Contrast-specific ultrasound techniques.

The advent of microbubble contrast agents has determined an important evolution of ultrasound (US) technology due to the introduction of contrast-specific US techniques. This was due to the fact that neither colour or power Doppler are suitable for correct management of the signals produced by microbubble insonation, as they are limited by the heavy presence of artefacts. Microbubbles may be insonated by a characteristic frequency named resonance or fundamental frequency (f (0)) by using a high or low transmit power. If insonated by a high transmit power, microbubbles produce a wideband harmonic signal due to microbubble destruction. If insonated by a low transmit power, microbubbles produce harmonic frequencies (2f, 3f, 4f) due to their nonlinear physical behaviour. Contrast-specific US techniques have recently undergone an important technical development with the introduction of innovative algorithms able to register selectively the harmonic signals produced by microbubbles and to suppress the signal produced by stationary tissues. The different contrast-specific US techniques may be distinguished by their basic principle into pseudo-Doppler, harmonic, phase-modulation, amplitude-modulation and phase-and amplitude-modulation techniques.