Subharmonic generation from ultrasonic contrast agents

Ultrasonic contrast agents are used to enhance backscatter from blood and t hus aid in delineating blood from surrounding tissue. However, behaviour of contrast agents in an acoustic field is nonlinear and leads to harmonic co mponents in the backscattered signal. Various research groups have investig ated second-harmonic emissions. In this work, the subharmonic emission from contrast agents is investigated with a view towards potential use in imagi ng. It is shown that the microbubbles with various surface properties, such as contrast agents, generate significant subharmonics under various insona ting conditions. Theoretical results as well as experimental results using Optison(TM) indicate the generation of strong subharmonics with burst inson ation at twice the resonant frequency of the microbubble. It is suggested t hat subharmonic imaging may provide a better modality than second-harmonic imaging to delineate blood from tissue and will be of significant importanc e for imaging deep vessels, such as in echocardiography and vascular diseas es, due to the high signal-to-clutter ratio of the subharmonic imaging.