Effects of transmit focusing on finite amplitude distortion based second harmonic generation

Generation of the second harmonic signal was studied for finite amplitude distortion based harmonic imaging. Acoustic field amplitudes along the range axis of a fixed focus transducer were measured using a PVDF needle hydrophone. Results indicated that on-axis amplitudes strongly depended on the f-number at both the fundamental and the second harmonic frequencies. Differences of the on-axis amplitudes between the two frequencies were also investigated. To explore the possibility of increasing harmonic generation by extending the depth of focus, a two-focus transducer was employed. Hydrophone measurements, pulse-echo imaging and simulations were performed. Although the increase in harmonic generation depended on specific imaging parameters, the effectiveness of improving the harmonic signal-to-noise ratio (SNR) by increasing the depth of field was clearly demonstrated. Degradation in contrast resolution associated with the two-focus transducer was also evaluated. It was found that the contrast resolution was still significantly better than that of the fundamental image at the same frequency. Results of the study using the two-focus transducers can be generalized to imaging systems with full dynamic transmit focusing capabilities. In other words, it is expected that dynamic transmit focusing can improve the SNR of finite amplitude distortion based second harmonic imaging while improving the contrast resolution over fundamental imaging.