臺灣博碩士論文加值系統

磁振擴散頻譜技術(Diffusion Spectrum MRI)能夠從磁振影像上每個像素內的水訊號去準確分析水分子的擴散分布情況，這有助於從磁振影像去觀察大腦神經之病變；然而，目前為止在任何文獻上卻都找不到跟擴散頻譜影像之取像參數相關的研究。
在本篇研究中，我們用蒙地卡羅的方法去模擬腦神經內外之訊號，之後再利用擴散頻譜技術去分析，看在怎麼樣的取像參數下可以得到最佳的分析結果。在模擬中神經的數目是被設定成兩根，神經的直徑為5μm，研究的目的是希望採取較簡化的模擬模型，進一步去探討在怎麼樣的擴散時間與擴散磁場持續時間之關係、多大的b值以及多少取像點數目下等方能得到較好的神經位置辨別率，與在分析結果上有較少的假神經出現。
模擬結果顯示，當取像是用比較低的b值時 (≦ 10000 s/mm²)，擴散時間必須遠大於擴散磁場持續時間方能得到較好分析結果。而取像若用較大的b值(> 10000 s/mm²)時，擴散時間與擴散磁場持續時間之關係則不用被限定，因為在較大的b值下，磁振造影系統對交叉神經纖維之訊號的分辨能力較強，如此可彌補擴散時間並非遠大於磁場持續時間這件事。而這樣的彌補特性會隨著b值愈大而益發明顯。除此之外，從模擬中我們發現最佳b值與建議的取像點數目分別為 [16000, 18000] s/mm²和 515，兩者皆是因為能夠得到最好的神經位置辨別率，與在分析結果上有較少的假神經出現方被認定為最佳參數。
在臨床上磁振系統有許多限制，例如擴散磁場強度不可超過5 Gauss/cm等，目前都尚未被突破。在目前這樣的磁振技術限制下，我們唯一能做的就是在取像時用最佳造影參數，以期能夠得到較好的影像分析結果。而本篇研究則是提供一個指標，讓大家了解什麼是擴散頻譜影像的最佳造影參數，日後在取影像時也能夠將其有所應用。

Diffusion Spectrum MRI (DSI) can be used to define complex oriented coherence accurately by mapping 3D orientation density function (ODF) of water molecular diffusion within each MRI voxel. However, the optimal imaging parameters for DSI to discriminate crossing fibers have not been studied yet. In this study, a Monte-Carlo simulation model is used to optimize the DSI imaging parameters such as the relationship between diffusion time and diffusion duration, b value, and the number of encoding points, for intersecting fiber mapping. Furthermore, the indices used for the optimization are Minimum Angular Resolution (MAR) and Length of Pseudo-Fiber (LPF), which represent the smallest crossing angle that could be distinguished by DSI and the longest ODF length of errors within DSI post-processing results, respectively. From the study, in lower b values (≦ 10000 s/mm²), diffusion time >> diffusion duration is requested to obtain better angular discrimination of crossing nerves; in higher b values (> 10000 s/mm²), diffusion time >> diffusion duration is not necessary because of the compensation of higher crossing discrimination ability due to higher b values. Besides, the optimal b value and the suggested number of encoding points are [16000, 18000] s/mm² and 515 respectively. In clinical, there are still many limitations, such as small maximum strength of bipolar gradients and difficult achievements of diffusion time >> diffusion duration, have not been broken through yet. Under the limitation, optimal imaging parameters seem to be the only way for acquiring images with the best results of DSI algorithms. The research could be of help to get gold standards for this manner.

1. Backgrounds..........................................15
1-1 Brownian Motion...................................15
1-2 Background........................................18
1-3 Diffusion Measurements by NMR.....................23
1-3-1 Spin Echo-EPI..................................23
1-3-2 Diffusion Spectrum Magnetic Resonance Imaging..25
2. Introductions........................................32
3. Methods..............................................36
4. Results..............................................44
The Comparisons Between (Δ>>δ) and (Δ~δ)............44
Optimal b Value Fitting in with Clinical Situations..46
Optimal Number of Encoding Directions...............49
Optimal SNR while Acquiring Diffusion Images........50
5. Discussions..........................................51
Verification of Hindered Diffusion in the Simulation.51
Verification of Restricted Diffusion in the Simulation..............................................53
The Comparisons Between (Δ>>δ) and (Δ~δ)...........55
Optimal b Value Fitting in with Clinical Situations.............................................57
Optimal Number of Encoding Directions..............58
5. Conclusions.........................................60
6. Reference...........................................78

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