ABSTRACT
New ultrasound-mediated drug delivery systems, such as acoustic cluster therapy or combined imaging and therapy systems, require transducers that can operate beyond the bandwidth limitation (~100%) of conventional piezoceramic transducers. In this article, a dual-frequency coupled resonator transducer (CRT) comprised of a polymeric coupling layer with a low acoustic impedance (2-5 MRayl) sandwiched between two piezoceramic layers is investigated. Depending on the electrical configuration, the CRT exhibits two usable frequency bands. The resonance frequency of the high-frequency (HF) band can be tailored to be ~3-5 times higher than that of the low-frequency (LF) band using the stiffness in the coupling layer. The CRT's LF band was analyzed analytically, and we obtained the closed-form expressions for the LF resonance frequency. A dual-frequency CRT was designed, manufactured, and characterized acoustically, and comparisons with theory showed good agreement. The HF band exhibited a center frequency of 2.5 MHz with a -3-dB bandwidth of 70% and is suited to manipulate microbubbles or for diagnostic imaging applications. The LF band exhibited a center frequency of 0.5 MHz with a -3-dB bandwidth of 13% and is suited to induce biological effects in tissue, therein manipulation of microbubbles.
ABSTRACT
New ultrasound imaging and therapeutic modalities may require transducer designs that are not readily facilitated by conventional design guidelines and analytical expressions. This motivates the investigation of numerical methods for complex transducer structures. Based on a mathematical theorem, we propose a new numerical design and optimization method for ultrasound transducers by linearizing the phase spectrum of transducer transfer functions. A gradient-based algorithm obtains the optimal transducer by varying a selected set of transducer parameters. To demonstrate the linear phase method, a simulated air-backed 4-MHz single-element imaging transducer with two matching layers, bondlines, and electrodes is optimized by varying the impedances and thicknesses of the matching layers. The magnitude spectrum resembles that of a Gaussian and, compared to a conventional transducer, the time-sidelobe level is reduced by more than 15-dB. Moreover, we apply the linear phase method to analyze and compensate for bondlines that resonate within the passband. Finally, we address the challenge of obtaining materials for the matching layers with the optimized impedance values by calculating alternative material pairs.
