超聲中的諧波HARMONIC技術
二次諧波可以去除旁瓣效應,增加圖象的對比分辨率，特別是對空腔臟器效果尤佳,。
二次諧波是超聲在組織傳播過程中由于組織的非線性產(chǎn)生的,。二次諧波的產(chǎn)生強度與組織的粘性、密度有直接的關系,，所以對比分辨率可以得到顯著的提高,。空腔臟器如膽囊,、膀胱,、子宮內(nèi)容物是液體，可以認為是無粘性的,，這些液體不產(chǎn)生二次諧波回聲,，所以應用二次諧波后實質組織圖像質量提升更為明顯。
旁瓣是伴隨主瓣存在的,，主瓣就是我們用來成像的聲束,，旁瓣聲束也會帶來回聲,，但這種回聲是我們不希望看到的，也就是噪聲,。在二次諧波模式下,，由于旁瓣的聲強比較小，不足以產(chǎn)生諧波,、而且諧波在聲源附近也不能產(chǎn)生,。綜合這幾點，二次諧波就可以明顯的抑制旁瓣噪聲了,。
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.
簡單的說,，由于非線性的關系，基波中含有很多雜亂的信號,，而主要由心臟瓣膜等低振幅高強度器官信號產(chǎn)生的二次諧波信號相對純凈的多,，里邊雜波少，但是信號也非常微弱,，因此對機器的要求也高,，需要機器有高的靈敏度，大的動態(tài)范圍,。
