基于CEST理論模型的優(yōu)化采樣方案研究
發(fā)布時間：2018-11-09 21:34
【摘要】：化學(xué)交換飽和轉(zhuǎn)移成像技術(shù)(chemical exchange saturation transfer MRI contrast,CEST)是一種全新的核磁共振對比成像技術(shù)。CEST不同于傳統(tǒng)的MR對比劑,它通過兩種或多種處于不同化學(xué)環(huán)境下的質(zhì)子之間的化學(xué)交換,產(chǎn)生成像對比。CEST的理論建模基于Bloch-Mc Connell方程。在實際的CEST成像中,通常在頻率軸上等間隔施加預(yù)飽和脈沖序列,采集信號得到的z譜會和CEST的理論模型進行匹配,從而估計我們需要了解的生理參數(shù)。這種采集信號的方式被稱為EDS(evenly distributed sampling)方式。EDS方式并不是最佳的采樣方式。相關(guān)研究證明,在某些特定頻率點上z譜信號對理論模型參數(shù)的微小變化具有相對更高的敏感度,如果在這些特定的頻率點上多放置采樣點,可以期待參數(shù)估計的精確度得到提高�；谝陨险J識,Y.K.Tee等研究者針對DIACEST提出了優(yōu)化采樣方案(osptimal sampling schedule,以下簡稱為OSS),并在DIACEST的雙池模型上進行了仿真驗證。結(jié)果表明,OSS下的參數(shù)估計精度相對于EDS確實有很大的提高。本文在此基礎(chǔ)上,進行了以下幾個方面的研究:第一,進一步改進了尋找OSS的算法。本文引入概率分布描述CEST重要模型參數(shù)的先驗知識,提出了基于參數(shù)先驗概率分布的平均歸一化敏感度方程。第二,利用平均歸一化敏感度方程提出了PARACEST雙池模型和DIACEST三池模型的優(yōu)化采樣方案OSS。第三,通過仿真從更加全面的角度對OSS的性能作出分析。包括OSS性能與模型參數(shù)真實值分布的聯(lián)系,OSS在不同噪聲環(huán)境中的性能以及在采樣點數(shù)目不斷減少的情況下,OSS的性能變化趨勢。第四,試圖利用與Fisher信息矩陣相聯(lián)系的Hessian矩陣行列式來預(yù)測OSS的性能表現(xiàn)。本研究發(fā)現(xiàn):第一,OSS在CEST對比劑化學(xué)位移附近的采樣點明顯增加,與平均歸一化敏感度曲線在此處的峰值相對應(yīng)。第二,OSS的性能與CEST模型參數(shù)真實值的分布有密切聯(lián)系,這主要是由于z譜信號在不同的模型參數(shù)真實值附近對模型參數(shù)的微小變化具有不同的敏感度。Hessian矩陣行列式可以定性地描述這種聯(lián)系。第三,在仿真限定的條件下,OSS的性能在不同的噪聲環(huán)境下相對于EDS都具有優(yōu)越性。第四,OSS相對EDS更能抵抗采樣點數(shù)目減少帶來的影響。
[Abstract]:Chemical exchange saturation transfer imaging (chemical exchange saturation transfer MRI contrast,CEST) is a new nuclear magnetic resonance imaging technique. CEST is different from traditional MR contrast agents. It produces imaging contrast by chemical exchange between two or more protons in different chemical environments. The theoretical modeling of CEST is based on Bloch-Mc Connell equation. In actual CEST imaging, presaturation pulse sequences are usually applied at equal intervals on the frequency axis, and the Z spectrum of the collected signals will match with the theoretical model of CEST to estimate the physiological parameters we need to know. This way of collecting signals is called EDS (evenly distributed sampling). EDS is not the best sampling method. Studies have shown that z spectrum signals are more sensitive to minor changes in theoretical model parameters at certain frequency points, if sampling points are placed at these particular frequency points, The accuracy of parameter estimation can be expected to be improved. Based on the above understanding, Y.K.Tee and other researchers proposed an optimized sampling scheme for DIACEST. (osptimal sampling schedule, is referred to as OSS), for short, and the simulation is carried out on the double-cell model of DIACEST. The results show that the precision of parameter estimation under OSS is much higher than that of EDS. On this basis, this paper studies the following aspects: first, the algorithm of finding OSS is further improved. In this paper, we introduce a priori knowledge of probability distribution to describe the parameters of important CEST model, and propose an average normalized sensitivity equation based on a priori probability distribution of parameters. Secondly, using the average normalized sensitivity equation, an optimal sampling scheme, OSS., for PARACEST two-cell model and DIACEST three-cell model is proposed. Thirdly, the performance of OSS is analyzed from a more comprehensive point of view through simulation. Including the relationship between OSS performance and real value distribution of model parameters, the performance of OSS in different noise environments and the trend of OSS performance under the condition of decreasing number of sampling points. Fourthly, the determinant of Hessian matrix associated with Fisher information matrix is used to predict the performance of OSS. It is found that: first, the sampling points of OSS near the chemical shift of CEST contrast agent increase obviously, corresponding to the peak value of the average normalized sensitivity curve here. Secondly, the performance of OSS is closely related to the distribution of real values of CEST model parameters. This is mainly due to the fact that the z spectrum signal has different sensitivity to the slight variation of the model parameters near the real values of different model parameters. The Hessian matrix determinant can describe this relation qualitatively. Thirdly, under the limited conditions of simulation, the performance of OSS is superior to that of EDS in different noise environments. Fourth, OSS is more resistant to the impact of a decrease in the number of sampling points than EDS.
【學(xué)位授予單位】：哈爾濱工業(yè)大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2014
【分類號】：R445.2
本文編號：2321617
[Abstract]:Chemical exchange saturation transfer imaging (chemical exchange saturation transfer MRI contrast,CEST) is a new nuclear magnetic resonance imaging technique. CEST is different from traditional MR contrast agents. It produces imaging contrast by chemical exchange between two or more protons in different chemical environments. The theoretical modeling of CEST is based on Bloch-Mc Connell equation. In actual CEST imaging, presaturation pulse sequences are usually applied at equal intervals on the frequency axis, and the Z spectrum of the collected signals will match with the theoretical model of CEST to estimate the physiological parameters we need to know. This way of collecting signals is called EDS (evenly distributed sampling). EDS is not the best sampling method. Studies have shown that z spectrum signals are more sensitive to minor changes in theoretical model parameters at certain frequency points, if sampling points are placed at these particular frequency points, The accuracy of parameter estimation can be expected to be improved. Based on the above understanding, Y.K.Tee and other researchers proposed an optimized sampling scheme for DIACEST. (osptimal sampling schedule, is referred to as OSS), for short, and the simulation is carried out on the double-cell model of DIACEST. The results show that the precision of parameter estimation under OSS is much higher than that of EDS. On this basis, this paper studies the following aspects: first, the algorithm of finding OSS is further improved. In this paper, we introduce a priori knowledge of probability distribution to describe the parameters of important CEST model, and propose an average normalized sensitivity equation based on a priori probability distribution of parameters. Secondly, using the average normalized sensitivity equation, an optimal sampling scheme, OSS., for PARACEST two-cell model and DIACEST three-cell model is proposed. Thirdly, the performance of OSS is analyzed from a more comprehensive point of view through simulation. Including the relationship between OSS performance and real value distribution of model parameters, the performance of OSS in different noise environments and the trend of OSS performance under the condition of decreasing number of sampling points. Fourthly, the determinant of Hessian matrix associated with Fisher information matrix is used to predict the performance of OSS. It is found that: first, the sampling points of OSS near the chemical shift of CEST contrast agent increase obviously, corresponding to the peak value of the average normalized sensitivity curve here. Secondly, the performance of OSS is closely related to the distribution of real values of CEST model parameters. This is mainly due to the fact that the z spectrum signal has different sensitivity to the slight variation of the model parameters near the real values of different model parameters. The Hessian matrix determinant can describe this relation qualitatively. Thirdly, under the limited conditions of simulation, the performance of OSS is superior to that of EDS in different noise environments. Fourth, OSS is more resistant to the impact of a decrease in the number of sampling points than EDS.
【學(xué)位授予單位】：哈爾濱工業(yè)大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2014
【分類號】：R445.2
【共引文獻】
相關(guān)博士學(xué)位論文 前3條
1 盧建華;化學(xué)交換飽和轉(zhuǎn)移及其在MRI中的應(yīng)用[D];廈門大學(xué);2014年
2 洪曉華;用酰胺質(zhì)子轉(zhuǎn)移磁共振成像技術(shù)評價鼠腦膠質(zhì)瘤放療療效[D];華中科技大學(xué);2014年
3 朱筱磊;環(huán)境響應(yīng)型CEST MRI造影劑的構(gòu)建及其性質(zhì)研究[D];中國科學(xué)院研究生院(武漢物理與數(shù)學(xué)研究所);2015年
，本文編號：2321617
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