驻波产生及消除

Fig. 6. Schematic diagram of the locations of the simulated four foci. The geometric focus of the phased array is located at the location, 3. The locationof sonication points, 1, 2, and 4 are [−12, −6, 6] mm from the geometric focus (3 ) on the acoustic axis, z.
Fig. 9. Comparison of the normalize pressure field measurements and sim ulation results. (a) 72 (apodization: 37.5%) element and (b) full (apodization: 100%) phased array.
Figure 4. Two-dimensional distribution of the ultrasound passing through a temporal bone fragment. The diagrams (a)–(d) are sinusoidal activation cases, and the diagrams (e)–(h) are RSBIC activation cases. The activating voltage was 50 Vpp. The other parameters are the same as those in Figure 3.
Fig. 10. Simulation results of sagittal views of the normalized pressure amplitude diቤተ መጻሕፍቲ ባይዱtribution inside the skull for eight different apodization levels when the focus is located at the geometric focus of the hemispherical phased array: (a) 12.5%, (b) 25.0%, (c) 37.5%, (d) 50.0%, (e) 62.5%, (f) 75.0%, (g) 87.5%,and (h) 100% apodization.
Figure 1. (a) A waveform of a sinusoidal wave. (b) The graph in the top panel is an example of a waveform of the RSBIC signal. The dotted line indicates the normal carrier, and the broken line indicates the inverse carriers. The RSBIC signal is generated by switching between the two carriers at random time intervals. The timing of the switching is determined by the zero-cross timing of the thermal noise, which is depicted in the bottom panel.
扫频信号-其输出的正弦波信号的频率随时间在 一定范围内反复扫描 随机调制信号-在输出信号上加入随机噪声信号 用以修饰
changing aperture size and f-number
Fig. 2.
Experimental apparatus for the pressure measurement in a human skull sample.
*Standing-Wave Suppression for Transcranial Ultrasound by Random Modulation.Sai Chun Tang∗, Member, IEEE, and Gregory T. Clement
(a)
(b)
(c)
Fig. 1. Frequency spectrum of the measured voltage of the transducer excited by the signals with (a) swept frequency, (b) random-signal modulation, and (c) single frequency without modulation.
驻波（Standing wave）：两个振幅、波长、周期 皆相同的正弦波相向行进干涉而成的合成波。此种 波的波形无法前进，因此无法传播能量，故名之。 由于节点静止不动，所以波形没有传播。能量以动 能和势能的形式交换储存，亦传播不出去。
v f
频率和振幅均相同、振动方向一致、 传播方向相反的两列波叠加后形成的 波。波在介质中传播时其波形不断向 前推进，故称行波；上述两列波叠加 后波形并不向前推进，故称驻波引。
To quantify the uniformity of the ultrasound intensity distribution, we propose a mathematical definition for UI. Here, we discuss the twodimensional case. The one-dimensional case is discussed in the Appendix. Let f(x,y) be the acoustic intensity (or pressure amplitude) of an ultrasound field on a twodimensional (x, y) plane. The two-dimensional uniformity index (UI) is defined by the following formula
Figure 3. Two-dimensional distribution of the ultrasound propagating freely in water. The diagrams, (a)–(e), are sinusoidal activation cases, and the diagrams, (f) –(j), are RSBIC activation cases. Z is the distance from the transducer surface. The activating voltage was 20 Vpp, and the carrier frequency was 500 kHz. Data were collected at intervals of 1 mm for both the X and Y directions. In the case of RSBIC activation, the intensity was averaged for 500 s. The lower cut-ofrequency of the noise was 50 kHz, and the upper cut-o frequency of the noise was 200 kHz. The uniformity indices are shown for each diagram.
Fig. 8. Normalized maximum pressure amplitude measurements in the YZ plane when the phased array focused at (0, 0, 0). (a) 72-element (apodization: 37.5%) array and full (Apodization: 100%) array (b) with or (c) without CTbased phase correction.