Research About Coded Excitation Ultrasonic Blood Flow Measurement System Based On Software Radio

The medical ultrasonic-inspection system has been widely applied in the hemodynamic parameters measurement due to its merits of noninvasiveness, low cost and real-time processing, which has proved to be a valuable tool for pathophysiology research and clinical diagnosis of vascular disease. With the rapid development of modern techniques, several topics recently become the hotspots in the research of the system, which include improving the measuring resolution, raising the signal to noise ratio of echoes, enhancing the sensitivity of the system, as well as designing the system with intelligentization and miniaturization.In this dissertation, several crucial problems in the measurement of blood velocity are studied and analyzed, and a complete intellectual system solution is proposed. In order to improve the traditional ultrasound measurement system, the idea of software radio (SR) and coded excitation technique are introduced into the hardware platform design. In addition, some new methods with improved performance are developed to flow velocity estimation. The research including hardware and software achieves the following innovative points:1) The SR idea is introduced into building the system platform. The high speed A/D converter is used to sample the ultrasonic radio frequency (RF) echo signals directly and the most of functions are accomplished by software. With the integrated circuit technique and the digital signal processing, a stable and all-purpose hardware platform is carried out for ultrasonic signals transmitting, receiving, sampling and post-processing.2) Coded excitation is applied to improve the ultrasonic transmit signal. After discoursing learnedly on the characteristics of the known codes which have been adapt to blood velocity measurement, a zero correlation window complementary (ZCWC) coded sequence is firstly presented for its prefect local autocoiTelation and cross correlation. The structure of ZCWC code and its good feature in measuring blood velocity profile are studied and analyzed. It is proved that ZCWC code provides higher performance by the mirror reflection experiment and the mix codes separation experiment.3) According to the ultrasonic echo signal mechanism of the human blood, a distributed model which represents the time delay effect of the scatters movement more accurately than the conventional model is setup. The distributed parameter is firstly utilized in the model, which is able to denote velocity extension in sampling unit.4) To cope with the estimation error caused by central frequency downshift and the wide frequency band, a two-dimensional frequency shift tracing (2-D FST) algorithm based on the spectral feature matching is proposed. The first step is to represent the received RF echo sequence signals in the two-dimensional spectrum by applying the 2-D FFT and then analyze the resulting spectral feature. After using the transmit signals to build dynamic templates, a simplified correlation coefficient tracing strategy is presented to acquire the coordinate of feature points. Finally, the velocity could be estimated by calculating the central frequency shift and the Doppler frequency shift.5)To improve the quality of the estimation in small sampling data situation, a time delay vector joint estimation (TVJE) method based on the distributed model motioned above is used to turn the conventional velocity estimation into the joint estimation of the central velocity and a distributed parameter. Then the velocity estimation is obtained from the central velocity and its validity is represented by the distributed parameter which denotes the velocity extension. After adjusting the sampling volume by the distributed parameter dynamically, the final velocity profile can be acquired.After building the hardware platform and debugging the software algorithm, the system was tested in the velocity measurement of simulated blood flow. It is proven that the system is capable of measuring the blood flow velocity profile, and the measurements showed a good agreement with the theoretical analysis. The system could provide good blood flow velocity measurements, especially under the circumstance of small sampling data.