P3P-7 Modeling of Lens Focused Piezoelectric Transducer for Medical Imaging

In previous work on acoustical lens effect, a model based on the annular decomposition of an axisymmetric focused piezoelectric transducer was described and found to be in agreement with FEM and experimental results. The design of a focused transducer must take into account both radiation and transduction properties. These complex models such as FEM, FDTD and PSTD methods take into account radial modes, but their influence is not significant compared to the thickness mode when the radius to thickness ratio of the piezoelectric source is high enough. This assumption was verified since the mean error was evaluated lower than 2% for the chosen lens materials and geometries. Therefore, an efficient transfer function to model the lens was proposed. This annular decomposition allows to determine the input acoustical impedance of the lens and it is shown that the lens can be considered as a semi-infinite propagation medium. For an experimental comparison, a transducer structure based on a 1-3 piezocomposite is chosen for its high thickness coupling factor k_t = 64% and low acoustical impedance Z = 14 MRa. This piezoelectric source is damped by an epoxy backing and matched to the propagation medium with two quarter wavelength matching layers. Based on this structure, different types of focusing techniques are investigated: geometrical, concave lens with high velocity material and convex lens with low velocity material. Experimental results obtained on the manufactured transducers are finally compared to simulated ones and the performance of the different focusing strategies is discussed