A dual pulse technique for improving the point spread function of superharmonic imaging systems

Nonlinear propagation causes the generation of higher harmonics of the emitted fundamental spectrum. Nowadays, medical echography employs reflections of the second harmonic, because this yields improved axial and lateral resolutions, and less reflections from nearby artifacts and grating lobes, as compared to fundamental imaging. To further exploit the benefits of higher harmonic imaging while keeping sufficient signal strength for detection, superharmonic imaging combines reflections of the third, fourth, and fifth harmonics. A drawback of adding harmonic reflections is the possible occurrence of ripples in the point spread function (PSF). Recently, a dual pulse technique was proposed for avoiding these ripples. This technique uses the emission of two consecutive pulses with a slightly different frequency, and performs imaging after summation of both superharmonic reflections. In this presentation, it is theoretically explained why this approach yields a better PSF than single pulse superharmonic imaging. For a superharmonic imaging system with an interleaved phased array, numerical results obtained with the iterative nonlinear contrast source method are shown. Moreover, these are validated against experimental results. The results confirm that the proposed technique significantly reduces ripple artifacts and gives a more compact PSF than the third harmonic alone. [Work supported by the STW and the NCF.]