利用穩定態影像達成之功能性磁振造影：在高磁場系統上的技術與應用

FMRI Utilizing Blood Oxygenation Sensitive Steady-state Imaging: Techniques and Applications at High Field

吳明龍(Ming-Long Wu)

指導教授 ： 鍾孝文

國立臺灣大學/電機資訊學院/電機工程學研究所/博士(2006年)

https://doi.org/10.6342/NTU.2006.00774

摘要

利用穩定態脈衝程序來達成功能性磁振造影是近年來發展出的一項新技術。本文主要的目的是探討穩定態功能性磁振造影技術的原理，以及在高場磁振造影系統上的應用。相較於已被廣泛使用的面回訊功能性磁振造影，穩定態功能性磁振造影具有較少的假影，更高的功能性靈敏度，以及達成高解析度功能性磁振造影的潛力。然而，由於穩定態功能性磁振造影的敏感區集中於極窄的頻帶，進行功能性磁振造影實驗過程中，若系統頻率偏移，將會失去功能性對比，而無法偵測到血氧濃度因腦部活動的改變。在本文當中我們提出一個改善頻率穩定度的方法，增進此技術在高磁場 3 Tesla系統上的可行性。在擷取每張影像之前，利用一個小角度射頻波得到頻率隨時間偏移的資訊，並即時更新系統頻率。我們更進一部採用無限脈衝響應濾波 (IIR filtering) 的方式，對小角度射頻波方法估計的頻率偏移做低通濾波，以減少更新頻率之震盪，增進信號之穩定度。實驗結果顯示利用 IIR filtering 的方式更新頻率，頻率震盪維持在 0.5 赫茲以內；在穩定態功能性磁振造影實驗過程當中，信號變異維持在 5% 以下。我們並進行高解析度功能性磁振造影實驗(單位像素長寬在一釐米以下)，在給予視覺刺激的實驗當中，因血氧濃度改變產生之信號變化達到 10-17%，偵測到的活化區域和視覺區灰質有良好的對應。因此，穩定態功能性磁振造影具有低假影及高解析力的特性；採用適當的穩定頻率的方法，則使得此一新技術，更適於高磁場磁振系統，達到更高的靈敏度。

關鍵字

血氧濃度敏感穩定態 ； 功能性磁振造影 ； 穩定態造影術 ； 頻率穩定技術 ； 無限脈衝響應濾波

並列摘要

One major challenge of Blood Oxygenation Sensitive Steady-state (BOSS) fMRI is that the frequency band corresponding to highest functional sensitivity is extremely narrow, leading to substantial loss of functional contrast in the presence of magnetic field drifts. In this thesis, a frequency stabilization scheme was proposed for BOSS fMRI at 3T, where an radio-frequency (RF) pulse with very small flip angle was applied before each image scan, with the initial phase of the excited free induction decay signals extracted to reflect the temporal field drifts. A simple infinite impulse response (IIR) filter was further employed to obtain a low-pass filtered estimate of the central reference frequency for the upcoming scan. Experimental results suggest that the proposed scheme was able to stabilize the central reference frequency settings in accordance with the magnetic drifts, with oscillation amplitudes less than 0.5Hz. Phantom studies show that both the slow drifts and the fast fluctuations were prominently reduced for the dynamic BOSS scans, resulting in less than 5% signal variations. Visual fMRI at sub-millimeter in-plane resolution further demonstrated 10-17% activation signals nicely registered in the microvessels within the sulci. It is concluded that the IIR-filtered frequency stabilization is effective for reliable BOSS fMRI at high fields.

並列關鍵字

blood oxygenation sensitive steady-state ； functional magnetic resonance imaging ； balanced steady-state free precession ； frequency stabilization ； infinite impulse response filter

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