Optimization of a phased array transducer for multiple harmonic imaging in medical applications: Frequency and topology
Matte, Guillaume × Van Neer, Paul Danilouchkine, Mike Huijssen, Jacobus Verweij, Martin D. De Jong, Nioc #
IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control vol:58 issue:3 pages:533-546
Second harmonic imaging is currently one of the standards in commercial echographic systems for diagnosis because of its high spatial resolution and low sensitivity to clutter and near-field artifacts. The use of nonlinear phenomena mirrors a great set of solutions to improve echographic image resolution. To further enhance the resolution and image quality the combination of the third to fifth harmonics – dubbed the superharmonics – could be used. However, this requires a bandwidth exceeding that of conventional transducers. A promising solution features a phased array design with interleaved low and high frequency elements for transmission and reception respectively. As the amplitude of the backscattered higher harmonics at the transducer surface is relatively low, it is highly desirable to increase the sensitivity in reception. Therefore, we investigated the optimization of the number of elements in the receiving aperture as well as their arrangement (topology). A variety of configurations was considered, including one transmit element for each receive element (1/2) up to one transmit for 7 receive elements (1/8). The topologies are assessed based on the ratio of the harmonic peak pressures in the main and grating lobes. Further, the higher harmonic level is maximized by optimization of the center frequency of the transmitted pulse. The achievable signal-to-noise-ratio (SNR) for a specific application is a compromise between the frequency dependent attenuation and nonlinearity at a required penetration depth. To calculate the SNR of the complete imaging chain, we use an approach analogous to the SONAR equation used in underwater acoustics. The generated harmonic pressure fields due to nonlinear wave propagation were modeled with the Iterative Nonlinear Contrast Source (INCS) method, the KZK or the Burger’s equation. The optimal topology for superharmonic imaging was an interleaved design with 1 transmit element per 7 receive elements. It improves the SNR by ~5 dB compared to the interleaved (1/2) design reported by Bouakaz et al. (Bouakaz, ten Cate et al. 2004) and van Neer et al. (van Neer, Matte et al. 2010). The optimal transmission frequency for superharmonic echocardiography was found to be 1.0 – 1.2 MHz. For superharmonic abdominal imaging this frequency was found to be 1.7 – 1.9 MHz. For second harmonic echocardiography the optimal transmission frequency of 1.8 MHz reported in the literature was corroborated with our simulation results.