Abstract
Harmonic imaging, the formation of ultrasound images from the harmonic components of the sonification signal, has been used in diagnostic ultrasound applications that utilize microbubble contrast agents. In the presence of microbubbles the harmonic signals are mainly due to nonlinear bubble oscillations. Recently, harmonic imaging has been applied without contrast agents, the harmonic signals arising from nonlinear propagation. Near-field artifacts and aberrations are reduced, and tissue borders are enhanced. The harmonic signals are at a lower amplitude and suffer more attenuation than the fundamental. However, the wide dynamic range, digital architecture, and signal processing capabilities of modern diagnostic ultrasound systems make it possible to utilize this tissue-generated harmonic energy for image formation. The nonlinear parabolic wave equation is used to model tissue propagation. A time domain numerical solution for a nonaxisymmetric source function is presented. Theory and experiments demonstrate harmonic generation in tissues. In vitro animal tissue experiments and in vivo collected radio frequency data are shown. A novel broadband pulsing scheme that uses two pulses with a phase difference of 180° is introduced, which results in cancellation of the propagated fundamental component and doubling of the second harmonic. Cardiac and abdominal harmonic images are shown.
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