Verbeek X A, Ledoux L A, Willigers J M, Brands P J, Hoeks A P
Department of Biophysics, Cardiovascular Research Institute Maastricht, Maastricht University, The Netherlands.
J Acoust Soc Am. 2000 Apr;107(4):2281-90. doi: 10.1121/1.428508.
The application of ultrasound contrast agents aims to detect low velocity blood flow in the microcirculation. To enhance discrimination between tissue and blood containing the contrast agent, harmonic imaging is used. Harmonic imaging requires the application of narrow-band signals and is obscured by high levels of native harmonics generated in an intervening medium. To improve discrimination between contrast agent and native harmonics, a pulse inversion technique has been proposed. Pulse inversion allows wide-band signals, thus preserving the axial resolution. The present study examines the interference of native harmonics and discusses the practical difficulties of wide-band pulse inversion measurements of harmonics by a single transducer. Native harmonics are not eliminated by pulse inversion. Furthermore, only even harmonics remain and are amplified by 6 dB, alleviating the requirement for selective filtering. Finally, it is shown that the contaminating third harmonic contained in the square wave activation signal leaks through in the emitted signal. The spectral location of the contaminating third harmonic is governed by the transducer spectral characteristics while the location of the native and contrast agent second harmonics is not. Thus the contaminating third harmonic and the native and contrast agent second harmonics may overlap and interfere. Optimal discrimination requires a balance between maximal sensitivity for the second harmonic at reception and minimal interference from the contaminating third harmonic.
超声造影剂的应用旨在检测微循环中的低速血流。为了增强含造影剂的组织与血液之间的分辨能力，采用了谐波成像技术。谐波成像需要应用窄带信号，并且会被中间介质中产生的高水平固有谐波所干扰。为了提高造影剂与固有谐波之间的分辨能力，人们提出了脉冲反转技术。脉冲反转允许使用宽带信号，从而保持轴向分辨率。本研究考察了固有谐波的干扰，并讨论了用单个换能器进行宽带脉冲反转谐波测量的实际困难。脉冲反转并不能消除固有谐波。此外，只有偶数谐波会保留下来，并被放大6dB，这减轻了选择性滤波的要求。最后，研究表明方波激活信号中所含的污染性三次谐波会在发射信号中泄漏出来。污染性三次谐波的频谱位置由换能器的频谱特性决定，而固有谐波和造影剂二次谐波的位置则不然。因此，污染性三次谐波与固有谐波和造影剂二次谐波可能会重叠并相互干扰。最佳分辨需要在接收时对二次谐波的最大灵敏度与污染性三次谐波的最小干扰之间取得平衡。
