Assessment of Side Lobe Estimation Accuracy Using Side Lobe Free Image

Abstract

In order to effectively suppress side lobes in ultrasound imaging, it is necessary to quantify the signal amplitude due to side lobes at imaging points. The received channel data before receive beamforming are modeled as a sum of sinusoids having different spatial frequencies depending on the angle incident on a receiving array. In the signal constituting the channel data, the signal due to the main lobe has a DC frequency, whereas the signal due to the side lobe has a spatial frequency that is approximately proportional to the incident angle. Taking the Fourier transform of the received channel data after zero padding, we can accurately estimate side lobes. In computer simulation of ultrasound images, we generate ultrasonic echoes returned from random scatterers. Because we already know the position of all reflectors, we can separate the signals reflected due to the main and side lobes so that the true side lobe level at each imaging point can be computed accurately. The main lobe image can be used as a gold standard for assessing a side lobe suppression filter. The true and estimated side lobes can be used in side lobe suppression filtering of ultrasound images. We obtained the conventional, true side lobe, and estimated side lobe images for an object containing a wire and a cyst using computer simulation in a 64 channel focusing system with a 5 MHz linear array transducer. In the hypoechoic cyst, the estimated side lobe is almost the same as the true side lobe. We confirmed that the estimated side lobe can effectively be used in side lobe suppression filtering. Therefore, it is feasible to quantitatively estimate side lobes from the channel data, and improve the performance of the side lobe suppression filter in ultrasound imaging.