超声谐波成像原理简介

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                    <section><section powered-by="gulangu"><section><section><p><img src="image/20201019/f8901522e84aa92184da4de60b65ec14_1.gif" /></p></section></section></section><section powered-by="gulangu"><section><section> <section> <section powered-by="gulangu"><section><section><p>谐波的概念</p></section></section></section> </section> </section></section></section><section powered-by="gulangu"><section><section><p>&nbsp; &nbsp; &nbsp; &nbsp;非正弦周期函数经过傅里叶变换，可以展开为常数与一系列具有共同周期的正弦函数（或余弦函数）之和。这一系列正弦函数中，频率等于原函数频率的称为基波；其余正弦函数的频率分别为基波频率的整数倍，称为高次谐波；基波频率二倍的正弦波称为二次谐波，依此类推。</p><p>&nbsp; &nbsp; &nbsp; &nbsp;组织内小振幅条件时超声线性传播，声场内任何距离上的质点都重复生源的振动规律。但当超声波不满足小振幅条件时将产生非线性传播，致使波形发生畸变，随传播距离增加必然伴随谐波的产生。因此，即使探头发射单纯频率的正弦波，经组织的非线性传播后也会波形畸变，产生谐波成分。与基波成分相比，谐波强度非常低，越是高次谐波，振幅越低。</p><p>&nbsp; &nbsp; &nbsp; &nbsp;传统的超声成像接收和发射相同频率的超声波，称为基波成像。而提取二次甚至高次谐波成分成像，可提高组织的分辨能力，减少干扰及伪像，改善图像质量，称为谐波成像（harmonic imaging）。对于各种方式的基波成像，均可扩展为谐波成像，例如二维灰阶、多普勒血流、能量多普勒及组织多普勒成像等。从临床应用区分，目前主要有造影剂谐波成像（contrast agents harmonic imaging）和自然组织谐波成像（native tissue harmonic imaging）。</p></section></section></section><section powered-by="gulangu"><section><section> <section> <section powered-by="gulangu"><section><section><p>造影剂谐波成像</p></section></section></section> </section> </section></section></section><section powered-by="gulangu"><section><section><p>&nbsp; &nbsp; &nbsp; 造影剂微泡除了散射基波外，在超声作用下发生振动而产生谐波，随外加声压不断增加微泡的非线性运动会更加复杂。谐波成像时通过带通滤波只提取二次谐波信号进行成像，由于造影剂微泡与周围组织声学特性差异较大，加上共振散射，造影剂微泡可产生丰富的二次谐波信号，能有效地抑制不含造影剂组织的基波信号（背景噪声），突出显示造影剂灌注区。</p></section></section></section><section powered-by="gulangu"><section><section> <section> <section powered-by="gulangu"><section><section><p>自然组织谐波成像</p></section></section></section> </section> </section></section></section><section powered-by="gulangu"><section><section><p>&nbsp; &nbsp; &nbsp; 超声波传播过程中受到组织的非线性作用而产生的谐波信号，被超宽频探头接收、提取并成像，称为自然组织谐波成像。</p><p>&nbsp; &nbsp; &nbsp; 由非线性传播产生的谐波信号有两个特点：</p><p>&nbsp; &nbsp; &nbsp; 1、基波强度随传播距离增加而线性衰减，但谐波强度随传播距离的变化却是非线性的；组织谐波产生于超声的传播过程中，因此谐波强度随着传播距离的增长而增加，直到传播距离产生的衰减作用占优势为止，超声成像中的干扰和伪像主要来源于体表或接近于体表的信号，这些信号因超声波传播距离短而只含有较少的谐波成分，采用滤波器滤除基波信号时这部分干扰也被消除，有利于浅表部位图像质量的提高。</p><p>&nbsp; &nbsp; &nbsp; 2、谐波能量与基波能量并非线性关系，旁瓣强度远低于主瓣强度，主瓣的谐波能量较强，但弱的基波却几乎没有谐波成分，旁瓣强度远低于主瓣强度，主瓣的谐波能量较强，但旁瓣却几乎没有谐波能量，因此，谐波成像时，旁瓣伪影干扰更少。</p></section></section></section><section powered-by="gulangu"><section><section> <section> <section powered-by="gulangu"><section><section><p>超声设备性能要求</p></section></section></section> </section> </section></section></section><section powered-by="gulangu"><section><section><p>&nbsp; &nbsp; &nbsp; 非线性现象产生的组织谐波很微弱，即使在最佳的环境下其能量也远远小于基波能量。因此，谐波成像依赖于接收系统的灵敏度和处理的先进性：</p><p>&nbsp; &nbsp; &nbsp; &nbsp;1、设备必须有足够宽的动态范围，因为谐波成像会损失10dB~20dB的信号强度，为保持信噪比，要有足够宽的动态范围接收这种相当弱的信号。</p><p>&nbsp; &nbsp; &nbsp; &nbsp;2、超宽频探头准备发射和接收宽频带信号。</p><p>&nbsp; &nbsp; &nbsp; &nbsp;3、高性能的滤波器和信号处理技术，仅使谐波频率通过。</p></section></section></section><section powered-by="gulangu"><section><section><p><br  /></p></section></section></section><section powered-by="gulangu"><section><section> <section powered-by="gulangu"><section><section><p>欢</p></section></section></section> </section><section> <section powered-by="gulangu"><section><section><p>迎</p></section></section></section> </section><section> <section powered-by="gulangu"><section><section><p>关</p></section></section></section> </section><section> <section powered-by="gulangu"><section><section><p>注</p></section></section></section> </section></section></section><section powered-by="gulangu"><section><section><p><br  /></p></section></section></section><section powered-by="gulangu"><section><section> <section powered-by="gulangu"><section><section><p><img src="image/20201019/9483a7d5bfcc9cbcb14a3083ac30b699_2.jpg" /></p></section></section></section> </section><section> <section powered-by="gulangu"><section><section> </section></section></section> </section><section> <section powered-by="gulangu"><section><section><p>关注我们</p><p>一起涨姿势！</p></section></section></section> </section></section></section></section>