本文選題：磁共振成像 + 逆方法　； 參考：《南方醫(yī)科大學(xué)》2012年碩士論文

【摘要】：磁共振成像(Nuclear Magnetic Resonance imaging, NMRI)是一種斷層成像技術(shù),它通過外部射頻場發(fā)射不同的射頻脈沖序列對生物體內(nèi)各個(gè)組織進(jìn)行激發(fā),通過接收探測生物體內(nèi)產(chǎn)生的核磁共振信號來對體內(nèi)組織器官的內(nèi)部生理與化學(xué)特征成像。相比其他的一些成像技術(shù),例如X-ray和CT (Computed Tomography),磁共振成像技術(shù)具有很多優(yōu)勢：首先,磁共振成像不需要將人體曝露在電離輻射的環(huán)境下,因此磁共振成像技術(shù)更具安全性。其次,磁共振為多參數(shù)成像,能夠提供許多診斷信息。第三,磁共振成像對比度高,可得到詳細(xì)的解剖圖譜。第四,磁共振具有對任意層面斷層成像的功能,可以從三維空間上直接觀察人體。第五,磁共振成像不會(huì)出現(xiàn)氣體及骨偽影等現(xiàn)象。 工程上實(shí)際的電磁場問題是很復(fù)雜的,如邊界形狀不規(guī)則,復(fù)雜的物質(zhì)結(jié)構(gòu),材料性能的非線性以及電磁場的分布性等等。因此,在計(jì)算機(jī)出現(xiàn)以前人們在實(shí)際的設(shè)計(jì)計(jì)算工作過程中,只能采取一些簡化措施,得出近似的解析解,或者用模擬實(shí)驗(yàn)的方法來滿足工程要求的近似結(jié)果。電子計(jì)算機(jī)的出現(xiàn),給解算復(fù)雜的電磁工程技術(shù)問題提供了強(qiáng)有力的工具。由于數(shù)字電子計(jì)算機(jī)具有運(yùn)算速度快,儲(chǔ)存容量大,計(jì)算功能強(qiáng)和準(zhǔn)確度高等優(yōu)點(diǎn),使得計(jì)算領(lǐng)域產(chǎn)生了驚人的發(fā)展。20世紀(jì)60年代以來,隨著電子計(jì)算機(jī)技術(shù)的發(fā)展,一些電磁場的數(shù)值計(jì)算方法發(fā)展起來,并得到廣泛地應(yīng)用,相對于經(jīng)典電磁理論而言,數(shù)值方法受邊界形狀的約束大為減少,可以解決各種類型的復(fù)雜問題。電磁學(xué)問題的數(shù)值求解方法主要分為時(shí)域和頻域兩大類。頻域技術(shù)主要有矩量法、有限差分方法等,頻域技術(shù)發(fā)展得比較早,也比較成熟。時(shí)域法主要有時(shí)域差分技術(shù)。時(shí)域法的引入是基于計(jì)算效率的考慮,某些問題在時(shí)域中討論起來計(jì)算量要小。但是,各種數(shù)值計(jì)算方法都有優(yōu)缺點(diǎn)：時(shí)域有限差分方法(FDTD)適合對非均勻復(fù)雜電磁參數(shù)電介質(zhì)(例如人體)進(jìn)行有效地分析,但是需要對整個(gè)三維空間進(jìn)行網(wǎng)格化,還要在空間邊緣設(shè)定邊界吸收條件,計(jì)算時(shí)間相對比較久,需要的物理內(nèi)存也相對比較大。相反,矩量法(MOM)在計(jì)算過程中不需要對整個(gè)空間網(wǎng)格化,也不許要設(shè)定吸收邊界,只需要對有電流分布的區(qū)域進(jìn)行離散化,計(jì)算相對快速,非常適用于對類似射頻線圈這樣的復(fù)雜結(jié)構(gòu)進(jìn)行電磁仿真分析。但同時(shí),MOM不能處理類似人體這樣的復(fù)雜介質(zhì)的負(fù)載。而逆方法則是反向考慮問題,由預(yù)設(shè)理想目標(biāo)場出發(fā)反演計(jì)算得出線圈結(jié)構(gòu)。一個(gè)復(fù)雜的問題往往難以依靠一種單一方法解決,常需要將多種方法結(jié)合起來,互相取長補(bǔ)短,因此混和方法日益受到人們的重視。 為了提高磁共振射頻線圈射頻場的性能,解決射頻線圈與人體組織復(fù)雜電磁作用問題,我們提出了以真實(shí)人體為基礎(chǔ)建立三維電磁模型的方法。以人體CT斷層掃描圖像為基礎(chǔ),采用精確的人工分割方式和體繪制三維重建方法,并賦上不同組織相應(yīng)的電磁參數(shù),建立真實(shí)人體三維電磁模型,作為射頻場FDTD域負(fù)載。將建立起來的真實(shí)人體三維電磁模型作為線圈負(fù)載,優(yōu)化磁共振射頻線圈的射頻場分布。 本本研究的目的在于增強(qiáng)磁共振射頻線圈仿真的真實(shí)性,提高射頻線圈設(shè)計(jì)水甲,從而提高線圈的成像質(zhì)量。充分利用三種方法的長處：矩量法(MOM)在計(jì)算復(fù)雜結(jié)構(gòu)線圈上電流分布的優(yōu)勢；時(shí)域有限差分(FDTD)方法在仿真人體模型等復(fù)雜的不均勻電磁介質(zhì)的優(yōu)勢；逆方法可以模擬理想目標(biāo)場。通過惠更斯等效面將MOM和FDTD以及逆方法和FDTD有機(jī)結(jié)合到一起,形成混合射頻線圈設(shè)計(jì)方法,并提出了一種新的真實(shí)人體電磁模型建立方法,融入到混合方法中,充分考慮線圈與真實(shí)人體組織之間復(fù)雜的電磁相互作用。其中,由MOM和FDTD混合方法設(shè)計(jì)的原型線圈掃描圖像的信噪比達(dá)到193.4dB,相比先前未考慮人體與線圈之間作用的方法得到的圖像信噪比提高了38.7dB,從而驗(yàn)證了仿真設(shè)計(jì)的正確性。
[Abstract]:Nuclear Magnetic Resonance imaging (NMRI) is a technique of tomography. It excuses the tissues of the organism by transmitting different radiofrequency pulses from the external radiofrequency field, and imaging the internal physiological and chemical characteristics of the organs and organs by receiving the MRI signals produced in the organism. Compared to some other imaging techniques, such as X-ray and CT (Computed Tomography), magnetic resonance imaging technology has many advantages: first, magnetic resonance imaging does not need to expose the human body to ionizing radiation, so magnetic resonance imaging technology is more secure. Secondly, MRI is a multi parameter imaging, which can provide a lot of diagnostic information. Third, the contrast of magnetic resonance imaging is high, and a detailed anatomical map can be obtained. Fourth, magnetic resonance has the function of tomography at any level, and can directly observe the human body from the three-dimensional space. Fifth, the magnetic resonance imaging will not appear gas and bone artifacts and so on.
The actual electromagnetic problem in engineering is very complicated, such as irregular boundary shape, complex material structure, nonlinear material property and distribution of electromagnetic field. So, in the process of actual design and calculation before the computer appears, some simplified measures can be taken to obtain approximate analytical solutions, or use models. The quasi experimental method meets the approximate results of the engineering requirements. The emergence of the electronic computer provides a powerful tool for solving the complex electromagnetic engineering technology problems. Because the digital electronic computer has the advantages of fast operation speed, large storage capacity, high computing function and high accuracy, the computing field has produced an amazing development of.2. Since the 60s zeroth Century, with the development of electronic computer technology, the numerical calculation method of some electromagnetic fields has been developed and widely used. Compared with the classical electromagnetic theory, the numerical method is greatly reduced by the boundary shape constraints, and can solve various types of complex problems. The numerical solution method of electromagnetics is the main method. It is divided into two categories in time domain and frequency domain. Frequency domain technology mainly includes moment method, finite difference method and so on. Frequency domain technology is developed early and mature. Time domain method is the main time difference technique. The introduction of time domain method is based on calculation efficiency, and some problems are discussed in time domain. The method has the advantages and disadvantages: the time domain finite difference method (FDTD) is suitable for the effective analysis of the inhomogeneous and complex electromagnetic parameter dielectric (such as the human body), but it needs to mesh the whole three-dimensional space and set the boundary absorption conditions at the edge of the space. The calculation time is relatively long, and the required physical memory is relatively large. In the calculation process, the method of moment (MOM) does not need to mesh the whole space, and does not have to set the absorption boundary. It only needs to discretize the area with current distribution, and is relatively fast. It is very suitable for electromagnetic simulation analysis of complex structures like radio frequency coils. But at the same time, MOM can not handle the similar human body. The load of complex medium is the inverse problem, and the coil structure is obtained from the presupposition ideal target field. A complex problem is often difficult to rely on a single method. It often needs to combine a variety of methods to complement each other, because this mixing method has been paid more and more attention.
In order to improve the performance of RF field of magnetic resonance radio frequency coil and solve the problem of complex electromagnetic action of radio frequency coil and body tissue, we put forward a method to establish a three-dimensional electromagnetic model based on real human body. Based on the CT tomography image of human body, we use accurate artificial segmentation method and body to draw three-dimensional reconstruction method. With the corresponding electromagnetic parameters of the organization, a three-dimensional electromagnetic model of the real human body is set up as the load of the FDTD field in the radio frequency field. The three-dimensional electromagnetic model of the real human body is set up as the coil load to optimize the RF field distribution of the magnetic resonance radio frequency coil.
The purpose of this study is to enhance the authenticity of the simulation of magnetic resonance radiofrequency coils, improve the design of water nail by the RF coil, and improve the imaging quality of the coils. The advantages of the method of moment (MOM) are fully utilized to calculate the current distribution on the coils of complex structures, and the time domain finite difference (FDTD) method is used to simulate the model of the human body. The advantage of heterogeneous electromagnetic medium, the inverse method can simulate the ideal target field. Through the combination of MOM and FDTD and the inverse method and FDTD, the design method of mixed radio frequency coil is formed through the Huygens equivalent surface. A new method for establishing the real human body electromagnetic model is put forward, and the coils are fully considered in the mixing method. The complex electromagnetic interaction with the real human tissue, in which the signal to noise ratio of the prototype coil scanned by the MOM and FDTD hybrid method is 193.4dB, and the image signal-to-noise ratio (38.7dB) is improved compared to the previously unconsidered method of the interaction between the human body and the coil, thus verifying the correctness of the simulation design.
【學(xué)位授予單位】：南方醫(yī)科大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2012
【分類號】：R310

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本文編號：1992163