Effect of frequency characteristic of excitation pulse on lateral spatial resolution in coded ultrasound imaging

Recently, portable ultrasonic diagnostic equipment has frequently been used in clinical situations. The use of portable ultrasonic diagnostic equipment expands various diagnosis areas, such as remote medical diagnosis, and emergent diagnosis at disaster. It is expected that portable ultrasonic diagnostic equipment will be used more frequently in the future. To make ultrasonic diagnostic equipment portable, the number of transducer elements in an ultrasonic probe should be reduced significantly. Therefore, the transmit-receive sensitivity of the ultrasonic probe is degraded. For the improvement of the signal-to-noise ratio (SNR) of the received ultrasonic echo, coded excitation was introduced in ultrasonic imaging. Owing to pulse compression applied to the received echo signal, its SNR significantly improved without the degradation of the range spatial resolution. However, the lateral spatial resolution in coded ultrasound imaging has not been investigated in previous studies. The present study showed that the lateral resolution in coded ultrasound imaging using a typical code, 5-bit Barker code, was worse than that using a conventional short pulse. Such degradation was discussed in terms of the frequency characteristics of the impulse response of the ultrasonic transducer and the excitation pulse. Also, the Gaussian phase coherence factor was introduced as one of the methods to overcome such degradation in lateral spatial resolution.