The present invention relates to a new composition comprising gas-filled microvesicles for contrast imaging which are particularly suitable for providing an effective echo response to at least two selected ultrasound waves having different frequencies. Said composition preferably comprises at least two different preparations of gas-filled microvesicles having respective peaks of non-liner echographic response differing by at least 2 MHz to each other, and preferably have respective size distributions with different mean sizes. In particular, said preparations preferably have size distributions with respective DV50 values differing from each other by at least 0.5 μm, more preferably at least 1.0 μm. Alternatively, said composition has a volume size distribution showing a value of Bowley skewness of 0.16 or higher. According to a preferred embodiment, at least 95% of the total volume of gas contained in said microvesicles, calculated on the population of microvesicles up to a diameter of 10 μm, is contained in microvesicles having a diameter of 8 micron or less.