Sidelobe reduction in high frame rate echocardiography

Abstract

The measurement of propagation of spontaneous vibration of the heart would be useful for evaluation of myocardial function and viscoelasticity. For this purpose, we have developed high frame rate echocardiography over 300 Hz using parallel receive beamforming with diverging beam transmission (PBFDB). However, sidelobe level increased due to the use of unfocused transmit beams. In the present study, a method for reduction of sidelobe level was developed. After applying delays to the signals received by transducer elements in receive beamforming, there is an almost linear relationship between the residual phase of the signal and element's lateral position when the signals are come from an echo from a scatterer very near to the focal point (in the direction of main lobe). On the other hand, there is not a linear relationship when the signals come from a scatterer away from the focal point (in the directions of sidelobes). In the present study, such difference in the signal phases was evaluated by the magnitude squared coherence function (MSCF) of the received signals for all the pairs of two neighboring elements. When the phases of received signals change linearly (echo from main lobe), the difference between signal phases of two neighboring elements are constant across the array. In such cases, MSCF becomes 1. MSCF decreases (minimum is 0) when the phase differences are not constant (echo from sidelobe). Therefore, by weighting beamformed RF signals using MSCF, echoes come from sidelobes can be suppressed. Sidelobe levels were evaluated using fine nylon wires. Using MSCF as a weighting function, the sidelobe level could be reduced significantly. The average sidelobe level of PBFDB with MSCF was lower than that of conventional beamforming (CB) by 4.8 dB. In in vivo measurement of the heart of a 23-year-old healthy male, the image contrast was significantly improved.