A method of acoustic imaging that potentially can improve the diagnostic capabilities of medical ultrasound is presented. The method, given the name SURF (Second order UltRasound Field) imaging, is achieved by processing the received signals from transmitted dual frequency band pulse complexes with at least partly overlapping high frequency (HF) and low frequency (LF) pulses. The transmitted HF pulses are used for image reconstruction, whereas the transmitted LF pulses are used to manipulate the elastic properties of the medium observed by the HF imaging pulses. The present paper discusses fundamental concepts in relation to the use of dual frequency band pulse complexes for medical ultrasound imaging.