Research On Endoscopic Doppler Ultrasonography System And Signal Processing

During the past decades, ultrasonic medical diagnostic instruments have advanced significantly as a result of the resourceful application of electronic-system and transducer designs to the problem of noninvasive information acquisition from clinical patients. Medical Doppler ultrasonic endoscope system bases on ultrasonic endoscope system, combining the advantage of ultrasound in diagnosis with the sensitivity of Doppler to moving objects together. The Pulsed Doppler Quantitative detection can be measured by Doppler blood velocity profile. In this way, we get histology characteristic of digestive organ wall's sections and the hemodynamics parameters on sections. The information helps us make judgments about pathological changes in upper gastrointestinal tract.This paper depicts the development of a Doppler ultrasonic endoscope imaging system based on FPGA. In order to capture the weak signal scattered from tissue, the ultrasound signal reception circuit with high gain and low noise level was designed. All-digital signal processing was utilized to process the weak signal in FPGA, and the quadrate demodulation and spectrum analysis were realized. The experiment platform based on the Doppler physical model was set up to verify the performance of the system. By comparing the sonogram under different situations, the system and signal processing method are proved reasonable and correct.This paper consists of 4 parts as follows:1. Introduce the theory and signal modal of ultrasonic pulse Doppler technique, design the integral system scheme based on hemodynamic features and its spectral influence. A distance switch was used in this system to extract information of blood flow rate. Furthermore, a quadrature demodulation module and a FIR high pass filter were adopted to depress noise from vessel and low velocity blood. Then dynamic spectral map was introduced to depict direction and velocity information of blood simultaneously.2. Currently, study of weak signal collected from small endoscope probe is still not popular. To overcome the low volume and weak amplitude problem in ultrasonic endoscope, this paper designs a totally digital process scheme for ultrasonic echo signals, and a receipt circuit with high gain and low noise. Quadrature demodulation and spectral analysis were operated in FPGA. The circuit shows advantages of well matching, high SNR, low volume and rapid process.3. A novel analysis frame on ultrasonic signal, which integrate wavelet analysis advantage on signal denoise and vessel noise removing, was developed to depress the influence of system noise and input artifacts. In this part we introduce wavelet transform firstly and then present the signal self adaptive description, which contains wavelet pack and wavelet frame. Finally we introduce the threshold denoising method based on wavelet frame transformation and time-invariant performance. To overcome the phase confusion in directly complex signal transformation, a denoise module integrated with non-fixed threshold method was designed and experiment results on Matlab was also presented.4. Construct the image experiment platform of Doppler modal and evaluate the correctness of system. In this part, constitutes and experiment method were introduced firstly. Three group experiments: static blood flow modal, dual path same direction blood flow modal, dual path different direction blood flow modal was operated in this part and the blood flow characters in these experiments were analyzed. Results of spectral map in blood velocity and direction changing shows that the circuit in this part has the ability to judge blood velocity and direction, specifically, it also has the distance switch function. Consenquently this scheme is reasonable and correct.Innovations of this paper are :1. For weak signal detection under ultrasonic endoscope micro probe, a receipt circuit with high gain and low noise was designed and realized. Quadrature demodulation and spectral analysis were operated in FPGA. The circuit of this system shows advantages of well matching, high SNR, low volume and rapid process.2. Under the consideration of the time-variance and complexity in mixed ultrasonic echo wave signals a novel signal optimization modal was designed, which combined nonorthogonal wavelet based on wavelet frame redundancy theory with interval power minimum optimization rules. Through improve the redundancy properly, this method overcome the limitation of orthogonal and biorthogonal wavelet on the denoising and reform the boundary distortion from the mismatch between wavelet frame approximate space and signal space. Results of value simulation expressed higher SNR, lower root mean square error of mean frequency waveform and spectral range waveform. Moreover, the experiment results also demonstrated this conclusion.3. Based on the multi scale wavelet character of blood ultrasonic Doppler signals, a novel semisoft threshold function was adopted in this paper to overcome the flaw of hard threshold and soft threshold algorithms. The comparison analysis with soft threshold, hard threshold and non-negetive Garrote threshold were operated combined with Quadratic B-Spline wavelet, the results showed that BWTF method based on semisoft threshold achieved the best performance.