Study On Pulse Compression Resolution Of Ultrasonic Linear Frequency Modulation Signal

Ultrasound imaging, which is one of the medical imaging technologies, has been widely used by its low intensity, the harmless, inexpensive, convenient, diverse display method, etc. In medical ultrasound imaging systems, image Signal to Norise Ratio (SNR) and resolution are two important factors. Since biological tissue has the ultrasound effect, the traditional single-pulse excitation pulse amplitude is limited by ultrasound imaging threshold. The image SNR and resolution are mutually contradictory. To solve this problem, the ultrasound imaging system gradually introduced radar systems applications---coded excitation in recent years. Coded excitation can guarantee SNR and detecting depth, improving image resolution at the same time. Compared to conventional ultrasonic pulse-echo system, the excitation ultrasound imaging system are much more complex. Increasing the pulse compression SNR and resolution are still one of the hot area.Based on coded excitation and pulse compression technology, this paper deduces the effect of chirp signal’s bandwidth, time width, amplitude envelope on the resolution of pulse compression, and verify the validity of theoretical analysis with the simulation and experimental method. Based on the characteristics of transducer frequency response, the impact of modulation parameters (bandwidth, scanning time, envelope, etc.) of ultrasonic chirp signal on pulse compression SNR and its resolution ware studied, in order to explore the optimal excitation method.This study covers:(1)The resulting expression is derived by analyzing the impact of the chirp signal bandwidth, time width amplitude envelope on the resolution of pulse compression theoretically through matched filter.(2)To choose five different bandwidths, five different time widths, six different coefficients of quadratic chirp signal amplitude modulated excitation pulse compression contrast pulse compression resolution by simulation. Design experiments chirp signal sets, which are used to excite the ultrasonic transducer, receive the ultrasonic echo signals through matched filter, contrast pulse compression resolution. The simulation and experimental ware conformed to the theoretical analysis.(3)Based on the characteristics of transducer frequency response, we choose five different bandwidth, different time widths, six different coefficients of quadratic chirp signal amplitude modulated excitation pulse compression to be pulse compression. Then we contrast excitation energy and the main lobe width before and after transducer, in order to explore the optimal excitation method.The results of this study show that(1)The main lobe width of chirp signal is inversely proportional to Bandwidth and time width, but as time increases the main lobe width is almost exclusively determined by the signal bandwidth.(2)The energy loss of chirp signal after transducer is proportional to the signal bandwidth signal, that is, the bigger chirp signal bandwidth is, the greater the SNR loss after transducer.(3)The resoution of amplitude envelope modulation chirp signal after pulse compression is the proportion of the relevant bands. The smaller proportion the center frequency is, the narrower beam width is after pulse compression, that is, the resolution is better. The band energy loss of the chirp signal after transducer is larger. Thus the larger proportion of chirp signal’s center frequency is, the smaller energy loss is, that is, the SNR is better.