Feasibility study of superharmonic imaging using chirps

Superharmonic imaging (SHI) targets a combination of the 3rd to 5th harmonics. It was proven to have certain advantages in comparison with the established imaging standards in medical utrasound. SHI enhances the spatial resolution and improves the quality of echographic images, mainly by eliminating reverberation artifacts at the chest wall. However, SHI suffers from ripple artifacts, originating from the spectral gaps in between harmonics, and degrading the temporal resolution. To solve this a chirp-based SHI protocol was employed and its characteristics investigated, i.e. point spread function (PSF). The protocol was implemented for an interleaved phased array probe (44+44 elements tuned at 1.0+3.7MHz), connected to a fully programmable ultrasound system. A linear chirp (center frequency 1MHz; bandwidth 40%) was used for excitation. To obtain the PSF, the RF traces were recorded at focus along the lateral axis and convolved with the decoding signal. This was computed using KZK simulations. A PSF comparison between a superharmonic chirp and the 3rd-harmonic of a 2.5-cycle Gaussian apodized sinus burst at 1MHz showed a decrease in axial pulse length of 46% at -6dB and 32% at the -20dB level in favor of SHI. Chirp based SHI is virtually free of ripple artifacts and therefore feasible.

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