The Study Of Ultrasound Harmonic Imaging

With the development of physics, ultrasonics, electronics, medicine, great progress were made in ultrasound imaging in the recent years. Due to its relative low price, no electromagnetic radiation, no destruction, multiple imaging method, ultrasound imaging has been widely used in medical imaging. Early ultrasound imaging were all based on the principles of linear acoustics. However, in the applications of sonography, acoustic wave’s nonlinear propagation in tissue and contrast bubble’s nonlinear oscillation will generate nonlinear acoustic signals (harmonic signals). In conventional linear ultrasound imaging, the nonlinear echo signal is of no use, even harmful. However, since the late1990s, a new imaging method based on the harmonic components emerged, namely the harmonic imaging. Plenty of simmulation and experimental researches indicated that harmonic imaging can provide better spatial resolution and contrast resolution, thus improving the image quality. Now harmonic imaging becomes the standard method in commercial B-mode imaging. The intensity of harmonic components is much weaker than the fundamental. The low signal-to-noise ratio will degrade the image. Medical diagnosis needs both higher image quality and enough signal intensity to acquire clear image, so that the accuracy can be ensured. So, how to improve both the image quality and signal-to-noise ratio became an important issue in the recent years.In this paper, several harmonic imaging methods have been studied. First, an experiment of second harmonic imaging on a tissue mimick phantom is conducted. Basically band pass filtering method is used to extract second harmonic components for imaging. To the low signal-to-noise ratio of second harmonic imaging, a new imaging method, sectional fast fourier transform imaging is proposed and investigated. In the sectional fast fourier transform imaging, each echo line was divided into many parts, and fast fourier transform was done on each parts. The value of second harmonic in frequency domain of each part is used to form a new matrix for imaging. Results proved that second harmonic imaging has better image quality than fundamental imaging. The sectional fast fourier transform imaging slightly degraded the lateral resolution, but its signal-noise-ratio, penetration depth were improved obviously, and this method has the potential to be applied in higher harmonics.We also made research about the pulse inversion technique both theoretically and experimentally. The image acquired by pulse inversion technique has been compared with that acquired by ordinary second harmonic imaging. The relative motion of the transducer surface and the tissue will bring motion artifacts in pulse inversion technique, and it has a great influence on the image quality. We investigated this issue and made a correction by computing the correlation coefficient to decrease the motion artifacts. Results indicated that pulse inversion can both improve signal-to-noise ratio, decrease speckles and ripples, and near field artifacts were reduced further. Comparing with the band pass filtering second harmonic imaging, pulse inversion received better image and relative motion apparently degraded the image quality.