Microbubbles consist of gas encapsulated by a shell. When they are exposed to an ultrasound field, they start oscillating during insonation, i.e., the microbubbles are compressed in the positive pressure phase and expand in the negative pressure phase. The compression against the encapsulated gas is smaller than the expansion, which creates a response (echo signal) which is not symmetrical (in linear relation to the insonation wave).

This is in particular the case, if the oscillation is in the range of the resonance frequency of the microbubbles, resulting in a strong non-linear response.