5G-4 Micro-Electro-Mechanical Systems (MEMS) Based Focused Ultrasound Transducers for High Resolution Second Harmonic Imaging Applications

Improvement of resolution in ultrasound based devices for tissue characterization has been of real interest in the field of minimally invasive imaging such as intravascular ultrasound. Harmonics obtained from the nonlinear properties of the tissue improve resolution and reduce artifacts. Unfortunately, the harmonics produced from the conventional unfocused transducers lie outside the arteries and hence the advantages with harmonic imaging cannot be utilized. A promising way where harmonics can be made to occur inside the arteries is by developing Micro-Electro-Mechanical Systems (MEMS) based focused transducers. Broadband (fractional bandwidths of 75%), high frequency (35-45 MHz) focused transducers were developed using polyvinylidene fluoride-trifluoroethylene (PVDF TrFE). Low pass (33 MHz cut off) and High pass (35 MHz cut off) filters were designed and developed for second harmonic imaging applications in pulse-echo mode. Second harmonic axial radiation patterns have been modeled for focused and unfocused sources and the focused source showed higher harmonic peak pressures. Second harmonic axial radiation patterns for various f-number transducers showed that the harmonic peaks occurred at corresponding focal lengths proving the possibility of obtaining harmonic images from desired locations by adjusting the focal length of the transducer. The second harmonic beam at 40 MHz for a 1 mm transducer was measured in pulse-echo mode and compared with the fundamental beam. Along the focal plane, the transmitted harmonic beam had a beam width of 65 mum whereas the fundamental had a beam width of 90 mum. The experimental values agree very closely with the theoretical calculations that the harmonic beam width should be 1/radic2 times the fundamental beam width. In order to demonstrate contrast enhancement using harmonics, silicon MEMS phantom consisting of fixed bars and spaces alternately with widths ranging from 20 mum to 160 mum was scanned using 20 MHz fundamental beam. It was shown that the harmonic beam exhibited better contrast, especially for 40 mum and 60 mum bars with about 4 dB and 7 dB greater signal strength compared to fundamental. Tissue harmonic imaging capabilities of the transducers were also displayed via successful in vitro imaging of a section of human aorta. The harmonic images provided additional information in comparison to fundamental. This work illustrates that focused transducers produce higher harmonic peak pressures and that the harmonics can be caused to occur as a function of focus. These results coupled with the demonstration of high-resolution, high-contrast harmonic images leads us to believe that this could be an ideal candidate for next generation second harmonic imaging applications