Adaptive bi-directional point-wise focusing method in ultrasonic imaging based on virtual source

Adaptive bi-directional point-wise focusing method in ultrasonic imaging based on virtual source is proposed to improve the resolution and contrast when the depth increases. Firstly, the ultrasonic transducer array is divided into sub-arrays and the ultrasonic RF scanning lines are collected using fixed-focus transmitting and receiving method. Secondly, these focuses are considered as virtual sources and the time delays between these virtual sources and the spatial points are calculated. The ultrasonic scanning lines are point-wise focused again using synthetic aperture method and weighted by the adaptive coherence factor. Then the high-quality image is obtained. Simulations based on spatial impulse response theory are used to verify the method. The imaging simulations for point and speckle targets are used to quantify the resolution and contrast. It proves that when the F number is 1.5 and the focus is at 10 mm (such a sub-aperture corresponds to only 17 elements of array), the imaging qualities at different depths are consistent for the proposed method and equal to that of fixed-focus transmitting and dynamic depth receiving method based on 64-channel imaging system. The phantom imaging experiment proves that the proposed method can obviously improve the imaging quality. The consistent resolution and contrast of the conventional imaging system can be obtained at all imaging depths and the image is much clearer on the whole.