Development Of Blood Flow Measurement System By Pulse Doppler Ultrasound

1. IntroductionIn the modern society, with the living level improvement and food habitchange, the incidence of heart and cerebral vascular diseases. As an importantpart of medical ultrasound , ultrasonic Doppler technology can proposefoundation for the disease of blood circulation system. Because of itsunwounded detection over human vessel, it plays a more and more importantpart in the clinic diagnoseThe ultrasound Doppler detection system can be divided into twoelementary parts: the Continuous Wave (CW) Doppler and the Pulse Wave (PW)Doppler system. Of the two, PW is suitable for those occasions when theparticular distance moving objects needs to be detected. PW combines theDoppler technique and pulse echo technique, and makes measurement on thedesired distance objects. The emission pulse is generated by the continuouswave through a control gate circuit. In the receiving circuit, a controlling circuitis also needed to receive the echo from the desired distance. The PW Dopplertechnique can be used to detect the moving objects in the heart and big vessels,and eliminate the occultation effect from the nearby vessels and movingorganization.The paper takes the artery blood flow as the study object and extracting theartery blood flow parameters as the target. According to the principle of PWDoppler, a pulse Doppler blood flow measurement system has been designed,which realize the detection on the artery blood flow. And software is designed tohelp the data analysis and results displaying.2. Research contentThe system designed in this paper is based on the ultrasound Doppler bloodflow measuring theory and method. It also pays more attention on theperformance requests on concrete applications. The hardware centers digitalsignal processor of TMS320VC5509A. Flex10k50 programmable gate array isused to extend the I/O of VC5509 and its controlling ability. According to thepractical function, the system can be partitioned into several parts: the analogparts which is used to complete transmission and reception of ultrasound, thedigital part which is used to undertake the echo signal acquisition andprocessing and the part which used as controlling and display module. Thesystem software is designed to assist the hardware realize the correspondingfunctions, including the transmission and reception of ultrasound, signalacquisition and processing of ultrasound echoes, and the realization ofman-machine interaction function. The main research work of the paper can bedivided into several pats as follows:(1) Ultrasound transmission and reception circuit designThe system adopts dual-plex transducer to complete ultrasoundtransmission and reception. The main clock generator generates high frequencysine wave, and the latter is transmitted after a power amplifier. In the receivingchannel the ultrasound echoes are received on the receiving transducers, throughthe isolation stage and into the preamplifier. So far, the echoes reception iscompleted. The echoes are so weak that more attention should be paid on thenoise restraint not only from the inside circuit but also from the outer space.Once the noise is introduced in this aspect, they will not be removed from thesystem any longer!(2)Controlling signal generating circuit designPulse Doppler system is sensitive to both frequency and phase. Both theoccasional phase of emission pulse and the swing of the sampling pulse phasecan introduce the noise into the system. So there is only one oscillator in thesystem, all the other oscillating signals and controlling signals includingtransmission pulse, reference signal of demodulating signal, or PRF are derivedfrom this oscillator. In the paper this function is realized by the frequencysynthesizer designed by the PLL.(3) Doppler frequency shift demodulation circuit designThe purpose of demodulation module is to extract the frequency shift signalfrom the echo signal. The pre-amplified echo signals are sent to the demodulator,where they are coherently demodulated with the carrier generated by the mainoscillator. As a result there will be a series of short output pulse, namely thesampled Doppler frequency shift signal output.(4) Signal shaping and data acquisitionThe main purpose of this part of circuit is to complete signal modetransformation of the demodulated signal, and make further shape andamplification. It first uses operation amplifier to complete the summation ofmultiplex signal with common characteristics and transform the signal modefrom current to voltage. Then a band-pass filter is used to remove the highfrequency of residual PRF component and the low frequency interference ofechoes from the blood vascular wall. After these processing, the demodulatedsignals make preparations for the following signal acquisition.(5) System control and data processingThe goal of this part of circuit is to control the transmission and receptionof the ultrasound, control the ultrasonic echo signal acquisition and makeprocessing over the sampled data. Meanwhile high-speed extern data memorycircuit for VC5509A has been designed to meet requests of speed at the occasionof large data amount.(6) Human-machine interact interface circuit designIn an actual application system, besides the basic data acquisition andprocessing function, an excellently designed human-machine interact interface isalso necessary. In this paper the control instructions are sent to the system by akey matrix of 4×4, and the results from the system displayed to the operator isgiven by the LCD.(7) Ultrasound Doppler system software designAccording to the systematic goal and the property of every part of circuit,corresponding software is designed, with which system realize the function fromthe transmission and reception of ultrasound to the displaying of the spectrum ofultrasound Doppler blood flow signals.3. ConclusionThe ultrasound Doppler blood flow measurement system designed in thispaper, can realize the measurement of blood flow of the peripheral artery vessels.It adopts the designing method of separations of analog and digital circuit whichimprove the stability and reliability of the system. High speed data acquisitionIC and DSP has been used which satisfies the real-time requirement of thesystem. In the software it realizes the systematic control, spectrum analysis anddisplaying of results. The experiments indicate that the system designed in thepaper has realized the scheduled goals.