Ultrasound harmonic imaging with contrast agents is an extremely useful method for observing blood perfusion in hard to image areas such as fine capillaries in the organs of the abdomen and cardiac imaging through the ribs Nonlinear acoustic scattering from microbubble contrast agents leads to 2 and subharmonics that are less attenuated than linearly scattered signals, exhibit higher lateral resolution due to reduced side lobes, and are less susceptable to artifacts such as color blooming and acoustic shadowing These effects make it possible to detect slow, small volume blood flow in the presense of the much more prevalent tissue, whose echo overwhelms the blood echo at the fundamental frequency. However, since the 2nd harmonic occurs at 2 times the fundamental and the subharmonic at 1/2 the fundamental transmitted frequency, transducers used for harmonic imaging must have very broad bandwidths (typically 150% or greater for both sub and 2nd harmonic imaging) To date bandwidths of approximately 100% can be achieved with 1-3 composite transducers, high impedance acoustic backings, and/or multiple matching layers Unfortunately all these methods also cause reduced transducer sensitivity Higher bandwidths with good sensitivity can be obtained with multilayer transducers, but this is difficult to implement for arrays Single crystal piezoelectrics which exhibit very high electromechanical coupling coefficients offer a means of producing harmonic imaging transducers with inherently large bandwidths For this Phase I SBIR program the feasibility of using single crystal transducers for harmonic imaging will be demonstrated by constructing devices with bandwidths >140% and testing their sensitivity to harmonic signals generated by constrast agents dispersed in water The transducers will be fabricated from 1-3 crystal-polymer compoites with moderate to high impedance backing layers and multiple matching layers Such structures are known to have larger bandwidth with equivalent or better sensitivity than conventional ceramic based transducers The proposed effort will be a collaboration between TRS Technologies, Blatek, Inc, Acoustic Sciences Associates, and Thomas Jefferson University.
