Characterizing collagen microstructure using high frequency ultrasound

Abstract

Collagen is the most abundant extracellular matrix protein in mammals and is widely investigated as a scaffold material for tissue engineering. Collagen provides structural properties for scaffolds and, importantly, the microstructure of collagen can affect key cell behaviors such as cell migration and proliferation. This study investigated the feasibility of using high-frequency quantitative ultrasound to characterize collagen microstructure, namely, collagen fiber density and size, nondestructively. The integrated backscatter coefficient (IBC) was employed as a quantitative ultrasound parameter to characterize collagen microstructure in 3-D engineered hydrogels. To determine the relationship between the IBC and collagen fiber density, hydrogels were fabricated with different collagen concentrations (1–4 mg/mL). Further, collagen hydrogels polymerized at different temperatures (22–37°C) were investigated to determine the relationship between the IBC and collagen microfiber size. The IBC was computed from measurements of the backscattered radio-frequency data collected using a single-element transducer (38-MHz center frequency, 13–47 MHz bandwidth). Parallel studies using second harmonic generation microscopy verified changes in collagen microstructure. Results showed that the IBC increased with increasing collagen concentration and decreasing polymerization temperature. Further, we demonstrated that parametric images of the IBC were useful for assessing spatial variations in collagen microstructure within hydrogels.