High frequency ultrasound for imaging and characterization of tissue engineered heart valves

Tissue-engineering heart valve equivalent (VE) structures are currently being developed for replacement of defective valves, especially in juvenile patients where mechanical valves have limited use. High-frequency (HF) transducers covering the range of 5--40 MHz are used to investigate the ultrasonic imaging characteristics of VEs under a variety of conditions. In particular, VE samples were imaged using a mechanically-scanned 20 MHz transducer to obtain 3D data sets for characterization of integrity and for building 3D computer models for flow channel simulation. In addition, a dual-element focused transducer was designed, fabricated, and tested for localized measurement of the viscoelastic properties of the VEs, potentially in situ. The confocal dual-element transducer is made of a 5 MHz highly focused pushing transducer capable of generating tissue displacements on the order of several micrometers using millisecond-long pulses. A shell transducer is used to monitor the tissue displacement in response to the pushing pulse along the axis of the transducer assembly. Initial experimental results indicate the feasibility of this measurement for actual VE samples as well as thin layers of tissue-mimicking phantoms. We have also tested the feasibility of real-time imaging of the VE samples using a small-part linear array probe. Initial tests have shown that a 2D speckle tracking of VE tissue undergoing uniaxial stretching is feasible. This thesis research addresses the design and implementation of algorithms and HF ultrasonic apparatus for characterization of VEs and other fine tissue structures with potential applications in vivo.; Studies on aortic heart valves in humans and other species suggest a universal design principle leading to a common layered structure suited for the mechanical environment in which they function. While it may not be necessary for VEs to mimic the structure of native valves, they must provide similar functionality under the same mechanical stresses in vivo. Therefore the elastic properties of VEs must be measured under realistic conditions related to their expected operating conditions. In addition, recent research efforts have proposed the use of 3D computer simulations (based on measured elastic properties) for guiding design improvements for enhancing the VE performance in vivo.