Detecting the changes of pH level using the CEST effect of MRI

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研究生：	吳怡君 Wu, Yi-Chun
論文名稱：	Detecting the changes of pH level using the CEST effect of MRI 運用磁振造影的化學交換飽和轉移效應量測酸鹼值的變化
指導教授：	王福年 Wang, Fu-Nien
口試委員:
學位類別：	碩士 Master
系所名稱：	原子科學院 - 生醫工程與環境科學系 Department of Biomedical Engineering and Environmental Sciences
論文出版年：	2010
畢業學年度：	98
語文別：	英文
論文頁數：	62
中文關鍵詞：	化學交換飽和轉移、磁振造影、磁振轉移比值、質子轉移比值
外文關鍵詞：	Chemical Exchange Saturation Transfer, MRI, asymmetry ratio of z-spectra, Proton Transfer Ratio
相關次數：	點閱：1 下載：0
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查詢本校圖書館目錄查詢臺灣博碩士論文知識加值系統勘誤回報

水分子上的質子與環境中其他化學物質上的質子進行化學交換的現象可透過磁振造影產生與傳統不同的對比影像，影像的對比來源為量測特定射頻頻率激發下飽和效應所引起的磁轉移比值，這種基於質子化學交換原理的磁振造影技術已被證明可以偵測到影像中不同酸鹼值的影響，可提供活體組織一非侵入性的酸鹼値監測。本研究目的嘗試以此新穎的技術應用於4.7 Tesla磁振造影儀中實現出酸鹼值變化的對比影像。
實驗的過程大致分成三個部份：(1) 磁振造影參數的最佳化，(2) 使用含有不同酸鹼值的假體進行實驗，(3) 動物實驗。為了可以順利發揮最大質子化學交換飽和轉移(CEST)於磁振造影，我們以連續波的激發方式並調整不同的射頻參數對假體進行磁振造影的掃描，得到最佳飽和效應的射頻參數設定，接著應用到含有不同酸鹼值的假體驗証CEST效應，在動物身上的酸鹼值調控則透過改變組織的生理條件達成，例如：吸入CO2氣體、死亡。
由於CEST效應容易受到不均勻磁場的影響造成估計誤差，因此我們的研究中額外收取一組磁場影像進行校正，可以逐一像素回復CEST 效應，並且探討運用磁場影像校正時的效益，在假體與動物實驗的效應評估分別使用磁振轉移比值(MTRasym)和質子轉移比值(proton transfer ratio)表示，從結果發現，當酸鹼值下降時，磁振造影偵測到的CEST效應亦隨之減小。此種快速、非侵入性的酸鹼值檢測技術具有潛力成為未來磁振掃描的臨床生理指標之一。

Chemical exchange saturation transfer (CEST) provides a novel contrast based on the chemical environment of protons. It has been shown its ability to detect the pH change in MRI. Here we try to reproduce the pH-sensitive contrast using our MR scanning system. Our study involves three parts: parameters optimization, in vitro and in vivo experiments of imaging the pH change.
To perform CEST imaging, concentration phantom experiments on a MRI scanner were first conducted to optimize the scanning parameters for CEST contrast. The pH phantom experiments were to verify the CEST effect with the optimal parameters. Then, in order to in vivo detect the pH changes, the brains of rats were scanned to monitor the biochemical variation under hypercapnia and cardiac arrested. However, the CEST effect is very sensitive to the magnetic field inhomogeneity. Concerning this interference, we developed the field map (FM) containing the local off-resonance frequency information in pixels to correct the measured CEST effect. The induced CEST effect was quantified by the asymmetry ratio of z-spectra (MTRasym) or proton transfer ratio (PTR) in this study.
The results showed the CEST effect decreased upon a lower pH level. Finally, the quantifications of FM efficiency were also included. A technique of imaging pH by endogenous contrast would allow an immediate, noninvasive clinical use of physiological indicator as part of a multi-modality MRI exam.

CONTENTS
致謝    i
摘要    ii
ABSTRACT    iii
CHAPTER 1    1
INTRODUCTION    1
CHAPTER 2    4
THEORY    4
2.1 CHEMICAL EXCHANGE PHENOMENON    4
2.2 HYDROGEN EXCHANGE    5
2.3 TWO-SITE EXCHANGE THEORY OF CEST    8
2.4 APPROXIMATION OF A WEAK SATURATION PULSE    11
2.5 STEADY-STATE SOLUTION UNDER THE WEAK SATURATION PULSE APPROXIMATION    12
2.6 TIME-DEPENDENT SOLUTIONS UNDER THE WEAK SATURATION PULSE APPROXIMATION    13
CHAPTER 3    16
MATERIALS AND METHODS    16
3.1 PREPARATIONS FOR THE EXPERIMENT    16
3.1.1 Phantom preparation    16
3.1.2 Animal preparation    18
3.2 MAGNETIC RESONANCE IMAGING    18
3.2.1 Sequence    18
3.2.2 B0 correction    20
3.2.3 Scan protocol    22
3.3 POST-PROCESSING    23
CHAPTER 4    25
RESULTS    25
4.1 PHANTOM    25
4.1.1 Optimization of experimental schemes including the RF power and duration    25
4.1.2 pH-sensitive CEST effect in vitro    36
4.2 RATS    40
4.2.1 pH-sensitive CEST effect in vivo    40
4.2.2 Quantifications of the field map (FM) efficiency    49
4.2.3 Mapping the CEST effect    52
CHAPTER 5    55
DISCUSSION    55
5.1 FIELD MAP (FM) USED FOR B0 FIELD CORRECTION    55
5.2 CEST EFFECTS IN HYPERCAPNIA CASE    56
CHAPTER 6    59
CONCLUSIONS    59
CHAPTER 7    61
REFERENCES    61

                                

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