Darbeli Doppler akış ölçeri

Abstract

Bu çalışmada, kan akış hızını, akış hızı profilini, hacimsel kan akış hızını ve damar çapını belirleyecek bir sistemde kullanılmak amacıyla, 2*5 MHz'lik bir darbeli Doppler kan akış ölçeri gerçekleştirilmiştir. Sistem tasarlanırken, halen kullanılmakta olan darbeli Doppler akış ölçeri örnek alınmıştır. Sistemin özellikleri şunlardır ; değişik darbe tekrarlama frekansının seçilebilmesi, gönderilecek darbe paket çiğindeki darbe sayısının değiştirilebilmesi, örnek alma kapı aralığının ayarlanabilir olması ve 90 faz farklı iki Doppler işareti elde edilerek kan akış yönünün belirlenebilmesi ne olanak vermesidir* Sistem temel olarak aşağıdaki bloklardan meydana gelmektedir : ana osilatör, güç kuvvetlendiricisi, radyo - frekans kuvvetlendiricisi, dönüştürücü, demodulator, band geçiren filtre ve sayısal kontrol devresi. Osilatör frekansı, darbe tekrarlama frekansının kontrol ettiği geçiş kapısında darbe paketçikleri haline gelir. Bu darbe paketçikleri güç kuvvetlendiricisi üzerinden ultrasonik dönüştürücü yardımıyla fizyolojik ortama gönderilir. Fizyolojik ortamdan geri dönen Doppler bileşenli işaret, aynı dönüştürücü ile alınarak radyo - frekans kuvvetlendiricisi ile kuvvetlendirilir. Demodülasyon ve filtreleme işlemlerinden sonra Doppler bilgisi elde edilir.

The measurement of various parameters of the biological systems has been one of the basic purposes of biomedical engineering» These parameters have a great importance for the diagnosis and the treatments of the human diseases» Because of the complexity of the biological systems and the object of the measurements is the human being, techniques to be used for the measurements are limited» The cardiovascular system is one of the major systems of the human body» Parameters of this system are the blood flow velocity, the velocity profile, the vessel diameter and the volumetric blood flow. Although various techniques are available for the measurement of the blood flow, ultrasonic Doppler techniques have considerable importance in clinical applications* The reason for this are many, but they perhaps chiefly derive from the ease and safety associated with its use. Doppler ultrasound is an important technique for non-invasively detecting and measuring the velocity of moving structures, and particularly blood, within the body. Doppler instrumentation has advanced to a stage where it is suitable for routine use outside the hospital research laboratory. The correct interpretation of the results obtained from Doppler equipment depends on the understanding of both the physical mechanisms and signal processing methods that result in Doppler signal. Sound can be described as periodic changes in the pressure of air. These pressure waves result from a force vi returning ultrasound signal, containing echoes from both stationary and moving targets, is fed to the radio frequency amplifier by the receiving crystal» This amplified signal is then demodulated and filtered to produce audio frequency signals whose frequencies and amplitudes provide information about motion within the ultrasound beam. PW Doppler systems are used to obtain Doppler information at a specific range from the face of the transducer. The main difference between a CW and a pulsed system is that the transducer is excited with bursts of pulses instead of being continuously excited. The burst of ultrasound travels into the body where it is Doppler shifted by moving structures. Returning echoes from both stationary and moving targets are received by the same transducer. This process repeated for the next burst of ultrasound. The Doppler shift information is extracted by the demodulation process. For directionallity of the system, a quadrature phase is added in order to discriminate whether the flow is toward or away from the transducer. Demodulation yields a signal containing sum and difference frequencies. This signal is sampled by the sample gate at the pulse repetition frequency and bandpass filtered to have the Doppler shift frequency. The choise of the optimum frequency for a desired depth is governed by two conflicting factors good penetration without high absorption, frequency is best since tissue attennuation proportionally to frequency. C2> For maximum power from the collection of red blood cells, frequency is best since each cell scatters by that is approximately proportional to fourth frequency. CI!) For a lower increases scattered a higher an amount power of Pulsed Doppler systems have one major drawback, they are able to detect velocities up to a finite maximum which is related to the depth of examination. The maximum Doppler shift frequency, a pulsed Doppler unit is able to detect is half the pulse repetition frequency. As the depth of the region of interest increases, the pulse repetition frequency must be decreased to allow the pulses sufficient time for the round journey, and thus for vessels deep within the body only lower velocities may be detected. The maximum velocity problem is of particular annoyance when examining high velocity jets in the heart. IX This thesis consists of tree sections. In the first section, a brief introduction is given. In the second section, basic principles of waves, blood and ultrasonic Doppler systems are described. In the third section, the pulsed Doppler flowmeter which is realized is introduced. The thesis is ended with a brief conclusion and r ec ommendat ion. returning ultrasound signal, containing echoes from both stationary and moving targets, is fed to the radio frequency amplifier by the receiving crystal» This amplified signal is then demodulated and filtered to produce audio frequency signals whose frequencies and amplitudes provide information about motion within the ultrasound beam. PW Doppler systems are used to obtain Doppler information at a specific range from the face of the transducer. The main difference between a CW and a pulsed system is that the transducer is excited with bursts of pulses instead of being continuously excited. The burst of ultrasound travels into the body where it is Doppler shifted by moving structures. Returning echoes from both stationary and moving targets are received by the same transducer. This process repeated for the next burst of ultrasound. The Doppler shift information is extracted by the demodulation process. For directionallity of the system, a quadrature phase is added in order to discriminate whether the flow is toward or away from the transducer. Demodulation yields a signal containing sum and difference frequencies. This signal is sampled by the sample gate at the pulse repetition frequency and bandpass filtered to have the Doppler shift frequency. The choise of the optimum frequency for a desired depth is governed by two conflicting factors good penetration without high absorption, frequency is best since tissue attennuation proportionally to frequency. C2> For maximum power from the collection of red blood cells, frequency is best since each cell scatters by that is approximately proportional to fourth frequency. CI!) For a lower increases scattered a higher an amount power of Pulsed Doppler systems have one major drawback, they are able to detect velocities up to a finite maximum which is related to the depth of examination. The maximum Doppler shift frequency, a pulsed Doppler unit is able to detect is half the pulse repetition frequency. As the depth of the region of interest increases, the pulse repetition frequency must be decreased to allow the pulses sufficient time for the round journey, and thus for vessels deep within the body only lower velocities may be detected. The maximum velocity problem is of particular annoyance when examining high velocity jets in the heart. IX This thesis consists of tree sections. In the first section, a brief introduction is given. In the second section, basic principles of waves, blood and ultrasonic Doppler systems are described. In the third section, the pulsed Doppler flowmeter which is realized is introduced. The thesis is ended with a brief conclusion and r ec ommendat ion.