Sources of 2nd harmonic generation in a medical ultrasound probe

Tissue harmonic imaging is often the preferred ultrasound imaging modality due to its ability to suppress reverberations. The method requires good control on the transmit stage of the ultrasound scanner, as harmonics in the transmitted ultrasound pulses will interfere with the harmonics generated in the tissue during nonlinear propagation, degrading the image quality. In this study, a medical ultrasound probe used in tissue harmonic imaging was experimentally characterized for transmitted 2nd harmonic distortion. To investigate the phenomenon and push the system to the limit, transmit powers well above conventional operation level were tested. We observed transmit levels at the 2nd harmonic frequency up to -20 dB relative to the fundamental frequency. The transmit stage consists of high-voltage output electronics, cable, tuning network, and the acoustic transducer stack. By separating and measuring at different stages in the ultrasound transmit chain, the main source of 2nd harmonics in transmitted ultrasound pulses was identified as nonlinearities in the acoustic stack