超音波對比劑聲學反應之優化

Abstract

The phospholipid coated microbubbles are widely used for ultrasound contrast imaging. The quality of imaging strongly depends on the resonance frequency of bubbles. When the transmitted frequency matches the resonance frequency of a contrast agent, conditions are optimal for producing higher scattering, which has been demonstrated to improve the contrast to tissue ratio (CTR). Therefore, a significant factor in ultrasound contrast imaging is knowledge of the resonance frequencies of bubbles. Usually, the natural frequency of a bubble is inversely proportional to its radius. However, in previous studies showed if a bubble size is smaller than 10 μm and its encapsulating material has high viscosity, its resonance frequency is very different from its natural frequency due to the high viscous damping coefficient. In order to research how the resonance frequencies of bubbles are affected by viscous damping over the whole range of microbubble sizes accompanied by the acoustic pressure increasing, we simulate the bubble dynamic oscillation for sizes varying between 1 to 4 μm with the Marmottant model. It showed the pressure-dependent resonance only occurred in bubble sizes larger than Rm (the bubble size at the maximal resonance frequency), and on the opposite side, at bubble sizes smaller than Rm it does not appear. While a bubble size that is smaller than Rm, and which has a larger and flatter spectrum without thresholding effect. The bubbles cease to resonate due to total damping larger than square root of 2. The understanding of the influence of shell parameters on the resonance frequency of lipid coated bubbles, which may aid to optimize pulse echoes for ultrasound contrast imaging. Second harmonic imaging is the principle technique used in the ultrasound contrast agent diagnosis. Quality of second harmonic images is limited by CTR. Techniques for improving the generation of the contrast harmonic are helpful in increasing the CTR of contrast images. Therefore, this study proposed the effects of the compressible shell on the second harmonic response due to ultrasonic excitation. Recent studies show that the ultrasound contrast agent with phospholipid shell may have compression-only behavior, which may improve the generation of harmonic responses due to the compressibility of shell. Analytical simulations were performed for understanding the influence of the compression-only behavior on the generation of second harmonic. A CER (Compression to Expansion Ratio) index is proposed to quantify the degree of the nonlinearity of microbubbles. Effects of the shell viscosity, shell elasticity and the derivative of shell elasticity on the generation of microbubble second harmonic are presented using the analytical solutions of the bubble dynamic equation. It is shown that the derivative of the effective shell elasticity is positively correlated with CER and SCS (Scattering Cross-Section) of second harmonic, and the CER provides a common ground for gauging the effect of different shell parameters on ultrasound contrast agent harmonic generation.