Ultrasound Imaging using the Low-Frequency Response of Microbubbles

When microbubbles are excited by two frequency signals, a nonlinear response signal may occur at the difference frequency. As these two frequencies are made close, this nonlinear response will reside at lowfrequency region. We have derived a theory for the weak short-pulse response based on a two-frequency approximation, which provides the pressure and the bandwidth dependence properties of the low-frequency response. Only microbubbles can produce this nonlinear response, therefore, it can provide good distinguishing ability between microbubbles and surrounding tissues in medical imaging. Because it is a short-pulse (i.e., wideband) response whose bandwidth is similar to that of the fundamental response, the axial resolution is close to that of the fundamental response and is better than the sub-harmonic response. It suffers less attenuation effect than second-harmonic response so it has better image quality especially in deeper region. Another advantage is due to that this response is a low-pressure response, contrast to other high-pressure harmonic imaging. We demonstrate these properties thorough experimental results, and use the signal-to-interference ratio (SIR) to denote power difference between microbubbles and surrounding tissues, which can be utilized as a discrimination index. In shallow region, the SIR can reach 3.79 dB in the fundamental response and 16.27 dB in the low-frequency response. It can be shown that the low-frequency image can produce improvement of 12.48 dB than the fundamental image. In deep region, another improvement of 1.79 dB can be achieved.