In PW Doppler scanning of peripheral vessels, the positive one-way blood flow is detected clearly, but obvious mirror image spectrum can be found in the spectrogram. Reducing the transmitting sound power only reduces the forward and reverse blood flow spectra to the same extent, but does not make the ghost disappear. Only when the emission frequency is adjusted, the difference can be found. The higher the emission frequency, the more obvious the mirror image spectrum. As shown in the following figure, the blood flow spectrum in the carotid artery presents obvious mirror spectra. The energy of the negative blood flow mirror image spectrum is only slightly weaker than the positive blood flow spectrum, and the flow velocity is higher. Why is this?
Before the study of ghosts, let’s examine the beam of the ultrasound scan. In order to obtain better directivity, the beam of ultrasonic scanning needs to be focused by different delay control of multi-element. The ultrasonic beam after focusing is divided into main lobe, side lobe and gate lobe. As shown below.
The main and side lobes always exist, but not the gating lobes, that is, when the gating lobe angle is greater than 90 degrees, there is no gating lobes. When the gating lobe angle is small, the amplitude of the gating lobe is often much larger than the side lobe, and may even be the same order of magnitude as the main lobe. The side-effect of of grating lobe and side lobe is that the interference signal which deviates from the scan line is superimposed on the main lobe, which reduces the contrast resolution of the image. Therefore, in order to improve the contrast resolution of the image, the side lobe amplitude should be small and the gating lobe angle should be large.
According to the formula of the main lobe angle, the larger the aperture (W) and the higher the frequency, the finer the main lobe is, which is beneficial to the improvement of the lateral resolution of B-mode imaging. On the premise that the number of channels is constant, the larger the element spacing (g) is, the larger the aperture (W) will be. However, according to the formula of gating angle, the gating angle will also decrease with the increase of frequency (wavelength decreases) and the increase of element spacing (g). The smaller the gating lobe angle, the higher the gating lobe amplitude. Especially when the scanning line is deflected, the amplitude of the main lobe will decrease with the position of the main lobe deviates from the center. At the same time, the position of the gating lobe will be closer to the center, so that the amplitude of the gating lobe will further increase, and even make multiple gating lobes into the imaging field of view.
Post time: Feb-07-2022
