1. A method for measuring a non-linear response from a target using an imaging apparatus, said apparatus comprising a transmitter, a transducer having a plurality N of excitable transducer elements excitable by the transmitter, a beamformer configured to control a focus for transmit and receive signals from the transducer elements, a summing/differencing module responsive to the beamformer and configured to determine a non-linear response, and a display configured to display a representation of the determined non-linear response; said method comprising: transmitting a signal towards a region of interest from the transducer at a first amplitude and measuring a first response thereto; transmitting, from the transducer towards the region of interest, at least one additional signal of the same phase as the first amplitude signal and at one or more different, lower amplitudes, and measuring at least one additional response thereto, wherein the at least one additional signal is transmitted using a random arrangement of subaperture transducer elements; determining a difference of a function of said measured at least one additional response from the first response to determine a non-linear response of the object of interest; and displaying a representation of the non-linear response on the display.