Influence of the transducer geometry on the phase of the signal used for reducing second harmonic during ultrasound propagation

Ultrasound contrast harmonic imaging is one of the most used contrast detection method in common system used for diagnostic. Its success comes from the higher non linearity properties of ultrasound contrast agents compared to tissue. A way to further enhance agent to tissue ratio would be to locally suppress second harmonic that is generated due to non linear propagation in tissue. Reducing higher harmonics from surrounding medium is a challenge that has been previously studied. Christopher [1] proposed to transmit inverted replica of back propagated signal, Krishnan [2], [3] back propagated only second harmonic and added inverted replica of it to excitation, Shen [4] proposed transmitting third harmonic to reduce the second harmonic. In our study, a custom made dual frequency phased array transducer [10], [11] is used to transmit 2 different signals: the imaging signal and the Second Harmonic Reduction Signal (SHRS). In this paper we study the what is optimal phase ® SHRS0 of SHRS needed to achieve a maximum 2nd harmonic frequency reduction in a region of interest, depending upon the several properties of transducer apertures. We have observed that as the size of the transducer and focal distance changes, also the optimal phase of the signal used for reducing the 2nd harmonic changes. If the phase of the signal is inappropriate, increase of the 2nd harmonic can be observed rather than decrease.

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