A Two-Element Annular Array of Short Fixed Focal Length for Dynamic Focusing

Abstract

A two-element annular array for dynamic focusing has been designed. It is combined with a lens of short, fixed focal length and is to be used in a mechanically scanned transducer for simultaneous 2D cardiographic echo imaging and 2D doppler flow chart mapping. The goal has been to minimize the number of elements, since it is difficult to make annular arrays with many elements having high sensitivity and equal impulse responses. High sensitivity is needed for the Doppler application and equal impulse responses are needed for dynamic focusing in the echo imaging application. A study of the on-axis spatial impulse response has led to a formula for the required number of annular elements. It has also suggested that one should combine the array with a lens of short fixed focal length to obtain best results in dynamic focusing. To test this new design principle experimentally we have made a two-element transducer and measured the impulse response and sensitivity of each element as well as the transient beam pattern. The measurements are in good agreement with calculations based on the transmission line model and the spatial impulse response method. The array produces a narrow beam close to the diffraction limit in the desired depth range.