Pulse inversion Doppler: A new method for detecting nonlinear echoes from microbubble contrast agents

A novel technique for the selective detection of ultrasound contrast agents , called pulse inversion Doppler, has been developed. In this technique, a conventional Doppler or color Doppler pulse sequence is modified by inverti ng every second transmit pulse. Either conventional or harmonic Doppler pro cessing is then performed on the received echoes. In the resulting Doppler spectra, Doppler shifts from linear and nonlinear scattering are separated into two distinct regions that can be analyzed separately or combined to es timate the ratio of nonlinear to linear scattering from a region of tissue. The maximum Doppler shift that can be detected is 1/2 the normal Nyquist l imit. This has the advantage over conventional harmonic Doppler that it can function over the entire bandwidth of the echo signal, thus achieving supe rior spatial resolution in the Doppler image. In vitro measurements compari ng flowing agent and cellulose particles suggest that pulse inversion Doppl er can provide 3 to 10 dB more agent to tissue contrast than harmonic imagi ng with similar pulses. Similar measurements suggest that broadband pulse i nversion Doppler can provide up to 16 dB more contrast than broadband conve ntional Doppler. Nonlinear propagation effects limit the maximum contrast o btainable with both harmonic and pulse inversion Doppler techniques.