Effects of Phospholipid Shell on the Generation of Second Harmonic of Ultrasound Contrast Agent Microbubble

Second harmonic imaging is the principle technique used in ultrasound contrast agent (UCA) diagnosis. Quality of second harmonic images is limited by contrast to tissue ratio (CTR). Techniques for improving the generation of contrast harmonic are helpful in increasing the CTR of contrast images. The purpose of this study is to exploit the effect of compressible shell on the second harmonic response due to ultrasonic excitation. Recent studies show that UCA 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 nonlinearity of UCA microbubble. Effects of the shell viscosity, shell elasticity and the derivative of shell elasticity on the generation of UCA second harmonic are presented using the analytical solution of the bubble dynamic equation. It is shown that the CER provides a common ground for gauging the effect of different shell parameters on UCA harmonic generation.