variations in lymph node (LN) microcirculation can be indicative of metastasis. Identifying and quantifying metastatic LNs remains essential for prognosis and treatment planning but a reliable non-invasive imaging technique is lacking. 3D super-resolution (SR) ultrasound has shown potential to noninvasively visualize microvascular networks in vivo.Purpose: To study the feasibility of 3D SR ultrasound imaging of rabbit lymph node (LN) microvascular structure and blood flow using microbubbles.Materials and Methods: In vivo studies were carried out to image popliteal LNs of two healthy male New Zealand White rabbits aged 6-8 weeks. 3D high frame rate contrast enhanced ultrasound was achieved by mechanically scanning a linear imaging probe. Individual microbubbles were identified, localized, and tracked to form 3D SR images and super-resolved velocity maps. Acoustic sub-aperture processing (ASAP)was used to improve image contrast and generateenhanced power Doppler (PD) and color Doppler (CD) images. Vessel size and blood flow velocity distributions were evaluated and assessed by Student’s paired t-test. Results:SR images revealed micro-vessels in the rabbitLN, with branches clearly resolved when separated by 30 μm, which is less than half of the acoustic wavelength and not resolvable by power or color Doppler. The apparent size distribution of most vessels in the SR images was below 80 μm and agrees with micro-CT data whereas most of those detected by Doppler techniques were larger than 80 μm. The blood flow velocity distribution indicated that most of the blood flow in the rabbit popliteal LN was at velocities lower than 5mm/s. Conclusion: 3D super-resolution ultrasound imaging using microbubbles allows non-invasive and non-ionizing visualization and quantification of rabbit lymph node microvascular structures and blood flow dynamics with resolution below the wave diffraction limit. This technology has potential for studying the physiological functions of the lymph system and for clinical detection of lymph node metastasis