Geliştirilmiş bir ultrasonik darbeli doppler kan akış ölçme düzeninde hata kaynaklarının analizi
The Analysis of error sources at a developed pulsed doppler blood flowmeter
- Tez No: 14186
- Danışmanlar: PROF.DR. ERTUĞRUL YAZGAN
- Tez Türü: Doktora
- Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1990
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 113
Özet
ÖZET Bu çalışmada, darbeli ultrasonik Doppler metodu kullanarak, gerçekleştirilen akış ölçerlere etki eden hata kaynaklarının teorik analizleri yapılarak deneysel sonuçlarla karşılaş tırılmıştır. Sistem sonucunu etkileyen hatalar iki temel başlık altında incelenmiştir. Bunlardan birincisi fizyolojik sistemden kaynaklanan hatalar, ikin cisi ise elektronik sistemden kaynaklanan hatalardır. Hastalarda doku yapılarının farklılık göstermesinden dolayı, farklı hastalarda aynı damarda yapılan ölçmeler de farklı neticeler vermektedir. Ayrı ca dokuları belirli yapıda ya da yapılarda kabul etmek de mümkün değildir. Bu durum ise nicel ölçme yapmayı engellemektedir. Fakat bütün bu olumsuz durumlara rağmen, genel bir doku modeli ele alına rak, Doppler ölçme sonucuna olan etkisi bulunup değiştirilen model parametreleriyle de yine ölçme sonucuna olan olumsuz katkıları göz lenebilir. Buna ait çalışma, bir çok araştırmacının kabul ettiği doku modeli ele alınarak bir bilgisayar simülasyonu ile yapılmıştır. Elektronik sistemden kaynaklanan hataların analizine ait de neysel sonuçlar, halen klinik çalışmalarda kullanılmakta olan 4 MHz lik bir darbeli Doppler akış ölçerine ilave elektronik donanım ge liştirilerek, geliştirilen bu akış ölçer üzerinden elde edilmiştir, ilave elektronik donanımları değişik darbe tekrarlama frekansının seçilebilmesi, gönderilen darbedeki titreşim sayısının seçilebilme si, örnek alma kapı genişliğinin ayarlanabilir olması, Doppler işa retinin 90° faz farklı elde edilmesine imkân vermesi ve değişik band genişliğinde darbeli Doppler dönüştürücülerini kullanabilmeye imkân veren empedans uydurma devresini içermektedir. Elektronik sistemde olması gereken bazı parametreler gerçek Doppler işaretinin elde edilmesini engellemektedir. Bu tezde girişim gürültüsü, Doppler filtresi, frekans örtüşmesi, çift yanband oluşumu ve örnek hacimde birden fazla damar olması halinde meydana gelen etkilerin analizleri yapılarak, deneysel sonuçlarla karşılaştırılmış ve bu alanda litera türde görülen önemli bir boşluk doldurulmuş bulunmaktadır. Bu çalışma,, şimdiye kadar darbeli Doppler sistemlerinin nicel ölçme yapmadığını ortaya koymakta, ancak yukarıda bahsedilen hata kaynaklarının dikkate alınmasıyla nicel ölçme yapılabileceğini be lirtmektedir. -vii-
Özet (Çeviri)
SUMMARY THE ANALYSIS OF ERROR SOURCES AT A DEVELOPED PULSED DOPPLER BLOOD FLOWMETER The cardiovascular system is one of the major systems of the human body. Its main purpose is to distribute blood to all parts of the body. Quantitave measurements of blood flow using ultrasonic Doppler techniques have considerable importance in clinical measurement. Since pulse wave Doppler has range resolution, and therefore can provide the information at a particular site of the vessel lumen, it is capable of measuring dynamic cardiovascular systems mode effectively than continuous wave Doppler. unfortunately because of its complexity, it requires a skilled operator to obtain blood flow information. For this reason, researchers have worked to reduce the dependence of PW Doppler on the operator. This can basically be achieved by developing more sophisticated PW Doppler systems. In this thesis, the principles of operation, design, measurement constrains, and applications of a pulsed ultrasound Doppler velocimeter are described. The elements necessary to construct a directional pulsed Doppler are described and signal processing methods required are stated. In the first section, a brief introduction of pulse wave Doppler unit is stated. No attempt has been made to provide a complete review of Doppler ultrasound or all of the limitations of Doppler flow measurement methods. One of the major accuracay problems with the current Doppler flow measurement techniques is that volume flow is estimated, not measured. The typical Doppler flowmeter measures the average flow velocity along the beam direction at a particular distance from the transducer. Both the vessel size and the measurement angle are unknown. The vessel size is estimated from a knowledge of an average size for the particular vessel being measured or by pulse-echo scanning methods. Since arterial vessel walls are constantly moving (due to the pulsatile flow or blood), it is difficult to determine the position of the vessel at a given time. Also, since the paths for each measurement are different, the two measurements could have different biases due to the different tissue effects. Since each patient has different size blood vessels and it is difficult to measure the Doppler angle with precision, quantitative comparisons between normal and abnormal flows are nearly impossible. -viii-In the second section, basic principles of ultrasonic Doppler system is introduced. Doppler flow systems depend on the interaction of impinging sonic energy with moving blood. It is known that the primary source of ultrasonic scattering is the red blood cells. At low concentrations of red blood cells, less than 10 %, the scat tering is a linear function of hematocrit. At higher concentrations, it was described a more complex relationship using a frequency of 5 MHZ. The Doppler frequency shift associated with the scattering from an interface, or a single cell, can be determined under certain conditions, since the frequency is defined as the rate of change of phase, the frequency of the backscattered energy is changed by a constant proportional to the velocity of the scatterer. This frequencies, is termed the Doppler shift. To easiest way of characterizing the backscattered Doppler signal is via a vector or phaser type of description. This electrical phaser approach will be helpful in understanding the relationships between the various components of the returned signal and its amplitude and modulation format. It will also be helpful in understanding the types of signal detection which can be used to drive the Doppler information. ultrasound travelling through the intervening tissue to the region of the blood vessel is absorbed at a rate proportional to the frequency while the backscattered signal intensity varies with the fourth power of the frequency. Based on this, it would appear that there is some optimal relationship between the ultrasound frequency and the depth of penetration for an optimal signal-to-noise ratio for a Doppler device. Seeking to maximize the signal-to-noise ratio by proper selection of the center frequency calls for a compromise between maximizing the scattering cross section, minimizing attenuation losses, and adjusting, system bandwidth. Recognizing that the wavelength of ultrasound is orders of magnitude greater than the dimensions of the scattering blood cells, the power level of the scattered sound follows the Rayleigh fourth power law assuming single scattering. In biological soft tissues the attenuation coefficient varies linearly with frequency. The rate of attenuation depends on the tissue type and ranges from 0.2 dB/MHz/cm to more than 2 dB/MHz/cm. It has been demonstrated that the performance of pulsed wave Doppler system depends on the average power level transmitted into the vessel of interest. The average power may be from 20 to 300 mW/crn^. The last part of second section is devoted to the theoretical analysis of color Doppler flow mapping. In this analysis, two parameters (mean flow velocity and flow turbulance) are derived by using statistical analysis of returned echoes. -ix-In the third section, the design of pulsed wave Doppler flowmeter is described. There are two levels of information relating to the design, application, and measurement constraints of pulsed Doppler flowmeters. First, the overall principle of operation of a pulsed Doppler blood flowmeter is described. A functional diagram of the complete system required to produce a Doppler signal is illustrated. Various design compromises and error sources are listed in order to facilitate user discrimination of the accuracy and applicability of the obtained blood velocity data. The first iteration in a two-tiered construct has been written to be easily accesible to the clinician desiring to understand the fundamentals and limits of pulsed Doppler system. Mathematical details have been de-emphasized, with the focus on salient features. Second, this section expands on the analytic details of pulsed Doppler designs and their electronic implementation. A pulsed Doppler flowmeter can be analyzed as a group of interacting blocks. A narrow band square wave signal of fundamental frequency f0 is generated by oscillator. This continuous signal is gated to yield a short pulse which is repeated at a lower frequency known as the pulse repetition frequency (hence the name pulsed Doppler). The gated pulse is amplified by a power amplifier so that a piezoelectric transducer can be driven to yield a sufficient ultrasonic signal to penetrate and return from tissue and blood chambers. Reflected and scattered ultrasonic energy is returned to the same piezoelectric transducer from which it is transmitted inducing a corresponding electric. signal which can be sensed from the transducer. This signal, which now includes components which have been shifted in frequency if the reflecting medium was moving, may require amplification by as much as 40 dB (100 times) by a radio-frequency amplifier, since it is usually quite small. This amplified signal is multiplied (mixed) with the original oscillator signal (a synchronous detector) in a process known as demodulation. For directionality of the system, a quadrature phase detector is addet in order to discriminate whether the flow is toward or away from the transducer. Demodulation yields a signal containing sum frequencies : fQ+(fo - *d)» an
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