Ultrasound contrast imaging with chirped excitation

Ultrasound contrast agents enhance echoes from the microvasculature and enable the visualization of flow in smaller vessels. Detection of contrast agents is a challenging task since it is difficult to differentiate contrast agent echoes from nonlinear components of tissue echoes. Coded excitation has been successfully used in medical ultrasound imaging to increase signal-to-noise ratio and penetration depth. The goal of this dissertation is to apply chirped excitation in ultrasound contrast agent imaging to increase signal-to-noise ratio and contrast-to-tissue ratio, achieve comparable resolution to conventional imaging and acceptable sidelobe levels, and improve biological safety.;Based on the fact that tissue can not produce subharmonic components resulting in good contrast-to-tissue ratio, subharmonic oscillation of microbubbles for high frequency chirped excitation was first investigated. Maximum and relative expansion was quantified as a function of initial diameter, determining the destruction threshold and demonstrating that using high frequency insonation was a safe method. Significantly more subharmonic components were observed for chirp insonation as compared to tone burst insonation over a range of transmission pressure, indicating that a single-pulse subharmonic strategy could be used to detect echoes from microbubbles.;In order to investigate the individual contrast agent oscillations and the resulting received echoes, a new simultaneous optical and acoustical system for observation of microbubbles was developed. With the new system, we evaluated the similarities and differences between echoes from increasing and decreasing frequency excitation, and the effect of diffusion on the quality of the predicted echoes. A new sophisticated microbubble detection technique was developed by designing pulsing schemes and exploiting any differences in the response to these excitations.;Based on the observation of the microbubble behavior for low frequency chirped excitation, two imaging methods were implemented with low transmitted intensities to reduce contrast agent destruction and tissue harmonics. The chirp excitation methods achieved higher contrast-to-tissue ratio than a tone burst method at the same low transmitted intensity. Axial resolution was degraded for chirped excitation compared with tone burst insonation, however a deconvolution process was proposed to improve the axial resolution and lower the sidelobe level.