Sources of difference frequency sound in a dual-frequency imaging system with implications for monitoring thermal surgery

Abstract

(cont.) parametric effect, which can be considered an imaging artifact. Additionally, it may be possible to use the nonlinear interaction of scattered waves to form images that rely on the presence of small scatterers; a technique that may be enhanced with the use of contrast agents containing small scattering micro-bubbles in vivo.

This thesis explores the nature of the ultrasound-stimulated vibro-acoustography (USVA) imaging method introduced by Fatemi and Greenleaf in 1998. The USVA method relies upon the generation of a difference frequency signal from the interaction of two pressure fields with a target. A thorough understanding of USVA will be necessary to further advance this dual-frequency method. Prior studies demonstrate a correlation between difference frequency signal response and tissue temperature, and difference frequency signal response and tissue coagulation, suggesting that USVA may be well suited for monitoring focused ultrasound surgery. This thesis explores three possible sources of the difference frequency signal: 1) the parametric effect, 2) linear reflection of the local difference frequency field, and 3) nonlinear interaction of linearly scattered waves. The research compares the relative significance of these three possible sources using mathematical analysis, computer simulations, and experimental results. The results set forth in this thesis suggest that the parametric effect may be the most significant source of difference frequency signal, reaching pressures of 1-10 Pa and significantly overshadowing the other two enumerated effects. The second effect, the linear reflection of the local evanescent difference frequency field, is undetectable experimentally. Finally, the third effect, the nonlinear interaction of linearly scattered waves for a single bubble, contributes to the difference frequency signal only slightly, albeit detectably, reaching levels of .1-1 Pa. These results have a number of implications for future implementations of USVA. In order to utilize USVA as a successful imaging tool, one must take measures to avoid the signal from the