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Ultrasound and photoacoustic imaging to monitor vascular growth in tissue engineered constructs

Quantitative and qualitative monitoring of neovascular growth is required in many vascular tissue engineering applications. For example, the contribution of progenitor cells in growing microvasculature has been demonstrated; however, the process of vascularization from progenitor cells is not well understood. Therefore, there is a need for an imaging technique that is consistent, easy to use, and can quantitatively assess the dynamics of vascular growth or regression in a three-dimensional environment. In this study, we evaluate the ability of combined ultrasound and photoacoustic imaging to assess the dynamics of vascular growth. The experiments were performed using hydrogels that spontaneously promote tube formation from implanted mesenchymal stem cells (MSCs). Specifically, PEGylated fibrin gels, supporting the development of capillary growth were implanted in a Lewis rat. After one week, the rat was euthanized and the gel implants were excised and positioned in water cuvettes for imaging. Simultaneous ultrasound and photoacoustic images were obtained using single-element, focused ultrasound transducers interfaced with a nanosecond pulsed laser source. To image samples, ultrasound transducers operating at either 25 MHz or 48 MHz and interfaced with laser sources operating at either 532 nm or within 680-800 nm wavelengths were used. The 3-D ultrasound and photoacoustic images were acquired by mechanically scanning the transducer over the region of interest and capturing spatially co-registered and temporally consecutive photoacoustic transients and ultrasound pulse-echo signals. The ultrasound and photoacoustic images agree well with the overall anatomy and vascular structure in the gel samples. The results suggest that the photoacoustic and ultrasound imaging could be used to sequentially monitor the growth of neovasculature in-vivo.