Combining multi-pulse excitation and chirp coding in contrast enhanced echographic imaging

Coded excitation is generally used in imaging to increase the signal to noise ratio (SNR) and the penetration depth. Only in recent years, coded excitation has been introduced for clinical ultrasound. The employment of this solution with contrast agents enhances the contrast to tissue ratio (CTR), since a signal with a long time duration is able to increase the backscattered echoes produced by microbubbles. However, nonlinear properties of contrast agents make decoding problematic in case of applying coded excitation to contrast imaging. An advantageous solution could be combining a multi-pulse technique, in particular one of the Contrast Pulse Sequences (CPS), with the chirp coding: a chirp excitation method for contrast agents based on the transmission and the opportune combination and post-processing of three chirp pulses is introduced (Chirp CPS3). The method proposed is compared to the Contrast Pulse Sequence technique based on the transmission of three uncoded signals (CPS3). Preliminary results are obtained considering simulated signals. The comparison of the methods reveals, under the same conditions, an increment of about 8 dB in the CTR and 17.5 dB in the SNR assured by the new technique. The simulation phase is followed by in-vitro tests that confirm the preliminary conclusions. The only drawback is represented in a loss of axial resolution: an 8 mm-diameter flow channel is overestimated of about 1 mm considering Chirp CPS3.