Evaluating <i>in vivo</i> tissue stiffness of capillaries using Microbubbles Under an Ultrasound Field

Introduction: We developed a new technique to study in vivo biomechanical properties of capillaries using microbubbles (MBs), ultrasound, and high-speed microscopic imaging. We hypothesized that diabetes stiffens capillaries and that MB acoustic behaviors within capillaries will change as wall stiffness changes. Methods: Type 1 diabetes was induced in rats using streptozotocin. Cremaster muscle was externalized in anesthetized rats. Definity® was intra-arterially injected (40–60 μl boluses). Capillaries were imaged microscopically at 8–12 Mfps, during delivery of one 6–15 cycle ultrasound pulse [f = 1 MHz, PNP = 0.5–2.0 MPa]. Capillary and MB diameters were obtained from image analysis and power spectra were derived. Stress–strain curves were generated using normal stress exerted by MB (derived from the linearized Euler’s equation) and circumferential vessel Green strain. Results: MBs in diabetic capillaries (n = 3) displayed greater subharmonic/fundamental power ratio (p = 0.03) and ultraharmonic/fundamental power ratio (p = 0.01) compared with MBs in healthy capillaries (n = 28). Preliminary elastic modulus analysis (n = 9 control and n = 2 diabetic) showed estimated elastic moduli of 1.7 MPa for healthy and 2.2 MPa for diabetic capillaries. Conclusions: Our preliminary data suggest that the MB frequency spectrum under the US may differ in diabetic capillaries, which may provide a basis for a novel approach to diagnose microvascular disease.