Bouakaz Ayache, Frigstad Sigmund, Ten Cate Folkert J, de Jong Nico
Experimental Echocardiography, Erasmus University, Rotterdam, The Netherlands.
Ultrasound Med Biol. 2002 Jan;28(1):59-68. doi: 10.1016/s0301-5629(01)00460-4.
For ultrasound contrast agents (UCA), nonlinear imaging now has become fundamental. All of the current contrast-imaging methods are dominantly based on the nonlinear response of UCA bubbles. The discrimination between the perfused tissue and the UCA is the challenge in the field of UCA-imaging. This differentiation is usually associated or expressed by the ratio of the scattered power from the contrast agent to the scattered power from the tissue and is termed "contrast-to-tissue ratio" (CTR). Second harmonic imaging showed a better discrimination between tissue and UCA than fundamental imaging because of a higher CTR. We demonstrate, in this study, that the CTR increases as a function of the order of the harmonic frequency. Currently, due to the limited bandwidth of the transducers, only the second harmonic is selectively imaged, resulting in images with a superior quality to fundamental images, but still degraded and not optimal because of the harmonic generation in the underlying tissue (due to nonlinear propagation) and hence giving a limited CTR. To increase the CTR and to take advantage of the higher harmonics (third, fourth, fifth and the ultraharmonics and termed here super harmonics), we have developed a new phased array transducer. The array transducer contains two different types of elements arranged in an interleaved pattern (odd and even elements). The total number of elements is 96. The elements can operate separately and at a distinct frequency, enabling separate transmission and reception modes. The odd elements (48) operate at typically 2.8 MHz center frequency and 80% bandwidth. The even elements (48) have a center frequency of 900 kHz with a bandwidth of 50%. In vitro measurements using the dual frequency probe show an increase of 40 dB in the CTR for super harmonic components over the conventional second harmonic system. The increase in CTR is in agreement with the calculations using existing models for the response of encapsulated bubbles and known theory of nonlinear propagation. Animal experiments have demonstrated the feasibility of this approach using commercially available UCA and showed a similar increase of the CTR.
对于超声造影剂（UCA）而言，非线性成像如今已成为基础。当前所有的造影成像方法主要都基于UCA微泡的非线性响应。灌注组织与UCA之间的区分是UCA成像领域的一项挑战。这种区分通常通过造影剂的散射功率与组织的散射功率之比来关联或表示，被称为“对比组织比”（CTR）。由于CTR更高，二次谐波成像在组织和UCA之间显示出比基波成像更好的区分度。在本研究中，我们证明CTR随谐波频率阶数的增加而增大。目前，由于换能器带宽有限，仅选择性地对二次谐波进行成像，从而得到质量优于基波图像的图像，但由于底层组织中的谐波产生（由于非线性传播），图像仍然会退化且并非最优，因此CTR有限。为了提高CTR并利用更高的谐波（三次、四次、五次谐波以及此处称为超谐波的超谐波），我们开发了一种新型相控阵换能器。该阵列换能器包含以交错模式排列的两种不同类型的元件（奇数和偶数元件）。元件总数为96个。这些元件可以单独运行且频率不同，实现单独的发射和接收模式。奇数元件（48个）通常在中心频率2.8 MHz和80%带宽下运行。偶数元件（48个）的中心频率为900 kHz，带宽为50%。使用双频探头进行的体外测量表明，与传统二次谐波系统相比，超谐波分量的CTR增加了40 dB。CTR的增加与使用现有封装微泡响应模型和已知非线性传播理论的计算结果一致。动物实验已经证明了使用市售UCA这种方法的可行性，并显示出CTR有类似的增加。
