SUBJECT: Ph.D. Proposal Presentation
BY: Toby Xu
TIME: Friday, April 25, 2014, 10:00 a.m.
PLACE: Love Building, 109
TITLE: Material and Array Design for CMUT Based Volumetric Intravascular and Intracardiac Ultrasound Imaging
COMMITTEE: Dr. F. Levent Degertekin, Chair (ME)
Dr. Oliver Brand (ECE)
Dr. Peter J. Hesketh (ME)
Dr. Karim Sabra (ME)
Dr. Todd Sulchek (ME)


Recent advances in medical imaging have greatly improved the success of cardiovascular and intracardiac interventions. This research aims to improve capacitive micromachined ultrasonic transducers (CMUT) based imaging catheters for intravascular ultrasound (IVUS) and intra-cardiac echocardiography (ICE) for 3-D volumetric imaging through integration of high-k thin film material into the CMUT fabrication and array design. CMUT-on-CMOS integration has been recently achieved and initial imaging of ex-vivo samples with adequate dynamic range for IVUS at 20MHz has been demonstrated; however, for imaging in the heart, higher sensitivities are needed for imaging up to 4-5cm depth at 20MHz and deeper at 10MHz. Consequently, one research goal is to design 10-20MHz CMUT arrays using integrated circuit (IC) compatible micro fabrication techniques and optimizing transducer performance through high-k dielectrics such as hafnium oxide (HfO2). This thin film material is electrically characterized for its dielectric properties and thermal mechanical stress is measured. Experiments on preliminary test CMUTs show a +6dB improvement in receive (Rx) sensitivity, and +6dB improvement in transmit sensitivity in (Pa/V) as compared to a CMUT using silicon nitride isolation layer. Transfer of the process to CMUT-on-CMOS is currently on-going work. On the array design side, a novel array configuration developed in this study addresses the limitations of forward looking IVUS array structures in imaging arterial cross sections accurately due to the limited forward looking acoustic beam angle. The possibility of achieving side looking imaging using the same forward looking CMUT IVUS array is explored for this purpose. The use of finite element analysis (FEA) simulations confirm the existence of evanescent acoustical cross-talk waves near the surface of the transducer that is near half the frequency (12.5MHz) of the bulk acoustic beam in the forward looking direction (20MHz). Proposed work will also investigate dual frequency volumetric imaging with 3 separate arrays: 20 and 40 MHz Rx and a broadband 30MHz Tx array. In these designs, better control of cross-talk and isolation of multiple CMUT arrays will be investigated by adding rigid barrier structures and “soft” periodic inter-element membranes.