Preliminary Experience with High-Resolution 3D Lymphangiovenulography: The First Success in Video Recording of the Lymphatic Pumping Using Photoacoustic Imaging in Man

Purpose: Photoacoustic imaging (PAI), which is based on photoacoustic (PA) technology, is an optical imaging that can image the distribution of light absorbing tissue components like hemoglobin or melanin, or optical absorption contrast imaging agents like Indocyanine green (ICG), with high spatial resolution.¹ The visualization of lymphatic system in mice with PAI technique has been demonstrated in previous report,² however, human lymphatic vasculature has not been visualized because the penetration depth was limited.
Recently, our group has reported PAI analysis of tumor-associated vasculatures in human breast cancer³ or palmar blood vessels.⁴ In this report, we introduce a new imaging technique of PA lymphangiovenulography to visualize human lymphatic vessels in three dimensions in detail.

Materials and Methods: We used the PAI-05 system with semi-spherical detector array, which was made by Canon Inc. (Japan), Hitachi, Ltd.(Japan) and Japan Probe Co, Ltd.(Japan). To image the lymphatic structures of the limbs in 4 healthy subjects, ICG was administered subcutaneously dorsal aspect of each foot or hand (some web spaces). A PA image was acquired by irradiating the tissue using a laser at wavelengths of near-infrared region. The voxel size was 0.125 mm.

Results: In the still images, the lymphatic vessels up to the diameter of 0.2 millimeters could be observed three-dimensionally with the blood vessels around them. In the videos, it was observed that lymphatic fluid including ICG was transported by spontaneous contraction of the collecting lymph vessels. The flow was observed intermittently with various intervals. The velocity of the flow was also varied from subject to subject. Lymph flow tended to be faster in the upper limbs than in the lower limbs.

Conclusions: In this study, three-dimensional high spatial and temporal resolution PA images were obtained using the PAI-05 system, allowing the visualization of fine lymphatic vasculature and its pumping movement . The system is a promising tool for more precise quantitative assessment of the pumping frequency and the velocity in the collecting lymphatic vessels in lymphedema patients or subclinical subjects.

Acknowledgement: This work was funded by the ImPACT Program of the Council for Science, Technology and Innovation (Cabinet Office, Government of Japan).

References:
1. Zackrisson S, van de Ven SMWY, Gambhir SS. Light In and Sound Out: Emerging Translational Strategies for Photoacoustic Imaging. Cancer Res. 2014;74(4):979-1004.
2. Forbrich A, Heinmiller A, Zemp RJ. Photoacoustic imaging of lymphatic pumping. J. Biomed. Opt. 2017;22(10), 106003. doi: 10.1117/1.JBO.22.10.106003.
3. Toi M, Asao Y, Matsumoto Y, et al. Visualization of tumor-related blood vessels in human breast by photoacoustic imaging system with a hemispherical detector array. Sci Rep. 2017;7(2):41970. doi: 10.1038/srep41970.
4. Matsumoto Y, Asao Y, Yoshikawa A, et al. Label-free photoacoustic imaging of human palmar vessels: a structural morphological analysis. Sci Rep. 2018;8(1):786. doi:10.1038/s41598-018-19161-z.