A method for ascertaining the presence of target-bound microbubbles in the context of ultrasound molecular imaging is taught. This method, referred to herein as dynamic scaling ultrasound molecular imaging, relies upon the time-varying behavior contrast agents within a region expressing a molecular imaging target and that within a reference region. Ultrasound contrast agents compositions that enable use of the method are also taught. The method is useful for the use of ultrasound molecular imaging in diagnosing and monitoring treatment.