A method for detecting echoes from microbubble contrast agents based on time-variance

Most ultrasound contrast agents, which are injected into the blood stream, use encapsulated microbubbles as acoustical scatterers in order to increase the reflectivity of the blood. The fact that the acoustic properties of microbubbles and those of scatterers in tissue differ significantly offers various possibilities to develop contrast agent specific imaging techniques. These new imaging modes may overcome the limitations of Doppler modes concerning their axial resolution and their sensitivity to motion artifacts and, therefore, improve the visualization of small vessels and the assessment of perfusion. Insonification can change the acoustic properties of bubbles, e.g., by breaking the shells and thus accelerating shrinkage and diffusion processes. Time-Variance Imaging (TVI), the new imaging mode the authors propose, detects these alterations by processing the echo signals resulting from sequences of broadband transmit pulses. TVI provides high resolution images which show the distribution of microbubbles.