STUDY ON THE IMPROVEMENT OF IMAGE DEGRADATION FOR ULTRASONIC ECHOGRAPHIC IMAGE BY USING HIGHER ORDER HARMONICS

本研究致力于利用超声在生物组织中传播时产生的高次谐波来改善超声回声图像的图像退化问题，由于超声图像中的斑点斑纹引起的图像退化一直是一个问题，因此对这些斑纹进行抑制以获得更高质量的图像是一项重要的任务。为了有效地检测回波信号中包含的高次谐波分量，除了发射换能器外，还需要一个专门用于宽带接收的换能器。利用这种PZT-PVDF双层超声探头构建了超声诊断系统的机械扇形扫描器，建立了可实时成像的超宽带超声成像系统样机。利用PZT-PVDF双层超声机械扇形探头获取体模回波信号进行实时成像，证实了其从基波到四次谐波的高次分量对散斑抑制的有效性。在此基础上，研制了一种新型换能器，该换能器发射双频超声脉冲，接收生物组织的多频回波。当超声脉冲OFF和4F0进入生物组织时，产生2F0、3F0和5F0分量作为二次波。我们在外径17 mm的PZT环上同轴放置了直径为7 mm的PZT圆盘样机，环形和圆形换能器分别发射2 MHz和8 MHz的脉冲。证实了该换能器在脱气水中形成了4、6和10 MHz的声束以及2和8 MHz的声束。然后对琼脂凝胶模型和猪肉进行了机械扫描成像实验。因此，我们也证实了通过降低相干斑噪声来改善图像退化。

This study concentrates on the improvement of image degradation of ultrasonic echographic images by using higher order harmonics which are generated while ultrasound waves propagate through the biological tissues., Since image degradation caused by speckle patterns in the echographic image has been a problem, it is an important task to reduce these patterns to obtain higher quality images. In order to detect the higher harmonic components contained in the echo signals efficiently, a specialized transducer for broadband reception, besides the transmitting transducer is necessary. By constructing a mechanical sector scanner of an ultrasonic diagnosis system using such a PZT-PVDF two-layered ultrasonic probe, an ultra-broadband ultrasonic imaging system capable of a real time imaging was prototyped. Thus, the real time imaging was carried out by obtaining the echo signal from phantoms using a PZT-PVDF two-layered ultrasonic mechanical sector probe, and the effectiveness of its higher harmonic components from the fundamental to the fourth harmonic was confirmed in speckle reduction. Then we prototyped the novel transducer which transmits ultrasonic pulse of dual-frequency and receives multi-frequency echoes from the biological tissues. When the ultrasonic pulse off and 4f_0 are transmitted into the biological tissues, 2f_0, 3f_0, and 5f_0 components are generated as the secondary waves. We have prototyped the PZT disk of 7 mm in diameter coaxially arranged in the PZT ring of 17 mm in outer diameter The ring and circular transducers transmit the pulses of 2 and 8 MHz, respectively. It was confirmed that this transducer formed the ultrasonic beams of 4, 6 and 10 MHz as well as the beams of 2 and 8 MHz in degassed water. Then the experiments of imaging the agar-gel phantom and pork meat were carried out by mechanical scanning. As a result, we also confirmed the improvement of image degradation by reducing the speckle noise.