Dual frequency contrast ultrasound angiography

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Acoustic angiography is a contrast imaging approach in which the superharmonics associated with low frequency excitation of microbubbles are received and processed over a broad bandwidth at a much higher frequency. As demonstrated by Dayton and colleagues, this technique facilitates the visualization of the microvasculature with a resolution typical of micro computed tomography and with superior contrast-to tissue-ratios than conventional nonlinear processing. To date dual frequency contrast imaging has been performed with a 2- 4 MHz annulus coaligned with 25-30MHz single element focused transducer operated by a Vevo770 (Visualsonics) scanner. This technology has limitations (mechanical scanning, single focus, limited focal depth) which hinder its clinical use but which could be alleviated using dualfrequency arrays (DFA). In this presentation we first describe the performance and applications of mechanical acoustic angiography and early results from the development of dual frequency array devices for acoustic angiography. The logical extension to array based designs is then explored. We built prototype dual frequency devices in both a vertical and horizonal topology. The horizonal topography consisted of a central 256 element high frequency array (HF, 20 MHz) flanked by two single element low frequency transducers(LF, 2 MHz) that extended for the length of the HF array. The vertical topography consisted of a 32-element LF transmitter array positioned behind a 256-element HF receiver array. Both arrays were built from piezoelectric composites. The HF backing material, between the two piezoelectric layers in the vertical design is weakly and highly attenuating at 2 MHz and 20 MHz, respectively. Two matching layers on the LF array and 4 matching layers on the HF array were used to improve transmission throughout the whole array structure. An elevation lens was added to focus the receive component at around 9 mm. Impedance and hydrophone measurements were performed to evaluate both configurations. Examples of super harmonic contrast imaging for each of the above configurations will be shown for phantom studies and preliminary in vivo images of animal models will be presented.