自動合焦振動子の指向特性の解析

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The usefulness of ultrasonics as a diagnostic medical tool is now an established fact. There have been, however, some limitations on spatial resolution, that is, fine azimuthal resolution can be obtained in sacrifice of target range, consequently some kind of compromise between them has been carried out in the design of ultrasonic transducers. In order to overcome this difficulty, electronic focusing of the ultrasonic beam has been introduced either on transmission or on reception. When focusing on reception, a dynamic focusing arrangement could be implemented to achieve virtually perfect focus at each range in turn with only a single transmitted pulse. The purpose of this paper is to analyze the directional pattern of dynamic focusing transducers (abbreviated by DFT) and to give some instructions for the design of DFT. The outline of the DFT analyzed in this paper is shown in Fig. 1, where an annular array of ultrasonic transducers is used to receive the reflected ultrasonic signals and an element of the annular array is used to transmit the ultrasonic pulses. The radii of the annular transducers are given by α√<i> (i=1, 2, …, n), where α is a constant and i is the number labeled to the transducer and n is the number of transducers. The received signals from the transducers are passed through the phase shifters so as to obtain coincidence of the phases of the received signals and the sum of the phase-shifted signals is used as the output of the DFT. The axial intensity of the DFT is shown in Fig. 2, where β-kα^2/4r_0 is used as a parameter (k:wavenumber of ultrasonic waves, r_0:radius of curvature of the DFT), and the target range (taken as the width between -20 dB points) of the curves in Fig. 2 is arranged in Fig. 3 (b), where the maximum target range is obtained in case of β=1. 5 if the total area of the DFT is kept constant. The half power width of the DFT is shown in Fig. 4, where the abscissa shows (n+2i_T-1)^<-1/2> (i_T is the number of the transducer used as the transmitter, 1≤i_T≤n), consequently a smaller half width is obtained if a larger valve of i_T is adopted. The sidelobe level of the DFT is shown in Fig. 5, where the parameter u=kα^2 (1/r-1/r_0)/2 is shown along the abscissa, and as can seen from these figures, it is possible to reduce the sidelobe level by properly choosing the number of the transmitting transducer. The influence of quantization on axial intensity, half power width, and sidelobe level is shown in Figs. 6〜8 respectively. In these figures, m shows the quantized number of the shifted phase and ⊿u shows the interval between the switching of the phase shifters and the maximum deviation of the directional characteristics due to the quantization is plotted in these figures.
社団法人日本音響学会の論文
1976-06-01

自動合焦振動子の指向特性の解析

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