Nonlinear ultrasound imaging of nanoscale acoustic biomolecules
Ultrasound imaging is widely used to probe the mechanical structure of tissues and visualize blood flow. However, the ability of ultrasound to observe specific molecular and cellular signals is limited. Recently, a unique class of gas-filled protein nanostructures called gas vesicles (GVs) was introduced as nanoscale (∼250 nm) contrast agents for ultrasound, accompanied by the possibilities of genetic engineering, imaging of targets outside the vasculature and monitoring of cellular signals such as gene expression. These possibilities would be aided by methods to discriminate GV-generated ultrasound signals from anatomical background. Here, we show that the nonlinear response of engineered GVs to acoustic pressure enables selective imaging of these nanostructures using a tailored amplitude modulation strategy. Finite element modeling predicted a strongly nonlinear mechanical deformation and acoustic response to ultrasound in engineered GVs. This response was confirmed with ultrasound measurements in the range of 10 to 25 MHz. An amplitude modulation pulse sequence based on this nonlinear response allows engineered GVs to be distinguished from linear scatterers and other GV types with a contrast ratio greater than 11.5 dB. We demonstrate the effectiveness of this nonlinear imaging strategy in vitro, in cellulo, and in vivo.
© 2017 American Institute of Physics. Received 31 October 2016; accepted 29 January 2017; published online 17 February 2017. The authors thank Dan Piraner for assistance with the GV phantom mold design, and Stuart Foster and Emmanuel Chérin for helpful discussions. This research was supported by the National Institutes of Health (R01-EB018975). D.M. is supported by the Human Frontiers Science Program Cross-Disciplinary Postdoctoral Fellowship (Award No. LT000637/2016). A.L. is supported by the NSF graduate research fellowship (Award No. 1144469). Y-L.N. is supported by the Taipei Veterans General Hospital / National Yang-Ming University Excellent Physician Scientists Cultivation Program, No. 103-Y-A-003. Research in the Shapiro laboratory is also supported by the Heritage Medical Research Institute, Burroughs Wellcome Career Award at the Scientific Interface, the Pew Scholarship in the Biomedical Sciences, and the Packard Fellowship for Science and Engineering.
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