1. A method for detection and imaging of ultrasound contrast agent in a region of soft tissue, comprising: transmitting at least two ultrasound pulse complexes toward said region, each of said at least two ultrasound pulse complexes includes a low frequency (LF) pulse and a high frequency (HF) pulse with the same or overlapping beam directions and that overlap in time at least for part of an image depth, wherein at least one of a frequency, an amplitude and a phase of said LF pulse relative to said HF pulse varies for each transmitted pulse complex; receiving scattered signals from the HF pulse component of each of the at least two ultrasound pulse complexes, the scattered signals being scattered by the ultrasound contrast agent in the region of the soft tissue; and time delay adjusting and combining the received scattered signals from at least two of said HF pulse components to provide image signals that essentially contain components scattered from contrast agent micro bubbles with strong suppression of signal components scattered from tissue.