On determination of focal laws for linear phased array probes as to the active and passive element size

Abstract

Ultrasonic linear phased array probes consist of several single transducers. By exciting each single transducer at its particular time wave fronts can be tilted, focused or both combined. The required set of delays is called "focal law". Hence, the shape of the resulting wave front depends signifcantly on focal law calculation. The typical single transducer in linear phased array probes has a rectangle shape with a width-to-length-ratio of approximately 0,1...0,5. The width of the short side is called "active aperture size", the larger one is called "passive aperture size". In state of the art calculations of the focal laws only the pitch between the single elements is considered. The elements are approximated with only a point source in the center of its aperture, that's why the real size geometry of the elements - neither the active size nor the passive size - has no further leverage. The CEFIT (cylindric elasto dynamic finite integration technique) in combination with PSS (point source synthesis) enables flexible and fast simulation of four-dimensional sound fields in homogeneous and layered half spaces, respectively. Thereby all wave physical effects like scattering and mode conversion will be considered. The calculations with different geometrical parameters for the probes show that both the active as well as the passive aperture size influence the time and frequency characteristic of the signal in the focus point. Based on the focal laws calculated with and without respect to the aperture size, sound fields were simulated for selected focus points. The results were qualitatively and quantitatively compared whereby the difference between both methods will be distinguishable. The feasibility of corrections for the focal laws with respect to time and frequency characteristics in the focus point as well as the improvement of resolution will be discussed. The results allow an estimation about the optimal width-to-length-ratio for the single transducers of linear phased arrays