Smart Microstystem for Diagnostic Imaging of Vulnerable Plaque

Smart Microsystems for Diagnostic Imaging in Medicine (SMiDA) is a project funded by the Research Council of Norway from 2004-2008. The objective is to develop robust technologies for medical diagnosis, in particular for identification of vulnerable plaque. A large percentage of heart attacks are caused by rupture of vulnerable plaques leading to the formation of blood clots, coronary stenosis and infarction. Technologies for diagnosis are not available today. Methods are being investigated to identify chemical alterations in atherosclerotic tissue, to measure locally temperature, blood pressure and blood flow velocity, guided by 3D ultrasound imaging. The vision is to develop a smart sensor integrated in a catheter for intravascular imaging. Miniaturization is one of the main challenges of the project as the catheter can have a maximum diameter of 1mm. This tough requirement is being met by utilizing nanoscale CMOS technology. New technology is being developed to integrate the array of approximately 7500 capacitor micromachined ultrasound transducers (CMUTs) with the electronics. To identify vulnerable plaque it is estimated that a spatial resolution of 50-150µm is required. The center frequency and thereby the size of the ultrasound transducers determines the resolution. First iteration in the design of the CMUTs has resulted in the following parameters: 5.7µm radius, 100nm membrane thickness, vibrational amplitude in the picometer range (10-12m) and approximately 35MHz center frequency. New processing techniques have been developed to fabricate these minute structures. The CMUTs are fabricated by transfer of the silicon nitride membrane with a silicon dioxide layer to a silicon wafer with pre-etched cavities using hydrophilic fusion bonding. To characterize these structures state-of-the-art measurement techniques have been developed to meet the challenging requirements. A heterodyne interferometer has been built for non-destructive characterization of micro- and nanostructures with picometer sensitivity. The setup is designed to measure absolute amplitude and phase in the entire frequency range 0-1.2GHz. A 3D map of the object modulation can be obtained by scanning in the x- and y-direction with sub-micrometer precision. By combining phase data and absolute amplitude, various signal processing can be performed e.g. two dimensional Fourier analysis. Both single and arrays of CMUTs have been characterized to optimize the manufacturing process and the design. Nonuniform response in resonance frequency, phase and vibrational amplitude of individual CMUTs has been identified. The technique is powerful for quality control and identification of defects. The SMiDA project is an exciting multidisciplinary project requiring research in the field of medicine, physics, electronics and optics. Support from electronic and ultrasound industry as well as close contact with cardiologists has been of great value.