Enhanced Contrast Detection Using Ultrasonic Pulse Inversion Imaging

Abstract

Ultrasonic contrast agents have been widely used to enhance the detection of blood. Most contrast agents are microbubble-based and act as harmonic oscillators when the impinging sound wave is near the bubble’s resonant frequency. An important application of ultrasonic contrast agents is to enhance the contrast between normal and diseased tissue. Contrast agents are also used to outline vessels and heart chambers. Despite the great potential, clinical applications of contrast agents are often limited because the tissue also produces significant echoes, thus degrading the contrast-to-tissue ratio. This paper reports pulse-inversion (PI) fundamental imaging for the enhancement of contrast detection. The PI technique involves two firings with inverted waveforms. When the returning echoes from the two firings are summed, the residual signal from the tissue is limited to even-order harmonics, but the fundamental signal from the bubbles is not cancelled completely, because the bubbles’ reaction under compression is different from that under rarefaction. The experimental results from PI imaging indicate that the contrast-to-tissue ratio is significantly enhanced compared to either conventional fundamental imaging or second-harmonic imaging. However, the performance of the nonlinear contrast detection is degraded when the frequency of the impinging sound wave is far from the bubble’s resonant frequency. The PI fundamental technique is highly sensitive to tissue motion, because the fundamental tissue signal is not cancelled in the presence of motion. We concluded that PI fundamental imaging is effective for contrast detection if the bubble’s resonant frequency is properly selected and corrections are made for motion.