1. A broadband intravascular ultrasound imaging system, comprising: a broadband ultrasound transducer configured to transmit ultrasound pulses towards a target, wherein the broadband ultrasound transducer comprises a micromachined piezoelectric composite body having a front side and a back side opposite to the front side, wherein the micromachined piezoelectric composite body is configured to convert electrical signals into ultrasound waves to be transmitted from the front side toward the target, and wherein the micromachined piezoelectric composite body is configured to convert the received echo signals into electrical signals, the micromachined piezoelectric composite body including a plurality of piezoelectric posts separated from each other by a plurality of filler members; and a dematching layer connected to the back side of the micromachined piezoelectric composite body and having higher acoustic impedance than an acoustic impedance of the micromachined piezoelectric composite body, the dematching layer not including the plurality of piezoelectric posts and the plurality of filler members; and a processing subsystem operatively coupled to the broadband ultrasound transducer and configured to: convolve one or more base ultrasound pulses corresponding to a particular frequency with a desired code to generate an extended excitation wave for driving the broadband transducer at a plurality of different frequencies; transmit the extended excitation wave to the broadband ultrasound transducer to be transmitted towards the target; receive echo signals reflected back from the target in response to the extended excitation wave; de-convolve the received echo signals to recover an original resolution of the broadband ultrasound transducer at the plurality of different frequencies; and generate a plurality of ultrasound images of the target corresponding to each of the plurality of different frequencies based on the de-convolved echo signals.