|
二次谐波可以去除旁瓣效应,增加图象的对比分辨率，特别是对空腔脏器效果尤佳。
二次谐波是超声在组织传播过程中由于组织的非线性产生的。二次谐波的产生强度与组织的粘性、密度有直接的关系，所以对比分辨率可以得到显著的提高。空腔脏器如胆囊、膀胱、子宫内容物是液体，可以认为是无粘性的，这些液体不产生二次谐波回声，所以应用二次谐波后实质组织图像质量提升更为明显。
旁瓣是伴随主瓣存在的，主瓣就是我们用来成像的声束，旁瓣声束也会带来回声，但这种回声是我们不希望看到的，也就是噪声。在二次谐波模式下，由于旁瓣的声强比较小，不足以产生谐波、而且谐波在声源附近也不能产生。综合这几点，二次谐波就可以明显的抑制旁瓣噪声了。
Tissue Harmonic Echo Tissue Harmonic Echo is a technique to pick up only the second harmonic (twice higher ultrasound frequency) for imaging. If the transmission frequency (fundamental frequency) of the probe is f0, the returned echo includes many harmonics (2fo or so) which are generated in the tissue. The system picks up only the second harmonic as others are negligible. The following merits are expected from the Tissue Harmonic Ech ?As the harmonic is generated only inside the body, influence of reverberation (multiple echoes) between the probe AND the skin, some organs AND vessels can be reduced. This leads to clear noise-free image at the shallow part of the image.?As the level of the harmonic is proportional to the square of the sound pressure, side lobe is greatly smaller than the main lobe. This also leads to clear noise-free image in the near field AND inside the heart cavity.?For the same reason as the above second point, the main lobe is much sharper, which results in improved spatial resolution AND contrast resolution.
f Fundamental wave 2f 2nd harmonic wave To take the above merits into consideration, imaging ability AND clearness of image for what is called "difficult patients" (obese people, the aged, heavy smokers) will increase. On the other hAND, the Tissue Harmonic Echo has the following demerit. Please take this in mind when you demonstrate it. ?As the power of the second harmonic is lower than that of the basic frequency, sensitivity decreases a little.
简单的说，由于非线性的关系，基波中含有很多杂乱的信号，而主要由心脏瓣膜等低振幅高强度器官信号产生的二次谐波信号相对纯净的多，里边杂波少，但是信号也非常微弱，因此对机器的要求也高，需要机器有高的灵敏度，大的动态范围。
| 
