High Frame Rate Contrast-Enhanced Ultrasound Imaging for Slow Lymphatic Flow: Influence of Ultrasound Pressure and Flow Rate on Bubble Disruption and Image Persistence

Contrast enhanced ultrasound (CEUS) utilising microbubbles shows great potential for visualising lymphatic vessels and identifying sentinel lymph nodes (SLN) which is valuable for axillary staging in breast cancer patients. However, current CEUS imaging techniques have limitations that affect the accurate visualisation and tracking of lymphatic vessels and SLN. (1) Tissue artefacts and bubble disruption can reduce the image contrast. (2) Limited spatial and temporal resolution diminishes the amount of information that can be captured by CEUS. (3) The slow lymph flow makes Doppler based approaches less effective. This work evaluates on a lymphatic vessel phantom the use of high frame-rate (HFR) CEUS for the detection of lymphatic vessels where flow is slow. Specifically the work particularly investigates the impact of key factors in lymphatic imaging, including ultrasound pressure and flow velocity as well as probe motion during vessel tracking, on bubble disruption and image contrast. A trail was also conducted to apply HFR CEUS imaging on vasculature in a rabbit popliteal lymph node (LN). Our results show that (1) HFR imaging and SVD filtering can significantly reduce tissue artefacts in the phantom; (2) the slow flow rate within the phantom makes image contrast and signal persistence more susceptible to changes in ultrasound amplitude/MI, and an MI value can be chosen to reach a compromise between images contrast and bubble disruption under slow flow condition; (3) probe motion significantly decreases image contrast of the vessel, which can be improved by applying motion correction prior to SVD filtering; (4) the optical observation of the impact of ultrasound pressure in HFR CEUS further confirm the importance of optimising ultrasound amplitude MI; (5) Vessels inside rabbit LN with blood flow less than 3 mm/s are clearly visualised