Compensation of the transducer response for high frequency coded excitation imaging

An enhanced approach in which the windowed chirp transmission is inverse filtered with the impulse response of an ultrasonic transducer is investigated to improve axial resolution and to reduce the side lobe level in high frequency coded excitation imaging. This method reduces side-lobe level more than the resolution enhancement compression (REC) method without using a mismatched filter. The proposed method modifies the number of samples of transmission signal to maintain the accuracy of convolution operation while performing inverse filtering. In this way, side-lobe level is minimized compared to the REC method without a loss of axial resolution. Both simulation and experimental measurements were carried out to validate the enhanced method. FieldII simulation was performed for a 34 MHz focused single element transducer. For the experimental measurement, a focused LiNbO₃ single element transducer at 34 MHz center frequency with 74% bandwidth was used. The two way impulse response of transducer was acquired by pulse-echo measurements. The frequency of the Hanning windowed chirp was from 20 MHz to 60 MHz. Images of a wire target and rabbit eyeball were acquired for the image evaluation. The simulation results showed that the axial resolution at -6dB was enhanced from 53¿m to 48¿m at the focal point of transducer whereas experimental results showed that the axial resolution at -6dB was enhanced from 52¿m to 46¿m and side-lobe level of proposed method was decreased by 4dB compared with the REC method.