Abstract
The detection of microbubble contrast agents in the vessels of the microcirculation is currently limited by the relatively low concentrations attainable without causing acoustic attenuation, set against the high clutter signal imposed by surrounding soft tissue. By inducing nonlinear resonant oscillation in a population of microbubbles in transit in the circulation, echoes at the second harmonic can be detected preferentially, thus segmenting the blood signal from that due to tissue. Real-time harmonic imaging and Doppler have been implemented using a clinical array imaging system, and it is shown that with bubble agents it can detect flow in 40 μm vessels of the kidney. At higher incident peak pressures (above about 0.5 Mpa at about 3 MHz), some microbubble contrast agents are irreversibly disrupted by ultrasound. They then produce transient echoes which are high in amplitude and rich in harmonics. Furthermore, transient echoes can be obtained repeatedly over a period of milliseconds, allowing correlation imaging. Such ‘‘transient Doppler’’ imaging can demonstrate microcirculatory blood in the capillaries of the moving myocardium, a new development with significant clinical potential. [Work supported by Medical Research Council of Canada.]
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