High-frequency ultrasound for microcirculation in metastatic mouse lymph node: contrast-free versus contrast-enhanced imaging.

Focal defects are one of the important features for the diagnosis of lymph node (LN) metastasis. In our previous study, an accurate method for detecting contrast agents was proposed. However, conventional B-mode and contrast-enhanced images via ultrasound contrast agents (UCAs) have the limitations of contrast and spatial resolution to visualize the microcirculation, such as focal defects to distinguish between benign and malignant LN. In the present study, we have developed a novel method based on clutter filtering with singular value decomposition (SVD) analysis using time-integrated amplitude envelope (TIAE) in high-frequency 40 MHz ultrasound for high contrast resolution of the microcirculation in LN tissue. A mouse LN was visualized in vivo without and with UCA to compare the contrast enhancement. A metastatic LN model was established with LM8-luc cells of C3H/HeJ-lpr/lpr and MXH54/Mo-lpr/lpr mice. Bioluminescence imaging and pathological observations were also conducted to evaluate tumor growth. It was found that clutter-filtered contrast-enhanced images with UCA could visualize the feature of the microcirculation in the control LN and focal defects in the metastatic LN. Consistent with histological findings of disrupted architecture and cellular heterogeneity, whereas clutter-filtered B-mode images without UCA failed to visualize the vascular circulation. TIAE provided images with high noise resistance, and the calculated vascular area in the LN showed a decreasing trend in the metastatic group compared to the control group. Our framework enables robust visualization and quantification of LN heterogeneity in microcirculation.