MAE PhD Defense – Sibo Li

TITLE: Micromachined Piezoelectric Material and Dual-layer Transducers for Ultrasound Imaging (Advisor: Dr. Jiang)

DATE: Tuesday, April 11, 2017

TIME & LOCATION: 10:30 AM   EB3 – 3115

ABSTRACT:

Medical ultrasound has been one of the main-stream diagnostic modalities, which are widely used in cardiology, obstetrics, gynecology, prostate evaluation, and blood vessels assessment. Two emerging areas of development within medical ultrasound are high-frequency intravascular ultrasound (IVUS) and microbubble contrast agent superharmonic imaging. Advances in both areas have been limited by difficulties in developing transducer arrays for good imaging performance. Arrays ultrasound are required for both researches to overcome the trade-off between resolutions, depth of field and frame rate in the single-element transducers. The problem encountered when developing arrays for high-frequency imaging is the piezoelectric material development which greatly influence the imaging performance. The problem encountered with the array ultrasound for superharmonic imaging is the broad bandpass requirement for fundamental transmission and superharmonic detection. The purpose of this thesis is to investigate and evaluate novel designs of arrays for high-frequency circular array for intravascular ultrasound and dual-frequency collinear array for microbubble superharmonic imaging that overcome the problems associated with conventional arrays. The designs are investigated using theoretical models based on piezoelectric material and transducer geometry to predict the imaging performance of the arrays.

For high-frequency ultrasound, micromachined single crystal 1-3 composites exhibits superior coupling coefficient (k t ~ 0.76), a decent acoustic impedance (Z ~20 MRayl), and a broad band response (FWHM bandwidth ~80%). Those properties showed great potential as an active material for transducer development. In this study, a 40 MHz micromachined PMN-PT 1-3 composite circular array was designed, fabricated and characterized for IVUS applications. The feature size of single crystal pillars was 18 μm in diameter. The kerf between pillars was less than 4 μm. A 50-element circular array transducer (radially outwards) with the pitch of 100 μm was wrapped around a needle resulting in an outer diameter of 1.7 mm. The array test showed that the center frequency was 39±2 MHz and −6-dB fractional bandwidth was 82±6%. The insertion loss was −41 dB, and crosstalk between adjacent elements was −24 dB.  A radial outward imaging testing with phantom wires (D = 50 μm) was conducted. The image was in a dynamic range of 30 dB to show a penetration depth of 6 mm by using the synthetic aperture method. The −6 dB beamwidth was estimated to be 60 μm in the axial direction at 3.1 mm distance away from the probe. The results suggest that the 40 MHz micromachined 1-3 composite circular array is promising for intravascular ultrasound imaging applications.

Ultrasound contrast agent based superharmonic imaging is the second topic discussed in the thesis. Based on the nonlinear responses of the microbubbles, conventional ultrasound with single frequency and limited bandpass cannot achieve the fundamental frequency transmission and superharmonic detection. To meet this requirement, the development of the array ultrasound with multi-center-frequency is proposed for the microbubbles superharmonic imaging. The probe consists of 64 transmit elements with a center frequency of 3 MHz and 128 receive elements with a center frequency of 15 MHz. The dimensions of the array are 18 mm in azimuth and 8 mm in elevation. The pitch is 280 μm for transmitting (TX) elements and 140 μm for receiving (RX) elements. Pulse-echo test of TX/RX elements and the acoustic field for focal beam were conducted and compared with the simulation results. Real-time contrast imaging was carried out using a Verasonics system on a tissue-mimicking phantom. Non-linear responses from microbubble contrast agents at a depth of 30 mm were clearly observed. The in-vivo animal imaging demonstrated the ability to detect individual vessels underneath the skin of a rat. These results indicate the potential use of the co-linear array for acoustic angiography imaging of prostate tumor and identification of regions of neovascularization for the guidance of prostate biopsies.

BIOGRAPHY:

Sibo Li received his B.S. degree in mechanical engineering from University of Science and technology of China in 2008, his M.S. in mechanical engineering from Stevens Institute of Technology in 2012. After that, He joined Dr. Xiaoning Jiang’s group in North Carolina State University. His current research majored in array transducer design and fabrication for prostate acoustic angiography, piezoelectric composite micromachined ultrasound transducers (PC-MUT) and tissue harmonic imaging.