Quantitative characterization of viscoelasticity of microbubbles in ultrasound contrast agent

Abstract

Ultrasound contrast agent (UCA) microbubbles have been commonly used in clinic to enhance the acoustic backscattering signals in ultrasound imaging diagnosis. With increasing demand for the continuous improvement of imaging resolution and sensitivity, new type UCAs (e.g., targeted microbubbles and multifunctional microbubbles) have attracted growing interest in both medical and scientific communities. Many efforts have been made to modify microbubble shell properties, which can strongly affect microbubble dynamic behaviors, so as to enable to create some new functionalities of UCAs. However, accurate characterization of the shell mechanical properties of UCAs has been recognized to be rather challenging. In previous work, microbubble’s mechanical properties are normally estimated by fitting measured dynamic response signals with coated-microbubble models. Inevitable uncertainty will be introduced in fitting results because there are more than one unknown shell parameters are adopted in these dynamic models. In the present paper, a comprehensive approach is developed to quantitatively characterize the visco-elasticity of the encapsulated microbubbles. By combining the techniques of atomic force microscopy (AFM), single particle optical sensing (SPOS), acoustic attenuation measurement, and the coated-bubble dynamics simulation, the size distribution, shell thickness, shell elasticity and viscosity of UCA microbubbles are determined one by one in sequence. To examine the validity of this approach, a kind of albumin-shelled microbubbles with diameters ranging from 1 to 5 μm are fabricated in our lab. Based on AFM technology, the microbubble effective shell stiffness and bulk elasticity modulus are measured to be 0.149±0.012 N/m and 8.31±0.667 MPa, respectively. It is noteworthy that the shell elastic property is shown to be independent of the initial size of microbubbles. Furthermore, the size distribution and acoustic attenuation measurements are also performed of these bubbles. Then, combined with microbubble dynamic model simulations, the UCA shell viscosity is calculated to be 0.374±0.003 Pa·s. Compared with previous estimation method, the current technology can be used as an effective tool to assess UCA shell visco-elasticity with improved accuracy and certainty. It is also shown that the feasibility to optimize the design and fabrication of UCAs can satisfy different requirements in ultrasound diagnostic and therapeutic applications.